Bash Script Debugging

Checking the syntax of a script with “-n”

The -n flag enables you to check the syntax of a script without having to execute it:

~> $ bash -n testscript.sh
testscript.sh: line 128: unexpected EOF while looking for matching `"'
testscript.sh: line 130: syntax error: unexpected end of file

Debugging usigh bashdb

Bashdb is a utility that is similar to gdb, in that you can do things like set breakpoints at a line or at a function, print content of variables, you can restart script execution and more.

You can normally install it via your package manager, for example on Fedora:

sudo dnf install bashdb

Or get it from the homepage. Then you can run it with your script as a paramater:

bashdb <YOUR SCRIPT>

Here are a few commands to get you started:

l - show local lines, press l again to scroll down
s - step to next line
print $VAR - echo out content of variable
restart - reruns bashscript, it re-loads it prior to execution.
eval - evaluate some custom command, ex: eval echo hi

b <line num> set breakpoint on some line
c - continue till some breakpoint
i b - info on break points
d <line #> - delete breakpoint at line #
shell - launch a sub-shell in the middle of execution, this is handy for manipulating variables

For more information, I recommend consulting the manual:
http://www.rodericksmith.plus.com/outlines/manuals/bashdbOutline.html

See also homepage:
http://bashdb.sourceforge.net/

Debugging a bash script with “-x”

Use “-x” to enable debug output of executed lines. It can be run on an entire session or script, or enabled programmatically within a script.

Run a script with debug output enabled:

$ bash -x myscript.sh

or

$ bash --debug myscript.sh

Turn on debugging within a bash script. It may optionally be turned back on, though debug output is automatically reset when the script exits.

#!/bin/bash
set -x # Enable debugging
# some code here
set +x # Disable debugging output.