bash Cookbook

Debugging Scripts

Problem

You can’t figure out what’s happening in your script and why it doesn’t work as expected.

Solution

Add set-x to the top of the script when you run it. Or use set-x to turn on xtrace before a troublesome spot and set+x to turn it off after. You may also wish to experiment with the $PS4 prompt (Customizing Your Prompt). xtrace also works on the interactive command line (Customizing Your Prompt). Here’s a script that we suspect is buggy:

#!/usr/bin/env bash
# cookbook filename: buggy
#

set -x

result=$1

[ $result = 1 ] \
  && { echo "Result is 1; excellent." ; exit 0;   } \
  || { echo "Uh-oh, ummm, RUN AWAY! " ; exit 120; }

Now we invoke this script, but first we set and export the value of the PS4 prompt. bash will print out the value of PS4 before each command that it displays during an execution trace (i.e., after a set -x ):

$ export PS4='+xtrace $LINENO:'
$ echo $PS4
+xtrace $LINENO:

$ ./buggy
+xtrace 4: result=
+xtrace 6: '[' = 1 ']'
./buggy: line 6: [: =: unary operator expected
+xtrace 8: echo 'Uh-oh, ummm, RUN AWAY! '
Uh-oh, ummm, RUN AWAY!

$ ./buggy 1
+xtrace 4: result=1
+xtrace 6: '[' 1 = 1 ']'
+xtrace 7: echo 'Result is 1; excellent.'
Result is 1; excellent.

$ ./buggy 2
+xtrace 4: result=2
+xtrace 6: '[' 2 = 1 ']'
+xtrace 8: echo 'Uh-oh, ummm, RUN AWAY! '
Uh-oh, ummm, RUN AWAY!

$ /tmp/jp-test.sh 3
+xtrace 4: result=3
+xtrace 6: '[' 3 = 1 ']'
+xtrace 8: echo 'Uh-oh, ummm, RUN AWAY! '
Uh-oh, ummm, RUN AWAY!

Discussion

It may seem odd to turn something on using ...