Raju asks:
How can I Debug a shell scripts?
This is most common question asked by new admins or UNIX user.
Shell scripting debugging can be boring job (read as not easy). There are various ways to debug a shell script.
-x option to debug a shell script
Run a shell script with -x option.
$ bash -x script-name
$ bash -x domains.sh
Use of set builtin command
Bash shell offers debugging options which can be turn on or off using set command.
- set -x : Display commands and their arguments as they are executed.
- set -v : Display shell input lines as they are read.
You can use above two command in shell script itself:
#!/bin/bash clear # turn on debug mode set -x for f in * do file $f done # turn OFF debug mode set +x ls # more commands |
You can replace the standard Shebang line:
#!/bin/bash
with the following (for debugging) code:
#!/bin/bash -xv
Use of intelligent DEBUG function
First, add a special variable called _DEBUG. Set _DEBUG to ‘on’ when you need to debug a script:
_DEBUG="on"
Put the following function at the beginning of the script:
function DEBUG() { [ "$_DEBUG" == "on" ] && $@ } |
Now wherever you need debugging simply use the DEBUG function as follows:
DEBUG echo "File is $filename"
OR
DEBUG set -x
Cmd1
Cmd2
DEBUG set +x
When done with debugging (and before moving your script to production) set _DEBUG to ‘off’. No need to delete debug lines.
_DEBUG="off" # set to anything but not to 'on'
Sample script:
#!/bin/bash _DEBUG="on" function DEBUG() { [ "$_DEBUG" == "on" ] && $@ } DEBUG echo 'Reading files' for i in * do grep 'something' $i > /dev/null [ $? -eq 0 ] && echo "Found in $i file" done DEBUG set -x a=2 b=3 c=$(( $a + $b )) DEBUG set +x echo "$a + $b = $c" |
Save and close the file. Run the script as follows:
$ ./script.sh
Output:
Reading files Found in xyz.txt file + a=2 + b=3 + c=5 + DEBUG set +x + '[' on == on ']' + set +x 2 + 3 = 5
Now set DEBUG to off (you need to edit the file):
_DEBUG="off"
Run script:
$ ./script.sh
Output:
Found in xyz.txt file 2 + 3 = 5
Above is a simple but quite effective technique. You can also try to use DEBUG as an alias instead of function.
Debugging Common Bash Shell Scripting Errors
Bash or sh or ksh gives various error messages on screen and in many case the error message may not provide detailed information.
End of file unexpected Error
If you are getting an End of file unexpected error message, open your script file and and make sure it has both opening and closing quotes. In this example, the echo statement has an opening quote but no closing quote:
#!/bin/bash ... .... echo 'Error: File not found ^^^^^^^ missing quote |
Also make sure you check for missing parentheses and braces ({}):
#!/bin/bash ..... [ ! -d $DIRNAME ] && { echo "Error: Chroot dir not found"; exit 1; ^^^^^^^^^^^^^ missing brace } ... |
Missing Keywords Such As fi, esac, ;;, etc.
If you missed ending keyword such as fi or ;; you will get an error such as as “xxx unexpected”. So make sure all nested if and case statements ends with proper keywords. See bash man page for syntax requirements. In this example, fi is missing:
#!/bin/bash echo "Starting..." .... if [ $1 -eq 10 ] then if [ $2 -eq 100 ] then echo "Do something" fi for f in $files do echo $f done # note fi is missing |
Tip#1: Send Debug Message To stderr
Standard error is the default error output device, which is used to write all system error messages. So it is a good idea to send messages to the default error device:
# Write error to stdout echo "Error: $1 file not found" # # Write error to stderr (note 1>&2 at the end of echo command) # echo "Error: $1 file not found" 1>&2 |
Tip#2: Turn On Syntax Highlighting
Most modern text editors allows you to set syntax highlighting option. This is useful to detect syntax and prevent common errors such as opening or closing quote. You can see bash script in different colors. This feature eases writing in a shell script structures and syntax errors are visually distinct. Highlighting does not affect the meaning of the text itself; it’s made only for you. In this example, vim syntax highlighting is used for my bash script:
See also:
- See how to turn on syntax highlighting under vim text editor.
- Bash man page
- BASH with Debugger and Improved Debug Support and Error Handling
24 comment
For running under “set -x” or “set +x”, why not just run it as:
bash -x ./script.sh
Also, instead of having _DEBUG in your script, just export it from your shell:
$ export _DEBUG=on
Now, you have to make absolutely no changes to enable or disable all debugging.
_ranger_,
Good idea however you may have to modify script little to check if _DEBUG is defined or not.
Appreciate your post.
I liked this debug method and implemented it in a simple script I wrote 🙂 It really does make for much cleaner code than what I have been doing before. Thanks mate
thanks,it is good idea
It’s a good idea. Thanks!
http://bashdb.sourceforge.net/
very nice, thanx
[ “$_DEBUG” == “on” ] && $@ || :
Why are you using last || operator?
Yes, it is not required.
Hi Vivek,
if this is not required, would you consider cleaning code – mainly because that may confuse novice programmers and may induce bugs … Thanks in advance! Nice topic, one more!
Philippe,
Thanks for your feedback. The post has been updated.
|| : will make sure the return code is true, which allows the script to run under -e mode without it breaking due to problems with debug commands. Using -e when debugging may be very helpful.
On a related note: You can set variables on a by-call basis like this:
_DEBUG=”on” command. No export required, thus no later unexport required. All nice and local.
The || should remain so it will work with “set -e”. To make it clearer, perhaps:
[ “$_DEBUG” == “on” ] && $@ || return 0
I am going to move from HP-Unix to Redhat Linux server. I have many bash shell programs in HP-Unix. Will they need change when i move to Linux (RHEL)? Kindly advice.
Excellent Topic
Very helpful. I have bookmarked this page, as it will help me port a Windows batch file into a Linux script.
why are we using this instruction >> “$_DEBUG” == “on” ] && $@
Very useful – thank you!
You can change the function to handle multiline statements with the following:
function DEBUG { if [ "$_DEBUG" == "on" ] ; then $@ else : $@ fi }Or, better:-
function DEBUG { if [ "$_DEBUG" == "on" ] ; then "$@" else : "$@" fi }This handles statements like:-
DEBUG printf "%12d %sn" 123 "hello"Hello,
First, many thanks for this very good HowTo.
About the function DEBUG:
In the form proposed
function DEBUG() { [ "$_DEBUG" == "on" ] && $@ }it’s OK for an echo command, but if I try
I obtain an error message.
So, I’m using this form :
function DEBUG() { [ "$_DEBUG" == "on" ] && eval $@ }which is OK for me.
More precisely, my debug function is
debug_cmd () { if [ ! -z "${DEBUG}" ] then msg_head=$(caller 0) echo "[ ${msg_head} ] $@" >&2 read -p "Run it [N/y] ? " if [ "$REPLY" = "y" ] || [ "$REPLY" = "Y" ] then eval $@ fi fi }This one allows me to view the command before choosing to run it or to ignore it.
It’s OK for a command like ‘a=2’, but fails with a command ‘echo “pouet (a=$a)”.
Not really a problem if we use debug_cmd only for commands like variable affectation, or function calls, not for simple echo commands.
Regards,
Laurent.
Thanks Excellent tutorials
good stuff #thank you
Thanks ..
It’s really helpful
I normally work on the production environment with no access to UAT/Dev servers.
If I use the -x option in production, will my script actually run while debugging or it will only debug.
Actually, I have a script, which does a lot of data changing. So I just wanted to debug without actually running the script.