Background

From time to time it’s useful if you can turn up the debugging messages that come from Bash, when working out either interactive or shell script problems. Here are 2 methods that can help in getting down to the details.

Solution

There are essentially 2 methods.

Method #1: -x method

When writing a shell script you’ll sometimes want to turn on line by line debugging. There’s basically 2 ways to to this.

Before we get started, suppose we have this sample script, myscript.bash:

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#!/bin/bash
 
echo "hi"
echo "bye"

First you can run your script like so:

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% bash -x myscript.bash
+ echo hi
hi
+ echo bye
bye

As an alternative you can add the following line, set -x to the top of our shell script to enable debugging as well:

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#!/bin/bash
 
set -x
echo "hi"
echo "bye"

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% ./myscript.bash
+ echo hi
hi
+ echo bye
bye

Method #2: env SHELLOPTS=xtrace …

This approach sets the env. variable SHELLOPTS=xtrace which has the same effect as using bash -x.

For example:

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% env SHELLOPTS=xtrace ./myscript.bash 
+ echo hi
hi
+ echo bye
bye

You can also use this technique to debug your bash environment (think .bashrc and .bash_profile) like so:

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% env SHELLOPTS=xtrace bash
...
...
+++++ line='complete -f -X
'\''!*.@(zip|[ejw]ar|exe|pk3|wsz|zargo|xpi|sxw|o[tx]t|od[fgpst]|epub|apk)'\''
unzip zipinfo'
+++++ line=' unzip zipinfo'
+++++ list=("${list[@]}" $line)
+++++ read line
+++++ '[' 'complete -f -X '\''*.Z'\'' compress znew' '!=' 'complete -f
-X '\''*.Z'\'' compress znew' ']'
+++++ line='complete -f -X '\''*.Z'\'' compress znew'
+++++ line='complete -f -X '\''*.Z'\'' compress znew'
+++++ line=' compress znew'
+++++ list=("${list[@]}" $line)
+++++ read line
+++++ '[' ' zcmp, zdiff, z*grep, zless, zmore intentionally not here,
see Debian: #455510' '!=' '# zcmp, zdiff, z*grep, zless, zmore
intentionally not here, see Debian: #455510' ']'
...
...

Here you can see every command getting executed from the system and user’s .bashrc and .bash_profile as bash starts up.

NOTE: For further details regarding my one-liner blog posts, check out my one-liner style guide primer.