Using set to debug scripts

You can use the set commands to help with debugging your scripts. There are two common options to set, x and v. Here is a description of each.

Note that a - activates the set while a + deactivates it. If that sounds backwards to you it is because it is backwards.

Use:

• set -x: to display the expanded trace before running the command

• set -v: to display the input line as it is parsed

Take a look at Script 5 which shows what set -x does:

Chapter 9 - Script 5 and Script 6

#!/bin/sh
#
# 6/7/2017
#
set -x                       # turn debugging on

echo "Chapter 9 - Script 5"

x=0
while [ $x -lt 5 ]
do
 echo "x: $x"
 let x++
done

echo "End of script5"
exit 0

And the output:

If this looks a little strange at first don't worry, it gets easier the more you look at it. In essence, the lines that start with a + are the expanded source code lines, and the lines without a + are the output of the script.

Look at the first two lines. It shows:

 + echo 'Chapter 9 - Script 5'
 Chapter 9...