Bash Command Error Handling
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and Signals and Traps (Oh My!) - Part 1 by William Shotts, Jr. In this lesson, we're going to look at handling errors during the execution of bash error handling best practices your scripts. The difference between a good program and a poor
Error Handling In Bash Shell Script
one is often measured in terms of the program's robustness. That is, the program's ability to handle situations powershell command error handling in which something goes wrong. Exit status As you recall from previous lessons, every well-written program returns an exit status when it finishes. If a program finishes successfully, the dos command error handling exit status will be zero. If the exit status is anything other than zero, then the program failed in some way. It is very important to check the exit status of programs you call in your scripts. It is also important that your scripts return a meaningful exit status when they finish. I once had a Unix system administrator
Bash Throw Error
who wrote a script for a production system containing the following 2 lines of code: # Example of a really bad idea cd $some_directory rm * Why is this such a bad way of doing it? It's not, if nothing goes wrong. The two lines change the working directory to the name contained in $some_directory and delete the files in that directory. That's the intended behavior. But what happens if the directory named in $some_directory doesn't exist? In that case, the cd command will fail and the script executes the rm command on the current working directory. Not the intended behavior! By the way, my hapless system administrator's script suffered this very failure and it destroyed a large portion of an important production system. Don't let this happen to you! The problem with the script was that it did not check the exit status of the cd command before proceeding with the rm command. Checking the exit status There are several ways you can get and respond to the exit status of a program. F
here for a quick overview of the site Help Center Detailed answers to any questions you might have Meta Discuss the workings and policies of this site About Us Learn more about Stack Overflow the company Business Learn more about hiring developers or posting ads with us Stack Overflow bash catch errors Questions Jobs Documentation Tags Users Badges Ask Question x Dismiss Join the Stack Overflow Community Stack Overflow is error handling in linux a community of 4.7 million programmers, just like you, helping each other. Join them; it only takes a minute: Sign up Is there a TRY CATCH
Bash Error Code
command in Bash up vote 67 down vote favorite 27 I'm writing a Shell script and need to check that a terminal app has been installed. I want to use a TRY/CATCH command to do this unless there is a neater way. bash http://linuxcommand.org/wss0150.php shell terminal share|improve this question asked Feb 25 '14 at 9:09 Lee Probert 2,35152335 Why not an if else ? –Thomas Ayoub Feb 25 '14 at 9:11 It might help if you could elaborate what problem you're attempting to solve. Seems that you're not exactly new here, but you still might want to visit the Help Center and see help on how to ask a good question. –devnull Feb 25 '14 at 9:11 That said, it seems that saying help test might http://stackoverflow.com/questions/22009364/is-there-a-try-catch-command-in-bash help you find the solution to your problem. –devnull Feb 25 '14 at 9:14 try/catch/finally block is not a command, it's a construct –Benedictus Oct 1 '14 at 13:14 possible duplicate of Linux shell try catch finally –blong Jul 23 '15 at 17:15 add a comment| 6 Answers 6 active oldest votes up vote 105 down vote accepted Is there a TRY CATCH command in Bash? No. Bash doesn't have as many luxuries as one can find in many programming languages. There is no try/catch in bash; however, one can achieve similar behavior using && or ||. Using ||: if command1 fails then command2 runs as follows command1 || command2 Similarly, using &&, command2 will run if command1 is successful The closest approximation of try/catch is as follows { # try command1 && #save your output } || { # catch # save log for exception } Also bash contains some error handling mechanisms, as well set -e It will immediately stop your script if a simple command fails. I think this should have been the default behavior. Since such errors almost always signify something unexpected, it is not really 'sane' to keep executing the following commands. And also why not if...else. It is your best friend. share|improve this answer edited Aug 5 at 1:53 strathallan 127 answered Feb 25 '14 at 9:50 Jayesh 8,30572140 6 With this, you need to take care that the code for #save your output does not fail, or the "catch" block will still execute. –chepner Feb 25 '14 at
Bash Prompts About Writing Robust Bash Shell Scripts Many people hack together shell scripts quickly to do simple tasks, but these soon take on a life of their own. Unfortunately shell scripts are full of subtle effects which result in scripts failing http://www.davidpashley.com/articles/writing-robust-shell-scripts/ in unusual ways. It's possible to write scripts which minimise these problems. In this article, I explain several techniques for writing robust bash scripts. Use set -u How often have you written a script that broke https://www.howtoforge.com/detailed-error-handling-in-bash because a variable wasn't set? I know I have, many times. chroot=$1 ... rm -rf $chroot/usr/share/doc If you ran the script above and accidentally forgot to give a parameter, you would have just deleted all of error handling your system documentation rather than making a smaller chroot. So what can you do about it? Fortunately bash provides you with set -u, which will exit your script if you try to use an uninitialised variable. You can also use the slightly more readable set -o nounset. david% bash /tmp/shrink-chroot.sh $chroot= david% bash -u /tmp/shrink-chroot.sh /tmp/shrink-chroot.sh: line 3: $1: unbound variable david% Use set -e Every script you write should include set -e at the top. This command error handling tells bash that it should exit the script if any statement returns a non-true return value. The benefit of using -e is that it prevents errors snowballing into serious issues when they could have been caught earlier. Again, for readability you may want to use set -o errexit. Using -e gives you error checking for free. If you forget to check something, bash will do it or you. Unfortunately it means you can't check $? as bash will never get to the checking code if it isn't zero. There are other constructs you could use: command if [ "$?"-ne 0]; then echo "command failed"; exit 1; fi could be replaced with command || { echo "command failed"; exit 1; } or if ! command; then echo "command failed"; exit 1; fi What if you have a command that returns non-zero or you are not interested in its return value? You can use command || true, or if you have a longer section of code, you can turn off the error checking, but I recommend you use this sparingly. set +e command1 command2 set -e On a slightly related note, by default bash takes the error status of the last item in a pipeline, which may not be what you want. For example, false | true will be considered t
Detailed Error Handling In Bash Summary The database The mechanism Caveat Invoking the error handler The full script Usage example References Detailed Error Handling In Bash by Willem Bogaerts, application smith at Kratz Business Solutions Summary Shell scripts are often running as background processes, doing useful things without running in a visible shell. Think, for example, of cron jobs or scripts that are fired from a program on a web server. To write such scripts can be quite painful, as all errors occur out of sight as well. Off course you can make use of a log file, but the ideal level of logging is hard to find. You often log way too much when the script is running fine and way too little when it unexpectedly fails. While log files can hold a lot of information, finding the relevant information is a bit trickier. My solution is to log only the errors with all the details to a small database. This database contains tables for the message, the corresponding stack trace and the important environment variables. I have chosen for an SQLite database in this howto, but the same principle works with other databases as well. The database SQLite needs some settings to work as I expect it to, and these settings can be put in an initializing script. These settings include the error behaviour of SQLite itself and its foreign key handling: .bail ON .echo OFF PRAGMA foreign_keys = TRUE; Off course, we also need a database and I do not want to rely on one to exist. Therefore, the first thing the bash script will do is to run an SQL "revive" script on the database file: if the database did not exist, it will be created and if it did, it will do nothing: CREATE TABLE IF NOT EXISTS ErrorLog (intErrorLogId INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, strMessage TEXT NOT NULL, tstOccurredAt DATE NOT NULL DEFAULT(CURRENT_TIMESTAMP) ); CREATE INDEX IF NOT EXISTS idxELOccurredAt ON ErrorLog(tstOccurredAt); CREATE TABLE IF NOT EXISTS ErrorStackTrace (intErrorStackTraceId INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, intErrorLogId INTEGER NOT NULL, strSourceFile TEXT NOT NULL, strFunction TEXT NOT NULL, intLine INTEGER NOT NULL, FOREIGN KEY(intErrorLogId) REFERENCES ErrorLog(intErrorLogId) ON DELETE CASCADE ON UPDATE CASCADE ); CREATE INDEX IF NOT EXISTS idxESTTraceErrorLogId ON ErrorStackTrace(intErrorLogId); CREATE TABLE IF NOT EXISTS ErrorEnvironment (intErrorEnvironmentId INTEGER NOT NULL PRIMARY KEY AUTOINCREMENT, intErrorLogId INTEGER NOT NULL, strVariable TEXT NOT NULL, strValue TEXT NULL, F