The Linux set command allows you to change the value of shell options or to display the names and values of shell variables. Rarely used, it is a bash builtin, but is quite a bit more complicated than most builtins.
If you use the command without any arguments, you will get a list of all the settings—the names and values of all shell variables and functions. Watch out though! You’ll end up with a torrent of output flowing down your screen. There are just short of 3,000 lines of output on my Fedora system:
$ set | wc -l 2954
The top of the list looks like what you see below, but the output gets considerably more complicated as you move through it.
$ set | head -10 BASH=/bin/bash BASHOPTS=cdspell:checkwinsize:cmdhist:complete_fullquote:expand_aliases:extglob:extquote:force_fignore:globasciiranges:histappend:interactive_comments:progcomp:promptvars:sourcepath BASH_ALIASES=() BASH_ARGC=([0]="0") BASH_ARGV=() BASH_CMDS=() BASH_COMPLETION_VERSINFO=([0]="2" [1]="11") BASH_LINENO=() BASH_SOURCE=() BASH_VERSINFO=([0]="5" [1]="1" [2]="0" [3]="1" [4]="release" [5]="x86_64-redhat-linux-gnu")
More practically, there are some very helpful things you can do with set. Let’s examine some of them.
Debugging your scripts
You can use the set -x command to do some script debugging. In other words, when you use this option, bash is going to show you a lot more output that displays what your script is up to. The script used in this example does a lot of checking and then prints the top lines of the selected file.
#!/bin/bash
set -x
if [ $# -lt 2 ]; then
echo “Usage: $0 lines filename”
exit 1
fi
if [ ! -f $2 ]; then
echo “Error: File $2 not found”
exit 3
else
echo top of file
head -$1 $2
fi
msg="bye"
echo $msg
Without the set -x, the script would display output like this:
$ script3 3 file top of file #!/bin/bash -x date bye
With the set -x, it shows each command as it’s being run as well as the output.
$ script3 3 file + ‘[‘ 2 -lt 2 ‘]’ + [[ 3 != [0-9]* ]] + ‘[‘ ‘!’ -f file ‘]’ + echo top of file top of file + head -3 file #!/bin/bash -x date + msg=bye + echo bye Bye
You can also invoke debugging by placing the -x on the “shebang line” (i.e., the top line of the script) like this:
#!/bin/bash -x
One of the benefits of using set -x and then set +x is that the first set starts the debugging and the second set turns it off, so you can see the verbose output for a small section of a script if that is all you need to focus on.
set -x
if [ ! -f $2 ]; then
echo “Error: File $2 not found”
exit 3
else
echo top of file
head -$1 $2
fi
set +x
Other set options can be turned on and then off again in this same manner.
Automatic exporting
Using set -a, you can cause any variable or function that you create to be automatically exported so that subshells and scripts can use them. Here’s an example:
$ set -a $ one=1 $ two=2 $ three=3 $ four=4 $ /bin/bash <== start new shell $ echo $one $two $three $four 1 2 3 4
Exit immediately if a command fails
The set -e command will cause a script to exit as soon as it runs into an error. In this example, the set -e command is invoked.
$ cat script1 #!/bin/bash set -e cat nosuchfile echo “So long!”
As you might suspect, the “nosuchfile” file doesn’t exist, so the script exits at that point with the help of set -e. The final echo command never gets a chance to run.
$ script1 cat: nosuchfile: No such file or directory
Don’t ignore non-existent variables
By default, bash will ignore any variables that don’t exist. For example, in the script below, bash will overlook the fact that $var2 has not been defined and will simply display $var1.
$ cat script1 #!/bin/bash var1="123" echo $var1 $var2 $ ./script1 123
If you add set -u to the script, it reports the problem.
$ cat script2 #!/bin/bash set -u var1="123" echo $var1 $var2
$ ./script2 ./script1: line 5: var2: unbound variable
Many additional set options are available, but these are some of the most useful.
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