If Error Code 0 Then
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exit codes, exit codes are important and this article describes how to use them in your scripts and understand them in general.
Bash If Exit Code Not 0
Written by Benjamin Cane on 2014-09-02 14:45:00| 4 min read Sponsored by Lately bash exit codes I've been working on a lot of automation and monitoring projects, a big part of these projects are taking
Linux Exit Code
existing scripts and modifying them to be useful for automation and monitoring tools. One thing I have noticed is sometimes scripts use exit codes and sometimes they don't. It seems like bash script exit on error exit codes are easy for poeple to forget, but they are an incredibly important part of any script. Especially if that script is used for the command line. What are exit codes? On Unix and Linux systems, programs can pass a value to their parent process while terminating. This value is referred to as an exit code or exit status. On POSIX systems the bash if not equal standard convention is for the program to pass 0 for successful executions and 1 or higher for failed executions. Why is this important? If you look at exit codes in the context of scripts written to be used for the command line the answer is very simple. Any script that is useful in some fashion will inevitably be either used in another script, or wrapped with a bash one liner. This becomes especially true if the script is used with automation tools like SaltStack or monitoring tools like Nagios, these programs will execute scripts and check the status code to determine whether that script was successful or not. On top of those reasons, exit codes exist within your scripts even if you don't define them. By not defining proper exit codes you could be falsely reporting successful executions which can cause issues depending on what the script does. What happens if I don't specify an exit code In Linux any script run from the command line has an exit code. With Bash scripts, if the exit code is not specified in the script itself the exit code
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 your scripts. The difference between a good program and a poor one is often measured in
Bash Set Exit Code
terms of the program's robustness. That is, the program's ability to handle situations in which exit bash shell something goes wrong. Exit status As you recall from previous lessons, every well-written program returns an exit status when it finishes. If a
Exit Code 0
program finishes successfully, the 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 http://bencane.com/2014/09/02/understanding-exit-codes-and-how-to-use-them-in-bash-scripts/ 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 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 http://linuxcommand.org/wss0150.php 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. First, you can examine the contents of the $? environment variable. $? will contain the exit status of the last command executed. You can see this work with the following: [me] $ true; echo $? 0 [me] $ false; echo $? 1 The true and false commands are programs that do nothing except return an exit status of zero and one, respectively. Using them, we can see how the $? environment variable contains the exit status of the previous program. So to check the exit status, we could write the script this way: # Check the exit status cd $s
Ryder In Bash scripting (and shell scripting in general), we often want to check the exit value of a command to decide an action to take after it https://sanctum.geek.nz/arabesque/testing-exit-values-bash/ completes, likely for the purpose of error handling. For example, to determine http://community.teradata.com/t5/General/BTEQ-IF-ERRORCODE-lt-gt-0-THEN-lt-QUERY-gt/td-p/31048 whether a particular regular expression regex was present somewhere in a file options, we might apply grep(1) with its POSIX -q option to suppress output and just use the exit value: grep -q regex options An approach sometimes taken is then to test the exit value with the $? exit code parameter, using if to check if it's non-zero, which is not very elegant and a bit hard to read: # Bad practice grep -q regex options if (($? > 0)); then printf '%s\n' 'myscript: Pattern not found!' >&2 exit 1 fi Because the if construct by design tests the exit value of commands, it's better to test the command directly, making the if error code expansion of $? unnecessary: # Better if grep -q regex options; then # Do nothing : else printf '%s\n' 'myscript: Pattern not found!\n' >&2 exit 1 fi We can precede the command to be tested with ! to negate the test as well, to prevent us having to use else as well: # Best if ! grep -q regex options; then printf '%s\n' 'myscript: Pattern not found!' >&2 exit 1 fi An alternative syntax is to use && and || to perform if and else tests with grouped commands between braces, but these tend to be harder to read: # Alternative grep -q regex options || { printf '%s\n' 'myscript: Pattern not found!' >&2 exit 1 } With this syntax, the two commands in the block are only executed if the grep(1) call exits with a non-zero status. We can apply && instead to execute commands if it does exit with zero. That syntax can be convenient for quickly short-circuiting failures in scripts, for example due to nonexistent commands, particularly if the command being tested already outputs its own error message. This
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