2.3. Debugging Bash scripts

2.3.1. Debugging on the entire script

When things don't go according to plan, you need to determine what exactly causes the script to fail. Bash provides extensive debugging features. The most common is to start up the subshell with the −x option, which will run the entire script in debug mode. Traces of each command plus its arguments are printed to standard output after the commands have been expanded but before they are executed.
This is the commented−script1.sh script ran in debug mode. Note again that the added comments are not visible in  the output of the script.

willy:~/scripts> bash −x script1.sh
+ clear
+ echo 'The script starts now.'
The script starts now.
+ echo 'Hi, willy!'
Hi, willy!
+ echo
+ echo 'I will now fetch you a list of connected users:'
I will now fetch you a list of connected users:
+ echo
+w

4:50pm     up 18 days, 6:49,          4 users, load average: 0.58, 0.62, 0.40
USER        TTY         FROM                     LOGIN@   IDLE   JCPU      PCPU WHAT
root        tty2        −                      Sat 2pm 5:36m 0.24s 0.05s −bash
willy       :0          −                      Sat 2pm    ?     0.00s      ?      −
willy       pts/3       −                      Sat 2pm 43:13 36.82s 36.82s BitchX willy ir
willy       pts/2       −                      Sat 2pm 43:13    0.13s 0.06s /usr/bin/screen
+ echo
+ echo 'I'\''m setting two variables now.'
I'm setting two variables now.
+ COLOUR=black
+ VALUE=9
+ echo 'This is a string: '
This is a string:
+ echo 'And this is a number: '
And this is a number:
+ echo
+ echo 'I'\''m giving you back your prompt now.'
I'm giving you back your prompt now.
+ echo
     Future Bash Features
     There is now a full−fledged debugger for Bash, available at SourceForge. Right now however, you need a patched version of bash−2.05. The new debugging features might become available in bash−3.0.

2.3.2. Debugging on part(s) of the script

Using the set Bash built−in you can run in normal mode those portions of the script of which you are sure they are without fault, and display debugging information only for troublesome zones. Say we are not sure what the w command will do in the example commented−script1.sh, then we could enclose it in the script like this:

set −x                         # activate debugging from here
w
set +x                         # stop debugging from here

Output then looks like this:

willy: ~/scripts> script1.sh
The script starts now.

Hi, willy!
I will now fetch you a list of connected users:
+w
5:00pm     up 18 days, 7:00,       4 users, load average: 0.79, 0.39, 0.33
USER         TTY        FROM                LOGIN@     IDLE     JCPU     PCPU WHAT
root         tty2       −                  Sat 2pm 5:47m 0.24s 0.05s −bash
willy        :0         −                  Sat 2pm     ?      0.00s      ?     −
willy        pts/3      −                  Sat 2pm 54:02 36.88s 36.88s BitchX willyke
willy        pts/2      −                  Sat 2pm 54:02      0.13s 0.06s /usr/bin/screen
+ set +x
I'm setting two variables now.
This is a string:
And this is a number:
I'm giving you back your prompt now.

willy: ~/scripts>
The table below gives an overview of other useful Bash options:

The dash is used to activate a shell option and a plus to deactivate it. Don't let this confuse you! In the example below, we demonstrate these options on the command line:

willy:~/scripts> set −v
willy:~/scripts> ls
ls
commented−scripts.sh           script1.sh
willy:~/scripts> set +v
set +v
willy:~/scripts> ls *
commented−scripts.sh           script1.sh
willy:~/scripts> set −f
willy:~/scripts> ls *
ls: *: No such file or directory
willy:~/scripts> touch *
willy:~/scripts> ls
*     commented−scripts.sh          script1.sh
willy:~/scripts> rm *
willy:~/scripts> ls
commented−scripts.sh           script1.sh

Alternatively, these modes can be specified in the script itself, by adding the desired options to the first line shell declaration. Options can be combined, as is usually the case with UNIX commands:

#!/bin/bash −xv

Once you found the buggy part of your script, you can add echo statements before each command of which you are unsure, so that you will see exactly where and why things don't work. In the example commented−script1.sh script, it could be done like this, still assuming that the displaying of users gives us problems:

echo "debug message: now attempting to start w command"; w

In more advanced scripts, the echo can be inserted to display the content of variables at different stages in the script, so that flaws can be detected:

 echo "Variable VARNAME is now set to $VARNAME."

2.4. Summary

A shell script is a reusable series of commands put in an executable text file. Any text editor can be used to
write scripts. Scripts start with #! followed by the path to the shell executing the commands from the script. Comments are added to a script for your own future reference, and also to make it understandable for other users. It is better
to have too many explanations than not enough. Debugging a script can be done using shell options. Shell options can be used for partial debugging or for analyzing the entire script. Inserting echo commands at strategic locations is also a common troubleshooting technique.

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