Using the Linux set command

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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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