dash — command interpreter (shell)


     dash [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o option_name]
      [+o option_name] [command_file [argument ...]]
     dash -c [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o option_name]
      [+o option_name] command_string [command_name [argument ...]]
     dash -s [-aCefnuvxIimqVEbp] [+aCefnuvxIimqVEbp] [-o option_name]
      [+o option_name] [argument ...]


     dash is the standard command interpreter for the system.  The current
     version of dash is in the process of being changed to conform with the
     POSIX 1003.2 and 1003.2a specifications for the shell.  This version has
     many features which make it appear similar in some respects to the Korn
     shell, but it is not a Korn shell clone (see ksh(1)).  Only features
     designated by POSIX, plus a few Berkeley extensions, are being
     incorporated into this shell.  This man page is not intended to be a
     tutorial or a complete specification of the shell.

     The shell is a command that reads lines from either a file or the
     terminal, interprets them, and generally executes other commands.  It is
     the program that is running when a user logs into the system (although a
     user can select a different shell with the chsh(1) command).  The shell
     implements a language that has flow control constructs, a macro facility
     that provides a variety of features in addition to data storage, along
     with built in history and line editing capabilities.  It incorporates
     many features to aid interactive use and has the advantage that the
     interpretative language is common to both interactive and non-interactive
     use (shell scripts).  That is, commands can be typed directly to the
     running shell or can be put into a file and the file can be executed
     directly by the shell.

     If no args are present and if the standard input of the shell is
     connected to a terminal (or if the -i flag is set), and the -c option is
     not present, the shell is considered an interactive shell.  An
     interactive shell generally prompts before each command and handles
     programming and command errors differently (as described below).  When
     first starting, the shell inspects argument 0, and if it begins with a
     dash ‘-’, the shell is also considered a login shell.  This is normally
     done automatically by the system when the user first logs in.  A login
     shell first reads commands from the files /etc/profile and .profile if
     they exist.  If the environment variable ENV is set on entry to an
     interactive shell, or is set in the .profile of a login shell, the shell
     next reads commands from the file named in ENV.  Therefore, a user should
     place commands that are to be executed only at login time in the .profile
     file, and commands that are executed for every interactive shell inside
     the ENV file.  To set the ENV variable to some file, place the following
     line in your .profile of your home directory

       ENV=$HOME/.shinit; export ENV

     substituting for “.shinit” any filename you wish.

     If command line arguments besides the options have been specified, then
     the shell treats the first argument as the name of a file from which to
     read commands (a shell script), and the remaining arguments are set as
     the positional parameters of the shell ($1, $2, etc).  Otherwise, the
     shell reads commands from its standard input.

   Argument List Processing
     All of the single letter options that have a corresponding name can be
     used as an argument to the -o option.  The set -o name is provided next
     to the single letter option in the description below.  Specifying a dash
     “-” turns the option on, while using a plus “+” disables the option.  The
     following options can be set from the command line or with the set
     builtin (described later).

       -a allexport     Export all variables assigned to.

       -c               Read commands from the command_string operand
                        instead of from the standard input.  Special
                        parameter 0 will be set from the command_name
                        operand and the positional parameters ($1, $2,
                        etc.)  set from the remaining argument operands.

       -C noclobber     Don't overwrite existing files with “>”.

       -e errexit       If not interactive, exit immediately if any
                        untested command fails.  The exit status of a
                        command is considered to be explicitly tested if
                        the command is used to control an if, elif, while,
                        or until; or if the command is the left hand
                        operand of an “&&” or “||” operator.

       -f noglob        Disable pathname expansion.

       -n noexec        If not interactive, read commands but do not
                        execute them.  This is useful for checking the
                        syntax of shell scripts.

       -u nounset       Write a message to standard error when attempting
                        to expand a variable that is not set, and if the
                        shell is not interactive, exit immediately.

       -v verbose       The shell writes its input to standard error as it
                        is read.  Useful for debugging.

       -x xtrace        Write each command to standard error (preceded by
                        a ‘+ ’) before it is executed.  Useful for

       -I ignoreeof     Ignore EOF's from input when interactive.

       -i interactive   Force the shell to behave interactively.

       -l               Make dash act as if it had been invoked as a login

       -m monitor       Turn on job control (set automatically when

       -s stdin         Read commands from standard input (set
                        automatically if no file arguments are present).
                        This option has no effect when set after the shell
                        has already started running (i.e. with set).

       -V vi            Enable the built-in vi(1) command line editor
                        (disables -E if it has been set).

       -E emacs         Enable the built-in emacs(1) command line editor
                        (disables -V if it has been set).

       -b notify        Enable asynchronous notification of background job
                        completion.  (UNIMPLEMENTED for 4.4alpha)

       -p priv          Do not attempt to reset effective uid if it does
                        not match uid. This is not set by default to help
                        avoid incorrect usage by setuid root programs via
                        system(3) or popen(3).

   Lexical Structure
     The shell reads input in terms of lines from a file and breaks it up into
     words at whitespace (blanks and tabs), and at certain sequences of
     characters that are special to the shell called “operators”.  There are
     two types of operators: control operators and redirection operators
     (their meaning is discussed later).  Following is a list of operators:

       Control operators:
             & && ( ) ; ;; | || <newline>

       Redirection operators:
             < > >| << >> <& >& <<- <>

     Quoting is used to remove the special meaning of certain characters or
     words to the shell, such as operators, whitespace, or keywords.  There
     are three types of quoting: matched single quotes, matched double quotes,
     and backslash.

     A backslash preserves the literal meaning of the following character,
     with the exception of ⟨newline⟩.  A backslash preceding a ⟨newline⟩ is
     treated as a line continuation.

   Single Quotes
     Enclosing characters in single quotes preserves the literal meaning of
     all the characters (except single quotes, making it impossible to put
     single-quotes in a single-quoted string).

   Double Quotes
     Enclosing characters within double quotes preserves the literal meaning
     of all characters except dollarsign ($), backquote (`), and backslash
     (\).  The backslash inside double quotes is historically weird, and
     serves to quote only the following characters:
       $ ` " \ <newline>.
     Otherwise it remains literal.

   Reserved Words
     Reserved words are words that have special meaning to the shell and are
     recognized at the beginning of a line and after a control operator.  The
     following are reserved words:

       !       elif    fi      while   case
       else    for     then    {       }
       do      done    until   if      esac

     Their meaning is discussed later.

     An alias is a name and corresponding value set using the alias(1) builtin
     command.  Whenever a reserved word may occur (see above), and after
     checking for reserved words, the shell checks the word to see if it
     matches an alias.  If it does, it replaces it in the input stream with
     its value.  For example, if there is an alias called “lf” with the value
     “ls -F”, then the input:

       lf foobar ⟨return⟩

     would become

       ls -F foobar ⟨return⟩

     Aliases provide a convenient way for naive users to create shorthands for
     commands without having to learn how to create functions with arguments.
     They can also be used to create lexically obscure code.  This use is

     The shell interprets the words it reads according to a language, the
     specification of which is outside the scope of this man page (refer to
     the BNF in the POSIX 1003.2 document).  Essentially though, a line is
     read and if the first word of the line (or after a control operator) is
     not a reserved word, then the shell has recognized a simple command.
     Otherwise, a complex command or some other special construct may have
     been recognized.

   Simple Commands
     If a simple command has been recognized, the shell performs the following

       1.   Leading words of the form “name=value” are stripped off and
            assigned to the environment of the simple command.
            Redirection operators and their arguments (as described below)
            are stripped off and saved for processing.

       2.   The remaining words are expanded as described in the section
            called “Expansions”, and the first remaining word is
            considered the command name and the command is located.  The
            remaining words are considered the arguments of the command.
            If no command name resulted, then the “name=value” variable
            assignments recognized in item 1 affect the current shell.

       3.   Redirections are performed as described in the next section.

     Redirections are used to change where a command reads its input or sends
     its output.  In general, redirections open, close, or duplicate an
     existing reference to a file.  The overall format used for redirection

       [n] redir-op file

     where redir-op is one of the redirection operators mentioned previously.
     Following is a list of the possible redirections.  The [n] is an optional
     number, as in ‘3’ (not ‘[3]’), that refers to a file descriptor.

       [n]> file   Redirect standard output (or n) to file.

       [n]>| file  Same, but override the -C option.

       [n]>> file  Append standard output (or n) to file.

       [n]< file   Redirect standard input (or n) from file.

       [n1]<&n2    Duplicate standard input (or n1) from file descriptor

       [n]<&-      Close standard input (or n).

       [n1]>&n2    Duplicate standard output (or n1) to n2.

       [n]>&-      Close standard output (or n).

       [n]<> file  Open file for reading and writing on standard input (or

     The following redirection is often called a “here-document”.

       [n]<< delimiter
             here-doc-text ...

     All the text on successive lines up to the delimiter is saved away and
     made available to the command on standard input, or file descriptor n if
     it is specified.  If the delimiter as specified on the initial line is
     quoted, then the here-doc-text is treated literally, otherwise the text
     is subjected to parameter expansion, command substitution, and arithmetic
     expansion (as described in the section on “Expansions”).  If the operator
     is “<<-” instead of “<<”, then leading tabs in the here-doc-text are

   Search and Execution
     There are three types of commands: shell functions, builtin commands, and
     normal programs -- and the command is searched for (by name) in that
     order.  They each are executed in a different way.

     When a shell function is executed, all of the shell positional parameters
     (except $0, which remains unchanged) are set to the arguments of the
     shell function.  The variables which are explicitly placed in the
     environment of the command (by placing assignments to them before the
     function name) are made local to the function and are set to the values
     given.  Then the command given in the function definition is executed.
     The positional parameters are restored to their original values when the
     command completes.  This all occurs within the current shell.

     Shell builtins are executed internally to the shell, without spawning a
     new process.

     Otherwise, if the command name doesn't match a function or builtin, the
     command is searched for as a normal program in the file system (as
     described in the next section).  When a normal program is executed, the
     shell runs the program, passing the arguments and the environment to the
     program.  If the program is not a normal executable file (i.e., if it
     does not begin with the "magic number" whose ASCII representation is
     "#!", so execve(2) returns ENOEXEC then) the shell will interpret the
     program in a subshell.  The child shell will reinitialize itself in this
     case, so that the effect will be as if a new shell had been invoked to
     handle the ad-hoc shell script, except that the location of hashed
     commands located in the parent shell will be remembered by the child.

     Note that previous versions of this document and the source code itself
     misleadingly and sporadically refer to a shell script without a magic
     number as a "shell procedure".

   Path Search
     When locating a command, the shell first looks to see if it has a shell
     function by that name.  Then it looks for a builtin command by that name.
     If a builtin command is not found, one of two things happen:

     1.   Command names containing a slash are simply executed without
      performing any searches.

     2.   The shell searches each entry in PATH in turn for the command.  The
      value of the PATH variable should be a series of entries separated
      by colons.  Each entry consists of a directory name.  The current
      directory may be indicated implicitly by an empty directory name, or
      explicitly by a single period.

   Command Exit Status
     Each command has an exit status that can influence the behaviour of other
     shell commands.  The paradigm is that a command exits with zero for
     normal or success, and non-zero for failure, error, or a false
     indication.  The man page for each command should indicate the various
     exit codes and what they mean.  Additionally, the builtin commands return
     exit codes, as does an executed shell function.

     If a command consists entirely of variable assignments then the exit
     status of the command is that of the last command substitution if any,
     otherwise 0.

   Complex Commands
     Complex commands are combinations of simple commands with control
     operators or reserved words, together creating a larger complex command.
     More generally, a command is one of the following:

     ·   simple command

     ·   pipeline

     ·   list or compound-list

     ·   compound command

     ·   function definition

     Unless otherwise stated, the exit status of a command is that of the last
     simple command executed by the command.

     A pipeline is a sequence of one or more commands separated by the control
     operator |.  The standard output of all but the last command is connected
     to the standard input of the next command.  The standard output of the
     last command is inherited from the shell, as usual.

     The format for a pipeline is:

       [!] command1 [| command2 ...]

     The standard output of command1 is connected to the standard input of
     command2.  The standard input, standard output, or both of a command is
     considered to be assigned by the pipeline before any redirection
     specified by redirection operators that are part of the command.

     If the pipeline is not in the background (discussed later), the shell
     waits for all commands to complete.

     If the reserved word ! does not precede the pipeline, the exit status is
     the exit status of the last command specified in the pipeline.
     Otherwise, the exit status is the logical NOT of the exit status of the
     last command.  That is, if the last command returns zero, the exit status
     is 1; if the last command returns greater than zero, the exit status is

     Because pipeline assignment of standard input or standard output or both
     takes place before redirection, it can be modified by redirection.  For

       $ command1 2>&1 | command2

     sends both the standard output and standard error of command1 to the
     standard input of command2.

     A ; or ⟨newline⟩ terminator causes the preceding AND-OR-list (described
     next) to be executed sequentially; a & causes asynchronous execution of
     the preceding AND-OR-list.

     Note that unlike some other shells, each process in the pipeline is a
     child of the invoking shell (unless it is a shell builtin, in which case
     it executes in the current shell -- but any effect it has on the
     environment is wiped).

   Background Commands -- &
     If a command is terminated by the control operator ampersand (&), the
     shell executes the command asynchronously -- that is, the shell does not
     wait for the command to finish before executing the next command.

     The format for running a command in background is:

       command1 & [command2 & ...]

     If the shell is not interactive, the standard input of an asynchronous
     command is set to /dev/null.

   Lists -- Generally Speaking
     A list is a sequence of zero or more commands separated by newlines,
     semicolons, or ampersands, and optionally terminated by one of these
     three characters.  The commands in a list are executed in the order they
     are written.  If command is followed by an ampersand, the shell starts
     the command and immediately proceed onto the next command; otherwise it
     waits for the command to terminate before proceeding to the next one.

   Short-Circuit List Operators
     “&&” and “||” are AND-OR list operators.  “&&” executes the first
     command, and then executes the second command iff the exit status of the
     first command is zero.  “||” is similar, but executes the second command
     iff the exit status of the first command is nonzero.  “&&” and “||” both
     have the same priority.

   Flow-Control Constructs -- if, while, for, case
     The syntax of the if command is

       if list
       then list
       [ elif list
       then    list ] ...
       [ else list ]

     The syntax of the while command is

       while list
       do   list

     The two lists are executed repeatedly while the exit status of the first
     list is zero.  The until command is similar, but has the word until in
     place of while, which causes it to repeat until the exit status of the
     first list is zero.

     The syntax of the for command is

       for variable [ in [ word ... ] ]
       do   list

     The words following in are expanded, and then the list is executed
     repeatedly with the variable set to each word in turn.  Omitting in word
     ... is equivalent to in "$@".

     The syntax of the break and continue command is

       break [ num ]
       continue [ num ]

     Break terminates the num innermost for or while loops.  Continue
     continues with the next iteration of the innermost loop.  These are
     implemented as builtin commands.

     The syntax of the case command is

       case word in
       [(]pattern) list ;;

     The pattern can actually be one or more patterns (see Shell Patterns
     described later), separated by “|” characters.  The “(” character before
     the pattern is optional.

   Grouping Commands Together
     Commands may be grouped by writing either



       { list; }

     The first of these executes the commands in a subshell.  Builtin commands
     grouped into a (list) will not affect the current shell.  The second form
     does not fork another shell so is slightly more efficient.  Grouping
     commands together this way allows you to redirect their output as though
     they were one program:

       { printf " hello " ; printf " world\n" ; } > greeting

     Note that “}” must follow a control operator (here, “;”) so that it is
     recognized as a reserved word and not as another command argument.

     The syntax of a function definition is

       name () command

     A function definition is an executable statement; when executed it
     installs a function named name and returns an exit status of zero.  The
     command is normally a list enclosed between “{” and “}”.

     Variables may be declared to be local to a function by using a local
     command.  This should appear as the first statement of a function, and
     the syntax is

       local [variable | -] ...

     Local is implemented as a builtin command.

     When a variable is made local, it inherits the initial value and exported
     and readonly flags from the variable with the same name in the
     surrounding scope, if there is one.  Otherwise, the variable is initially
     unset.  The shell uses dynamic scoping, so that if you make the variable
     x local to function f, which then calls function g, references to the
     variable x made inside g will refer to the variable x declared inside f,
     not to the global variable named x.

     The only special parameter that can be made local is “-”.  Making “-”
     local any shell options that are changed via the set command inside the
     function to be restored to their original values when the function

     The syntax of the return command is

       return [exitstatus]

     It terminates the currently executing function.  Return is implemented as
     a builtin command.

   Variables and Parameters
     The shell maintains a set of parameters.  A parameter denoted by a name
     is called a variable.  When starting up, the shell turns all the
     environment variables into shell variables.  New variables can be set
     using the form


     Variables set by the user must have a name consisting solely of
     alphabetics, numerics, and underscores - the first of which must not be
     numeric.  A parameter can also be denoted by a number or a special
     character as explained below.

   Positional Parameters
     A positional parameter is a parameter denoted by a number (n > 0).  The
     shell sets these initially to the values of its command line arguments
     that follow the name of the shell script.  The set builtin can also be
     used to set or reset them.

   Special Parameters
     A special parameter is a parameter denoted by one of the following
     special characters.  The value of the parameter is listed next to its

     *            Expands to the positional parameters, starting from one.
              When the expansion occurs within a double-quoted string it
              expands to a single field with the value of each parameter
              separated by the first character of the IFS variable, or by
              a ⟨space⟩ if IFS is unset.

     @            Expands to the positional parameters, starting from one.
              When the expansion occurs within double-quotes, each
              positional parameter expands as a separate argument.  If
              there are no positional parameters, the expansion of @
              generates zero arguments, even when @ is double-quoted.
              What this basically means, for example, is if $1 is “abc”
              and $2 is “def ghi”, then "$@" expands to the two arguments:

                    "abc" "def ghi"

     #            Expands to the number of positional parameters.

     ?            Expands to the exit status of the most recent pipeline.

     - (Hyphen.)  Expands to the current option flags (the single-letter
              option names concatenated into a string) as specified on
              invocation, by the set builtin command, or implicitly by the

     $            Expands to the process ID of the invoked shell.  A subshell
              retains the same value of $ as its parent.

     !            Expands to the process ID of the most recent background
              command executed from the current shell.  For a pipeline,
              the process ID is that of the last command in the pipeline.

     0 (Zero.)    Expands to the name of the shell or shell script.

   Word Expansions
     This clause describes the various expansions that are performed on words.
     Not all expansions are performed on every word, as explained later.

     Tilde expansions, parameter expansions, command substitutions, arithmetic
     expansions, and quote removals that occur within a single word expand to
     a single field.  It is only field splitting or pathname expansion that
     can create multiple fields from a single word.  The single exception to
     this rule is the expansion of the special parameter @ within double-
     quotes, as was described above.

     The order of word expansion is:

     1.   Tilde Expansion, Parameter Expansion, Command Substitution,
      Arithmetic Expansion (these all occur at the same time).

     2.   Field Splitting is performed on fields generated by step (1) unless
      the IFS variable is null.

     3.   Pathname Expansion (unless set -f is in effect).

     4.   Quote Removal.

     The $ character is used to introduce parameter expansion, command
     substitution, or arithmetic evaluation.

   Tilde Expansion (substituting a user's home directory)
     A word beginning with an unquoted tilde character (~) is subjected to
     tilde expansion.  All the characters up to a slash (/) or the end of the
     word are treated as a username and are replaced with the user's home
     directory.  If the username is missing (as in ~/foobar), the tilde is
     replaced with the value of the HOME variable (the current user's home

   Parameter Expansion
     The format for parameter expansion is as follows:


     where expression consists of all characters until the matching “}”.  Any
     “}” escaped by a backslash or within a quoted string, and characters in
     embedded arithmetic expansions, command substitutions, and variable
     expansions, are not examined in determining the matching “}”.

     The simplest form for parameter expansion is:


     The value, if any, of parameter is substituted.

     The parameter name or symbol can be enclosed in braces, which are
     optional except for positional parameters with more than one digit or
     when parameter is followed by a character that could be interpreted as
     part of the name.  If a parameter expansion occurs inside double-quotes:

     1.   Pathname expansion is not performed on the results of the expansion.

     2.   Field splitting is not performed on the results of the expansion,
      with the exception of @.

     In addition, a parameter expansion can be modified by using one of the
     following formats.

     ${parameter:-word}    Use Default Values.  If parameter is unset or null,
                       the expansion of word is substituted; otherwise,
                       the value of parameter is substituted.

     ${parameter:=word}    Assign Default Values.  If parameter is unset or
                       null, the expansion of word is assigned to
                       parameter.  In all cases, the final value of
                       parameter is substituted.  Only variables, not
                       positional parameters or special parameters, can be
                       assigned in this way.

     ${parameter:?[word]}  Indicate Error if Null or Unset.  If parameter is
                       unset or null, the expansion of word (or a message
                       indicating it is unset if word is omitted) is
                       written to standard error and the shell exits with
                       a nonzero exit status.  Otherwise, the value of
                       parameter is substituted.  An interactive shell
                       need not exit.

     ${parameter:+word}    Use Alternative Value.  If parameter is unset or
                       null, null is substituted; otherwise, the expansion
                       of word is substituted.

     In the parameter expansions shown previously, use of the colon in the
     format results in a test for a parameter that is unset or null; omission
     of the colon results in a test for a parameter that is only unset.

     ${#parameter}         String Length.  The length in characters of the
                       value of parameter.

     The following four varieties of parameter expansion provide for substring
     processing.  In each case, pattern matching notation (see Shell
     Patterns), rather than regular expression notation, is used to evaluate
     the patterns.  If parameter is * or @, the result of the expansion is
     unspecified.  Enclosing the full parameter expansion string in double-
     quotes does not cause the following four varieties of pattern characters
     to be quoted, whereas quoting characters within the braces has this

     ${parameter%word}     Remove Smallest Suffix Pattern.  The word is
                       expanded to produce a pattern.  The parameter
                       expansion then results in parameter, with the
                       smallest portion of the suffix matched by the
                       pattern deleted.

     ${parameter%%word}    Remove Largest Suffix Pattern.  The word is
                       expanded to produce a pattern.  The parameter
                       expansion then results in parameter, with the
                       largest portion of the suffix matched by the
                       pattern deleted.

     ${parameter#word}     Remove Smallest Prefix Pattern.  The word is
                       expanded to produce a pattern.  The parameter
                       expansion then results in parameter, with the
                       smallest portion of the prefix matched by the
                       pattern deleted.

     ${parameter##word}    Remove Largest Prefix Pattern.  The word is
                       expanded to produce a pattern.  The parameter
                       expansion then results in parameter, with the
                       largest portion of the prefix matched by the
                       pattern deleted.

   Command Substitution
     Command substitution allows the output of a command to be substituted in
     place of the command name itself.  Command substitution occurs when the
     command is enclosed as follows:


     or (“backquoted” version):


     The shell expands the command substitution by executing command in a
     subshell environment and replacing the command substitution with the
     standard output of the command, removing sequences of one or more
     ⟨newline⟩s at the end of the substitution.  (Embedded ⟨newline⟩s before
     the end of the output are not removed; however, during field splitting,
     they may be translated into ⟨space⟩s, depending on the value of IFS and
     quoting that is in effect.)

   Arithmetic Expansion
     Arithmetic expansion provides a mechanism for evaluating an arithmetic
     expression and substituting its value.  The format for arithmetic
     expansion is as follows:


     The expression is treated as if it were in double-quotes, except that a
     double-quote inside the expression is not treated specially.  The shell
     expands all tokens in the expression for parameter expansion, command
     substitution, and quote removal.

     Next, the shell treats this as an arithmetic expression and substitutes
     the value of the expression.

   White Space Splitting (Field Splitting)
     After parameter expansion, command substitution, and arithmetic expansion
     the shell scans the results of expansions and substitutions that did not
     occur in double-quotes for field splitting and multiple fields can

     The shell treats each character of the IFS as a delimiter and uses the
     delimiters to split the results of parameter expansion and command
     substitution into fields.

   Pathname Expansion (File Name Generation)
     Unless the -f flag is set, file name generation is performed after word
     splitting is complete.  Each word is viewed as a series of patterns,
     separated by slashes.  The process of expansion replaces the word with
     the names of all existing files whose names can be formed by replacing
     each pattern with a string that matches the specified pattern.  There are
     two restrictions on this: first, a pattern cannot match a string
     containing a slash, and second, a pattern cannot match a string starting
     with a period unless the first character of the pattern is a period.  The
     next section describes the patterns used for both Pathname Expansion and
     the case command.

   Shell Patterns
     A pattern consists of normal characters, which match themselves, and
     meta-characters.  The meta-characters are “!”, “*”, “?”, and “[”.  These
     characters lose their special meanings if they are quoted.  When command
     or variable substitution is performed and the dollar sign or back quotes
     are not double quoted, the value of the variable or the output of the
     command is scanned for these characters and they are turned into meta-

     An asterisk (“*”) matches any string of characters.  A question mark
     matches any single character.  A left bracket (“[”) introduces a
     character class.  The end of the character class is indicated by a (“]”);
     if the “]” is missing then the “[” matches a “[” rather than introducing
     a character class.  A character class matches any of the characters
     between the square brackets.  A range of characters may be specified
     using a minus sign.  The character class may be complemented by making an
     exclamation point the first character of the character class.

     To include a “]” in a character class, make it the first character listed
     (after the “!”, if any).  To include a minus sign, make it the first or
     last character listed.

     This section lists the builtin commands which are builtin because they
     need to perform some operation that can't be performed by a separate
     process.  In addition to these, there are several other commands that may
     be builtin for efficiency (e.g.  printf(1), echo(1), test(1), etc).


     true   A null command that returns a 0 (true) exit value.

     . file
        The commands in the specified file are read and executed by the

     alias [name[=string ...]]
        If name=string is specified, the shell defines the alias name with
        value string.  If just name is specified, the value of the alias
        name is printed.  With no arguments, the alias builtin prints the
        names and values of all defined aliases (see unalias).

     bg [job] ...
        Continue the specified jobs (or the current job if no jobs are
        given) in the background.

     command [-p] [-v] [-V] command [arg ...]
        Execute the specified command but ignore shell functions when
        searching for it.  (This is useful when you have a shell function
        with the same name as a builtin command.)

        -p     search for command using a PATH that guarantees to find all
               the standard utilities.

        -V     Do not execute the command but search for the command and
               print the resolution of the command search.  This is the
               same as the type builtin.

        -v     Do not execute the command but search for the command and
               print the absolute pathname of utilities, the name for
               builtins or the expansion of aliases.

     cd -

     cd [-LP] [directory]
        Switch to the specified directory (default HOME).  If an entry for
        CDPATH appears in the environment of the cd command or the shell
        variable CDPATH is set and the directory name does not begin with
        a slash, then the directories listed in CDPATH will be searched
        for the specified directory.  The format of CDPATH is the same as
        that of PATH.  If a single dash is specified as the argument, it
        will be replaced by the value of OLDPWD.  The cd command will
        print out the name of the directory that it actually switched to
        if this is different from the name that the user gave.  These may
        be different either because the CDPATH mechanism was used or
        because the argument is a single dash.  The -P option causes the
        physical directory structure to be used, that is, all symbolic
        links are resolved to their respective values.  The -L option
        turns off the effect of any preceding -P options.

     echo [-n] args...
        Print the arguments on the standard output, separated by spaces.
        Unless the -n option is present, a newline is output following the

        If any of the following sequences of characters is encountered
        during output, the sequence is not output.  Instead, the specified
        action is performed:

        	      A backspace character is output.

        \c      Subsequent output is suppressed.  This is normally used at
                the end of the last argument to suppress the trailing
                newline that echo would otherwise output.

        \f      Output a form feed.

        \n      Output a newline character.

        \r      Output a carriage return.

        \t      Output a (horizontal) tab character.

        \v      Output a vertical tab.

                Output the character whose value is given by zero to three
                octal digits.  If there are zero digits, a nul character
                is output.

        \\      Output a backslash.

        All other backslash sequences elicit undefined behaviour.

     eval string ...
        Concatenate all the arguments with spaces.  Then re-parse and
        execute the command.

     exec [command arg ...]
        Unless command is omitted, the shell process is replaced with the
        specified program (which must be a real program, not a shell
        builtin or function).  Any redirections on the exec command are
        marked as permanent, so that they are not undone when the exec
        command finishes.

     exit [exitstatus]
        Terminate the shell process.  If exitstatus is given it is used as
        the exit status of the shell; otherwise the exit status of the
        preceding command is used.

     export name ...

     export -p
        The specified names are exported so that they will appear in the
        environment of subsequent commands.  The only way to un-export a
        variable is to unset it.  The shell allows the value of a variable
        to be set at the same time it is exported by writing

              export name=value

        With no arguments the export command lists the names of all
        exported variables.  With the -p option specified the output will
        be formatted suitably for non-interactive use.

     fc [-e editor] [first [last]]

     fc -l [-nr] [first [last]]

     fc -s [old=new] [first]
        The fc builtin lists, or edits and re-executes, commands
        previously entered to an interactive shell.

        -e editor
               Use the editor named by editor to edit the commands.  The
               editor string is a command name, subject to search via the
               PATH variable.  The value in the FCEDIT variable is used as
               a default when -e is not specified.  If FCEDIT is null or
               unset, the value of the EDITOR variable is used.  If EDITOR
               is null or unset, ed(1) is used as the editor.

        -l (ell)
               List the commands rather than invoking an editor on them.
               The commands are written in the sequence indicated by the
               first and last operands, as affected by -r, with each
               command preceded by the command number.

        -n     Suppress command numbers when listing with -l.

        -r     Reverse the order of the commands listed (with -l) or
               edited (with neither -l nor -s).

        -s     Re-execute the command without invoking an editor.


        last   Select the commands to list or edit.  The number of
               previous commands that can be accessed are determined by
               the value of the HISTSIZE variable.  The value of first or
               last or both are one of the following:

                      A positive number representing a command number;
                      command numbers can be displayed with the -l option.

                      A negative decimal number representing the command
                      that was executed number of commands previously.
                      For example, -1 is the immediately previous command.

               A string indicating the most recently entered command that
               begins with that string.  If the old=new operand is not
               also specified with -s, the string form of the first
               operand cannot contain an embedded equal sign.

        The following environment variables affect the execution of fc:

        FCEDIT    Name of the editor to use.

        HISTSIZE  The number of previous commands that are accessible.

     fg [job]
        Move the specified job or the current job to the foreground.

     getopts optstring var
        The POSIX getopts command, not to be confused with the Bell Labs
        -derived getopt(1).

        The first argument should be a series of letters, each of which
        may be optionally followed by a colon to indicate that the option
        requires an argument.  The variable specified is set to the parsed

        The getopts command deprecates the older getopt(1) utility due to
        its handling of arguments containing whitespace.

        The getopts builtin may be used to obtain options and their
        arguments from a list of parameters.  When invoked, getopts places
        the value of the next option from the option string in the list in
        the shell variable specified by var and its index in the shell
        variable OPTIND.  When the shell is invoked, OPTIND is initialized
        to 1.  For each option that requires an argument, the getopts
        builtin will place it in the shell variable OPTARG.  If an option
        is not allowed for in the optstring, then OPTARG will be unset.

        optstring is a string of recognized option letters (see
        getopt(3)).  If a letter is followed by a colon, the option is
        expected to have an argument which may or may not be separated
        from it by white space.  If an option character is not found where
        expected, getopts will set the variable var to a “?”; getopts will
        then unset OPTARG and write output to standard error.  By
        specifying a colon as the first character of optstring all errors
        will be ignored.

        A nonzero value is returned when the last option is reached.  If
        there are no remaining arguments, getopts will set var to the
        special option, “--”, otherwise, it will set var to “?”.

        The following code fragment shows how one might process the
        arguments for a command that can take the options [a] and [b], and
        the option [c], which requires an argument.

              while getopts abc: f
                      case $f in
                      a | b)  flag=$f;;
                      c)      carg=$OPTARG;;
                      \?)     echo $USAGE; exit 1;;
              shift `expr $OPTIND - 1`

        This code will accept any of the following as equivalent:

              cmd -acarg file file
              cmd -a -c arg file file
              cmd -carg -a file file
              cmd -a -carg -- file file

     hash -rv command ...
        The shell maintains a hash table which remembers the locations of
        commands.  With no arguments whatsoever, the hash command prints
        out the contents of this table.  Entries which have not been
        looked at since the last cd command are marked with an asterisk;
        it is possible for these entries to be invalid.

        With arguments, the hash command removes the specified commands
        from the hash table (unless they are functions) and then locates
        them.  With the -v option, hash prints the locations of the
        commands as it finds them.  The -r option causes the hash command
        to delete all the entries in the hash table except for functions.

     pwd [-LP]
        builtin command remembers what the current directory is rather
        than recomputing it each time.  This makes it faster.  However, if
        the current directory is renamed, the builtin version of pwd will
        continue to print the old name for the directory.  The -P option
        causes the physical value of the current working directory to be
        shown, that is, all symbolic links are resolved to their
        respective values.  The -L option turns off the effect of any
        preceding -P options.

     read [-p prompt] [-r] variable [...]
        The prompt is printed if the -p option is specified and the
        standard input is a terminal.  Then a line is read from the
        standard input.  The trailing newline is deleted from the line and
        the line is split as described in the section on word splitting
        above, and the pieces are assigned to the variables in order.  At
        least one variable must be specified.  If there are more pieces
        than variables, the remaining pieces (along with the characters in
        IFS that separated them) are assigned to the last variable.  If
        there are more variables than pieces, the remaining variables are
        assigned the null string.  The read builtin will indicate success
        unless EOF is encountered on input, in which case failure is

        By default, unless the -r option is specified, the backslash “\”
        acts as an escape character, causing the following character to be
        treated literally.  If a backslash is followed by a newline, the
        backslash and the newline will be deleted.

     readonly name ...

     readonly -p
        The specified names are marked as read only, so that they cannot
        be subsequently modified or unset.  The shell allows the value of
        a variable to be set at the same time it is marked read only by

              readonly name=value

        With no arguments the readonly command lists the names of all read
        only variables.  With the -p option specified the output will be
        formatted suitably for non-interactive use.

     printf format [arguments ...]
        printf formats and prints its arguments, after the first, under
        control of the format.  The format is a character string which
        contains three types of objects: plain characters, which are
        simply copied to standard output, character escape sequences which
        are converted and copied to the standard output, and format
        specifications, each of which causes printing of the next
        successive argument.

        The arguments after the first are treated as strings if the
        corresponding format is either b, c or s; otherwise it is
        evaluated as a C constant, with the following extensions:

              ·   A leading plus or minus sign is allowed.
              ·   If the leading character is a single or double quote,
                  the value is the ASCII code of the next character.

        The format string is reused as often as necessary to satisfy the
        arguments.  Any extra format specifications are evaluated with
        zero or the null string.

        Character escape sequences are in backslash notation as defined in
        ANSI X3.159-1989 (“ANSI C89”).  The characters and their meanings
        are as follows:

      Write a <bell> character.

              	      Write a <backspace> character.

              \f      Write a <form-feed> character.

              \n      Write a <new-line> character.

              \r      Write a <carriage return> character.

              \t      Write a <tab> character.

              \v      Write a <vertical tab> character.

              \\      Write a backslash character.

              \num    Write an 8-bit character whose ASCII value is the
                      1-, 2-, or 3-digit octal number num.

        Each format specification is introduced by the percent character
        (``%'').  The remainder of the format specification includes, in
        the following order:

        Zero or more of the following flags:

                #       A `#' character specifying that the value should
                        be printed in an ``alternative form''.  For b, c,
                        d, and s formats, this option has no effect.  For
                        the o format the precision of the number is
                        increased to force the first character of the
                        output string to a zero.  For the x (X) format, a
                        non-zero result has the string 0x (0X) prepended
                        to it.  For e, E, f, g, and G formats, the result
                        will always contain a decimal point, even if no
                        digits follow the point (normally, a decimal point
                        only appears in the results of those formats if a
                        digit follows the decimal point).  For g and G
                        formats, trailing zeros are not removed from the
                        result as they would otherwise be.

                -       A minus sign `-' which specifies left adjustment
                        of the output in the indicated field;

                +       A `+' character specifying that there should
                        always be a sign placed before the number when
                        using signed formats.

                ‘ ’     A space specifying that a blank should be left
                        before a positive number for a signed format.  A
                        `+' overrides a space if both are used;

                0       A zero `0' character indicating that zero-padding
                        should be used rather than blank-padding.  A `-'
                        overrides a `0' if both are used;

        Field Width:
                An optional digit string specifying a field width; if the
                output string has fewer characters than the field width it
                will be blank-padded on the left (or right, if the left-
                adjustment indicator has been given) to make up the field
                width (note that a leading zero is a flag, but an embedded
                zero is part of a field width);

                An optional period, ‘.’, followed by an optional digit
                string giving a precision which specifies the number of
                digits to appear after the decimal point, for e and f
                formats, or the maximum number of bytes to be printed from
                a string (b and s formats); if the digit string is
                missing, the precision is treated as zero;

                A character which indicates the type of format to use (one
                of diouxXfwEgGbcs).

        A field width or precision may be ‘*’ instead of a digit string.
        In this case an argument supplies the field width or precision.

        The format characters and their meanings are:

        diouXx      The argument is printed as a signed decimal (d or i),
                    unsigned octal, unsigned decimal, or unsigned
                    hexadecimal (X or x), respectively.

        f           The argument is printed in the style [-]ddd.ddd where
                    the number of d's after the decimal point is equal to
                    the precision specification for the argument.  If the
                    precision is missing, 6 digits are given; if the
                    precision is explicitly 0, no digits and no decimal
                    point are printed.

        eE          The argument is printed in the style [-]d.ddde±dd
                    where there is one digit before the decimal point and
                    the number after is equal to the precision
                    specification for the argument; when the precision is
                    missing, 6 digits are produced.  An upper-case E is
                    used for an `E' format.

        gG          The argument is printed in style f or in style e (E)
                    whichever gives full precision in minimum space.

        b           Characters from the string argument are printed with
                    backslash-escape sequences expanded.
                    The following additional backslash-escape sequences
                    are supported:

                    \c      Causes dash to ignore any remaining characters
                            in the string operand containing it, any
                            remaining string operands, and any additional
                            characters in the format operand.

                    \0num   Write an 8-bit character whose ASCII value is
                            the 1-, 2-, or 3-digit octal number num.

        c           The first character of argument is printed.

        s           Characters from the string argument are printed until
                    the end is reached or until the number of bytes
                    indicated by the precision specification is reached;
                    if the precision is omitted, all characters in the
                    string are printed.

        %           Print a `%'; no argument is used.

        In no case does a non-existent or small field width cause
        truncation of a field; padding takes place only if the specified
        field width exceeds the actual width.

     set [{ -options | +options | -- }] arg ...
        The set command performs three different functions.

        With no arguments, it lists the values of all shell variables.

        If options are given, it sets the specified option flags, or
        clears them as described in the section called Argument List
        Processing.  As a special case, if the option is -o or +o and no
        argument is supplied, the shell prints the settings of all its
        options.  If the option is -o, the settings are printed in a
        human-readable format; if the option is +o, the settings are
        printed in a format suitable for reinput to the shell to affect
        the same option settings.

        The third use of the set command is to set the values of the
        shell's positional parameters to the specified args.  To change
        the positional parameters without changing any options, use “--”
        as the first argument to set.  If no args are present, the set
        command will clear all the positional parameters (equivalent to
        executing “shift $#”.)

     shift [n]
        Shift the positional parameters n times.  A shift sets the value
        of $1 to the value of $2, the value of $2 to the value of $3, and
        so on, decreasing the value of $# by one.  If n is greater than
        the number of positional parameters, shift will issue an error
        message, and exit with return status 2.

     test expression

     [ expression ]
        The test utility evaluates the expression and, if it evaluates to
        true, returns a zero (true) exit status; otherwise it returns 1
        (false).  If there is no expression, test also returns 1 (false).

        All operators and flags are separate arguments to the test

        The following primaries are used to construct expression:

        -b file       True if file exists and is a block special file.

        -c file       True if file exists and is a character special file.

        -d file       True if file exists and is a directory.

        -e file       True if file exists (regardless of type).

        -f file       True if file exists and is a regular file.

        -g file       True if file exists and its set group ID flag is

        -h file       True if file exists and is a symbolic link.

        -k file       True if file exists and its sticky bit is set.

        -n string     True if the length of string is nonzero.

        -p file       True if file is a named pipe (FIFO).

        -r file       True if file exists and is readable.

        -s file       True if file exists and has a size greater than

        -t file_descriptor
                      True if the file whose file descriptor number is
                      file_descriptor is open and is associated with a

        -u file       True if file exists and its set user ID flag is set.

        -w file       True if file exists and is writable.  True indicates
                      only that the write flag is on.  The file is not
                      writable on a read-only file system even if this
                      test indicates true.

        -x file       True if file exists and is executable.  True
                      indicates only that the execute flag is on.  If file
                      is a directory, true indicates that file can be

        -z string     True if the length of string is zero.

        -L file       True if file exists and is a symbolic link.  This
                      operator is retained for compatibility with previous
                      versions of this program.  Do not rely on its
                      existence; use -h instead.

        -O file       True if file exists and its owner matches the
                      effective user id of this process.

        -G file       True if file exists and its group matches the
                      effective group id of this process.

        -S file       True if file exists and is a socket.

        file1 -nt file2
                      True if file1 and file2 exist and file1 is newer
                      than file2.

        file1 -ot file2
                      True if file1 and file2 exist and file1 is older
                      than file2.

        file1 -ef file2
                      True if file1 and file2 exist and refer to the same

        string        True if string is not the null string.

        s1 = s2       True if the strings s1 and s2 are identical.

        s1 != s2      True if the strings s1 and s2 are not identical.

        s1 < s2       True if string s1 comes before s2 based on the ASCII
                      value of their characters.

        s1 > s2       True if string s1 comes after s2 based on the ASCII
                      value of their characters.

        n1 -eq n2     True if the integers n1 and n2 are algebraically

        n1 -ne n2     True if the integers n1 and n2 are not algebraically

        n1 -gt n2     True if the integer n1 is algebraically greater than
                      the integer n2.

        n1 -ge n2     True if the integer n1 is algebraically greater than
                      or equal to the integer n2.

        n1 -lt n2     True if the integer n1 is algebraically less than
                      the integer n2.

        n1 -le n2     True if the integer n1 is algebraically less than or
                      equal to the integer n2.

        These primaries can be combined with the following operators:

        ! expression  True if expression is false.

        expression1 -a expression2
                      True if both expression1 and expression2 are true.

        expression1 -o expression2
                      True if either expression1 or expression2 are true.

        (expression)  True if expression is true.

        The -a operator has higher precedence than the -o operator.

     times  Print the accumulated user and system times for the shell and for
        processes run from the shell.  The return status is 0.

     trap [action signal ...]
        Cause the shell to parse and execute action when any of the
        specified signals are received.  The signals are specified by
        signal number or as the name of the signal.  If signal is 0 or
        EXIT, the action is executed when the shell exits.  action may be
        empty (''), which causes the specified signals to be ignored.
        With action omitted or set to `-' the specified signals are set to
        their default action.  When the shell forks off a subshell, it
        resets trapped (but not ignored) signals to the default action.
        The trap command has no effect on signals that were ignored on
        entry to the shell.  trap without any arguments cause it to write
        a list of signals and their associated action to the standard
        output in a format that is suitable as an input to the shell that
        achieves the same trapping results.



        List trapped signals and their corresponding action

              trap '' INT QUIT tstp 30

        Ignore signals INT QUIT TSTP USR1

              trap date INT

        Print date upon receiving signal INT

     type [name ...]
        Interpret each name as a command and print the resolution of the
        command search.  Possible resolutions are: shell keyword, alias,
        shell builtin, command, tracked alias and not found.  For aliases
        the alias expansion is printed; for commands and tracked aliases
        the complete pathname of the command is printed.

     ulimit [-H | -S] [-a | -tfdscmlpn [value]]
        Inquire about or set the hard or soft limits on processes or set
        new limits.  The choice between hard limit (which no process is
        allowed to violate, and which may not be raised once it has been
        lowered) and soft limit (which causes processes to be signaled but
        not necessarily killed, and which may be raised) is made with
        these flags:

        -H          set or inquire about hard limits

        -S          set or inquire about soft limits.  If neither -H nor
                    -S is specified, the soft limit is displayed or both
                    limits are set.  If both are specified, the last one

        The limit to be interrogated or set, then, is chosen by specifying
        any one of these flags:

        -a          show all the current limits

        -t          show or set the limit on CPU time (in seconds)

        -f          show or set the limit on the largest file that can be
                    created (in 512-byte blocks)

        -d          show or set the limit on the data segment size of a
                    process (in kilobytes)

        -s          show or set the limit on the stack size of a process
                    (in kilobytes)

        -c          show or set the limit on the largest core dump size
                    that can be produced (in 512-byte blocks)

        -m          show or set the limit on the total physical memory
                    that can be in use by a process (in kilobytes)

        -l          show or set the limit on how much memory a process can
                    lock with mlock(2) (in kilobytes)

        -p          show or set the limit on the number of processes this
                    user can have at one time

        -n          show or set the limit on the number files a process
                    can have open at once

        -r          show or set the limit on the real-time scheduling
                    priority of a process

        If none of these is specified, it is the limit on file size that
        is shown or set.  If value is specified, the limit is set to that
        number; otherwise the current limit is displayed.

        Limits of an arbitrary process can be displayed or set using the
        sysctl(8) utility.

     umask [mask]
        Set the value of umask (see umask(2)) to the specified octal
        value.  If the argument is omitted, the umask value is printed.

     unalias [-a] [name]
        If name is specified, the shell removes that alias.  If -a is
        specified, all aliases are removed.

     unset [-fv] name ...
        The specified variables and functions are unset and unexported.
        If -f or -v is specified, the corresponding function or variable
        is unset, respectively.  If a given name corresponds to both a
        variable and a function, and no options are given, only the
        variable is unset.

     wait [job]
        Wait for the specified job to complete and return the exit status
        of the last process in the job.  If the argument is omitted, wait
        for all jobs to complete and return an exit status of zero.

   Command Line Editing
     When dash is being used interactively from a terminal, the current
     command and the command history (see fc in Builtins) can be edited using
     vi-mode command-line editing.  This mode uses commands, described below,
     similar to a subset of those described in the vi man page.  The command
     ‘set -o vi’ enables vi-mode editing and place sh into vi insert mode.
     With vi-mode enabled, sh can be switched between insert mode and command
     mode.  The editor is not described in full here, but will be in a later
     document.  It's similar to vi: typing ⟨ESC⟩ will throw you into command
     VI command mode.  Hitting ⟨return⟩ while in command mode will pass the
     line to the shell.


     Errors that are detected by the shell, such as a syntax error, will cause
     the shell to exit with a non-zero exit status.  If the shell is not an
     interactive shell, the execution of the shell file will be aborted.
     Otherwise the shell will return the exit status of the last command
     executed, or if the exit builtin is used with a numeric argument, it will
     return the argument.


     HOME       Set automatically by login(1) from the user's login directory
            in the password file (passwd(4)).  This environment variable
            also functions as the default argument for the cd builtin.

     PATH       The default search path for executables.  See the above
            section Path Search.

     CDPATH     The search path used with the cd builtin.

     MAIL       The name of a mail file, that will be checked for the arrival
            of new mail.  Overridden by MAILPATH.

     MAILCHECK  The frequency in seconds that the shell checks for the arrival
            of mail in the files specified by the MAILPATH or the MAIL
            file.  If set to 0, the check will occur at each prompt.

     MAILPATH   A colon “:” separated list of file names, for the shell to
            check for incoming mail.  This environment setting overrides
            the MAIL setting.  There is a maximum of 10 mailboxes that can
            be monitored at once.

     PS1        The primary prompt string, which defaults to “$ ”, unless you
            are the superuser, in which case it defaults to “# ”.

     PS2        The secondary prompt string, which defaults to “> ”.

     PS4        Output before each line when execution trace (set -x) is
            enabled, defaults to “+ ”.

     IFS        Input Field Separators.  This is normally set to ⟨space⟩,
            ⟨tab⟩, and ⟨newline⟩.  See the White Space Splitting section
            for more details.

     TERM       The default terminal setting for the shell.  This is inherited
            by children of the shell, and is used in the history editing

     HISTSIZE   The number of lines in the history buffer for the shell.

     PWD        The logical value of the current working directory.  This is
            set by the cd command.

     OLDPWD     The previous logical value of the current working directory.
            This is set by the cd command.

     PPID       The process ID of the parent process of the shell.





     csh(1), echo(1), getopt(1), ksh(1), login(1), printf(1), test(1),
     getopt(3), passwd(5), environ(7), sysctl(8)


     dash is a POSIX-compliant implementation of /bin/sh that aims to be as
     small as possible.  dash is a direct descendant of the NetBSD version of
     ash (the Almquist SHell), ported to Linux in early 1997.  It was renamed
     to dash in 2002.


     Setuid shell scripts should be avoided at all costs, as they are a
     significant security risk.

     PS1, PS2, and PS4 should be subject to parameter expansion before being


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