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3.24. Debugging

The Bash shell contains no debugger, nor even any debugging-specific commands or constructs. Syntax errors or outright typos in the script generate cryptic error messages that are often of no help in debugging a non-functional script.

Example 3-80. test23, a buggy script



if [$a -gt 27 ]
  echo $a

exit 0

Output from script:
./test23: [37: command not found

What's wrong with the above script (hint: after the if)?

What if the script executes, but does not work as expected? This is the all too familiar logic error.

Example 3-81. test24, another buggy script


# This is supposed to delete all filenames
# containing embedded spaces in current directory,
# but doesn't.  Why not?

badname=`ls | grep ' '`

# echo "$badname"

rm "$badname"

exit 0

To find out what's wrong with Example 3-81, uncomment the echo "$badname" line. Echo statements are useful for seeing whether what you expect is actually what you get.

Summarizing the symptoms of a buggy script,

  1. It bombs with an error message syntax error, or

  2. It runs, but does not work as expected (logic error)

  3. It runs, works as expected, but has nasty side effects (logic bomb).

Tools for debugging non-working scripts include

  1. echo statements at critical points in the script to trace the variables, and otherwise give a snapshot of what is going on.

  2. using the tee filter to check processes or data flows at critical points.

  3. setting option flags -n -v -x

    sh -n scriptname checks for syntax errors without actually running the script. This is the equivalent of inserting set -n or set -o noexec into the script. Note that certain types of syntax errors can slip past this check.

    sh -v scriptname echoes each command before executing it. This is the equivalent of inserting set -v or set -o verbose in the script.

    sh -x scriptname echoes the result each command, but in an abbreviated manner. This is the equivalent of inserting set -x or set -o xtrace in the script.

    Inserting set -u or set -o nounset in the script runs it, but gives an unbound variable error message at each attempt to use an undeclared variable.

  4. trapping at exit

    The exit command in a script actually sends a signal 0, terminating the process, that is, the script itself. It is often useful to trap the exit, forcing a "printout" of variables, for example. The trap must be the first command in the script.


Specifies an action on receipt of a signal; also useful for debugging.

Note: A signal is simply a message sent to a process, either by the kernel or another process, telling it to take some specified action (usually to terminate). For example, hitting a Control-C, sends a user interrupt, an INT signal, to a running program.

trap 2 #ignore interrupts (no action specified) 
trap 'echo "Control-C disabled."' 2

Example 3-82. Trapping at exit


trap 'echo Variable Listing --- a = $a  b = $b' EXIT
# EXIT is the name of the signal generated upon exit from a script.



exit 0
# Note that commenting out the 'exit' command makes no difference,
# since the script exits anyhow after running out of commands.

Example 3-83. Cleaning up after Control-C


# A quick 'n dirty script to check whether you are on-line yet.

# Note that $LOGFILE must be readable (chmod 644 /var/log/messages).
# Create a "unique" temp file name, using process id of the script.
# At logon, the line "remote IP address" appended to /var/log/messages.

# Cleans up the temp file if script interrupted by control-c.


while [ $TRUE ]  #Endless loop.
  # Saves last line of system log file as temp file.
  search=`grep $KEYWORD $TEMPFILE`
  # Checks for presence of the "IP address" phrase,
  # indicating a successful logon.

  if [ ! -z "$search" ] # Quotes necessary because of possible spaces.
     echo "On-line"
     rm -f $TEMPFILE  # Clean up temp file.
     exit $ONLINE
     echo -n "." # -n option to echo suppresses newline,
                 # so you get continuous rows of dots.

  sleep 1  

# Note: if you change the KEYWORD variable to "Exit",
# this script can be used while on-line to check for an unexpected logoff.

# Exercise: Change the script, as per the above note,
#           and prettify it.

exit 0

Note: trap '' SIGNAL (two adjacent apostrophes) disables SIGNAL for the remainder of the script. trap SIGNAL restores the functioning of SIGNAL once more. This is useful to protect a critical portion of a script from an undesirable interrupt.

	trap '' 2  # Signal 2 is Control-C, now disabled.
	trap 2     # Reenables Control-C

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