try
try
NAMESYNOPSIS
DESCRIPTION
EXAMPLES
SEE ALSO
KEYWORDS
___________________________
NAME
try − Trap and process errors and exceptions
SYNOPSIS
try body ?handler...? ?finally script? ___________________________
DESCRIPTION
This command executes the script body and, depending on what the outcome of that script is (normal exit, error, or some other exceptional result), runs a handler script to deal with the case. Once that has all happened, if the finally clause is present, the script it includes will be run and the result of the handler (or the body if no handler matched) is allowed to continue to propagate. Note that the finally clause is processed even if an error occurs and irrespective of which, if any, handler is used.
The
handler clauses are each expressed as several words,
and must have one of the following forms:
on code variableList script
This clause matches if the evaluation of body completed with the exception code code. The code may be expressed as an integer or one of the following literal words: ok, error, return, break, or continue. Those literals correspond to the integers 0 through 4 respectively.
trap pattern variableList script
This clause matches if the evaluation of body resulted in an error and the prefix of the −errorcode from the interpreter’s status dictionary is equal to the pattern. The number of prefix words taken from the −errorcode is equal to the list-length of pattern, and inter-word spaces are normalized in both the −errorcode and pattern before comparison.
The variableList word in each handler is always interpreted as a list of variable names. If the first word of the list is present and non-empty, it names a variable into which the result of the evaluation of body (from the main try) will be placed; this will contain the human-readable form of any errors. If the second word of the list is present and non-empty, it names a variable into which the options dictionary of the interpreter at the moment of completion of execution of body will be placed.
The script word of each handler is also always interpreted the same: as a Tcl script to evaluate if the clause is matched. If script is a literal “−” and the handler is not the last one, the script of the following handler is invoked instead (just like with the switch command).
Note that handler clauses are matched against in order, and that the first matching one is always selected. At most one handler clause will selected. As a consequence, an on error will mask any subsequent trap in the try. Also note that on error is equivalent to trap {}.
If an exception (i.e. any non-ok result) occurs during the evaluation of either the handler or the finally clause, the original exception’s status dictionary will be added to the new exception’s status dictionary under the −during key.
EXAMPLES
Ensure that a file is closed no matter what:
set f [open
/some/file/name a]
try {
puts $f "some message"
# ...
} finally {
close $f
}
Handle different reasons for a file to not be openable for reading:
try {
set f [open /some/file/name w]
} trap {POSIX EISDIR} {} {
puts "failed to open /some/file/name: it’s a
directory"
} trap {POSIX ENOENT} {} {
puts "failed to open /some/file/name: it doesn’t
exist"
}
SEE ALSO
catch(n), error(3), return(n), throw(n)
KEYWORDS
cleanup, error, exception, final, resource management
Opportunity
Personal Opportunity - Free software gives you access to billions of dollars of software at no cost. Use this software for your business, personal use or to develop a profitable skill. Access to source code provides access to a level of capabilities/information that companies protect though copyrights. Open source is a core component of the Internet and it is available to you. Leverage the billions of dollars in resources and capabilities to build a career, establish a business or change the world. The potential is endless for those who understand the opportunity.
Business Opportunity - Goldman Sachs, IBM and countless large corporations are leveraging open source to reduce costs, develop products and increase their bottom lines. Learn what these companies know about open source and how open source can give you the advantage.
Free Software
Free Software provides computer programs and capabilities at no cost but more importantly, it provides the freedom to run, edit, contribute to, and share the software. The importance of free software is a matter of access, not price. Software at no cost is a benefit but ownership rights to the software and source code is far more significant.
Free Office Software - The Libre Office suite provides top desktop productivity tools for free. This includes, a word processor, spreadsheet, presentation engine, drawing and flowcharting, database and math applications. Libre Office is available for Linux or Windows.
Free Books
The Free Books Library is a collection of thousands of the most popular public domain books in an online readable format. The collection includes great classical literature and more recent works where the U.S. copyright has expired. These books are yours to read and use without restrictions.
Source Code - Want to change a program or know how it works? Open Source provides the source code for its programs so that anyone can use, modify or learn how to write those programs themselves. Visit the GNU source code repositories to download the source.
Education
Study at Harvard, Stanford or MIT - Open edX provides free online courses from Harvard, MIT, Columbia, UC Berkeley and other top Universities. Hundreds of courses for almost all major subjects and course levels. Open edx also offers some paid courses and selected certifications.
Linux Manual Pages - A man or manual page is a form of software documentation found on Linux/Unix operating systems. Topics covered include computer programs (including library and system calls), formal standards and conventions, and even abstract concepts.
