Debugging under shell script.

1) -x option to debug a shell script

Run a shell script using -x option

$bash -x script-name

$bash -x temp.sh farhat jahan
+ echo 'File Name: temp.sh'
File Name: temp.sh
+ echo 'First Parameter : farhat'
First Parameter : farhat
+ echo 'Second Parameter : jahan'
Second Parameter : jahan
+ echo 'Passed Parameters: farhat' jahan
Passed Parameters: farhat jahan
+ echo 'Passes Values : farhat jahan'
Passes Values : farhat jahan
+ echo 'Total no. of Parameters : 2'

Total no. of Parameters : 2

Note: You can even write 
       $bash -xv script-name

2)Use of set builtin command

Bash shell offers debugging option which can be turn on or off using set command.

1. set -x : Display commands and their argument as the are executed.
2. set -v : Display shell input lines as they are read.

Example:

#!/bin/bash

# turn on debug mode

set -x

echo "File Name: $0"

echo "First Parameter : $1"

echo "Second Parameter : $2"

echo  "Passed Parameters: $@"

echo "Passes Values : $*"

# turn OFF debug mode

set +x

echo "Total no. of Parameters : $#"

$./temp.sh farhat jahan

+ echo 'File Name: ./temp.sh'

File Name: ./temp.sh

+ echo 'First Parameter : farhat'

First Parameter : farhat

+ echo 'Second Parameter : jahan'

Second Parameter : jahan

+ echo 'Passed Parameters: farhat' jahan

Passed Parameters: farhat jahan

+ echo 'Passes Values : farhat jahan'

Passes Values : farhat jahan

+ set +x

Total no. of Parameters : 2

Note: you can even replace #!/bin/bash with #!/bin/bash -xv for debugging

3)Use of intelligent DEBUG function.

Add special variable _DEBUG. Set it "on" when you need to debug a script

                    

                    _DEBUG="on"

Keep the following function at the beginning of the script.

function DEBUG()

{

[ "$_DEBUG" == "on" ] &&  $@

}

Now whenever u want debug simply call the function.

 DEBUG echo "ERROR:: file not found"

Note:When debugging is done you can set _DEBUG to off.

Example:

#!/bin/bash

_DEBUG="on"

function DEBUG()

{

 [ "$_DEBUG" == "on" ] &&  $@ 

}

DEBUG echo "Calculating"

DEBUG set -x

a=5

b=3

c=$(( $a - $b ))

DEBUG set +x

echo "$a - $b = $c"

OUTPUT:

$./temp.sh

Calculating

+ a=5

+ b=3

+ c=2

+ DEBUG set +x

+ '[' on == on ']'

+ set +x

5 - 3 = 2