Debugging

Advanced Bash-Scripting Guide:
Prev |  | Next

# Chapter 32. Debugging

| Debugging is twice as hard as writing the code in the first place. Therefore, if you write the code as cleverly as possible, you are, by definition, not smart enough to debug it. --Brian Kernighan

The Bash shell contains no built-in debugger, and only bare-bones debugging-specific commands and 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 32-1. A buggy script

    #!/bin/bash
    # ex74.sh

    # This is a buggy script.
    # Where, oh where is the error?

    a=37

    if [$a -gt 27 ]
    then
      echo $a
    fi

    exit $?   # 0! Why?

Output from script:

    ./ex74.sh: [37: command not found

What's wrong with the above script? Hint: after the if.

Example 32-2. Missing keyword

    #!/bin/bash
    # missing-keyword.sh
    # What error message will this script generate? And why?

    for a in 1 2 3
    do
      echo "$a"
    # done     # Required keyword 'done' commented out in line 8.

    exit 0     # Will not exit here!

    # === #

    # From command line, after script terminates:
      echo $?    # 2

Output from script:

    missing-keyword.sh: line 10: syntax error: unexpected end of file

Note that the error message does not necessarily reference the line in which the error occurs, but the line where the Bash interpreter finally becomes aware of the error.

Error messages may disregard comment lines in a script when reporting the line number of a syntax error.

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

Example 32-3. test24: another buggy script

    #!/bin/bash

    #  This script is supposed to delete all filenames in current directory
    #+ containing embedded spaces.
    #  It doesn't work.
    #  Why not?

    badname=`ls | grep ' '`

    # Try this:
    # echo "$badname"

    rm "$badname"

    exit 0

Try to find out what's wrong with Example 32-3 by uncommenting the `echo "$badname"` line. Echo statements are useful for seeing whether what you expect is actually what you get.

In this particular case, `rm "$badname"` will not give the desired results because `$badname` should not be quoted. Placing it in quotes ensures that rm has only one argument (it will match only one filename). A partial fix is to remove to quotes from `$badname` and to reset `$IFS` to contain only a newline, `IFS=$'\n'`. However, there are simpler ways of going about it.

    # Correct methods of deleting filenames containing spaces.
    rm *\ *
    rm *" "*
    rm *' '*
    # Thank you. S.C.

Summarizing the symptoms of a buggy script,

1. It bombs with a "syntax error" message, 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. Inserting echo statements at critical points in the script to trace the variables, and otherwise give a snapshot of what is going on.

   | Even better is an echo that echoes only when debug is on.
       ### debecho (debug-echo), by Stefano Falsetto ###
       ### Will echo passed parameters only if DEBUG is set to a value. ###
       debecho () {
         if [ ! -z "$DEBUG" ]; then
            echo "$1" >&2
            #         ^^^ to stderr
         fi
       }

       DEBUG=on
       Whatever=whatnot
       debecho $Whatever   # whatnot

       DEBUG=
       Whatever=notwhat
       debecho $Whatever   # (Will not echo.)

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.

   The `-n` and `-v` flags work well together. `sh -nv scriptname` gives a verbose syntax check.

   `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 and aborts the script.

       set -u   # Or   set -o nounset

       # Setting a variable to null will not trigger the error/abort.
       # unset_var=

       echo $unset_var   # Unset (and undeclared) variable.

       echo "Should not echo!"

       # sh t2.sh
       # t2.sh: line 6: unset_var: unbound variable

4. Using an "assert" function to test a variable or condition at critical points in a script. (This is an idea borrowed from C.)

   Example 32-4. Testing a condition with an assert

       #!/bin/bash
       # assert.sh

       #######################################################################
       assert ()                 #  If condition false,
       {                         #+ exit from script
                                 #+ with appropriate error message.
         E_PARAM_ERR=98
         E_ASSERT_FAILED=99

         if [ -z "$2" ]          #  Not enough parameters passed
         then                    #+ to assert() function.
           return $E_PARAM_ERR   #  No damage done.
         fi

         lineno=$2

         if [ ! $1 ]
         then
           echo "Assertion failed:  \"$1\""
           echo "File \"$0\", line $lineno"    # Give name of file and line number.
           exit $E_ASSERT_FAILED
         # else
         #   return
         #   and continue executing the script.
         fi
       } # Insert a similar assert() function into a script you need to debug.
       #######################################################################

       a=5
       b=4
       condition="$a -lt $b"     #  Error message and exit from script.
                                 #  Try setting "condition" to something else
                                 #+ and see what happens.

       assert "$condition" $LINENO
       # The remainder of the script executes only if the "assert" does not fail.

       # Some commands.
       # Some more commands . . .
       echo "This statement echoes only if the \"assert\" does not fail."
       # . . .
       # More commands . . .

       exit $?

5. Using the $LINENO variable and the caller builtin.

6. Trapping at exit.

   The exit command in a script triggers a signal 0, terminating the process, that is, the script itself. [1] 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.

Trapping signals

trap

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

  A simple instance:

      trap '' 2
      # Ignore interrupt 2 (Control-C), with no action specified.

      trap 'echo "Control-C disabled."' 2
      # Message when Control-C pressed.

Example 32-5. Trapping at exit

    #!/bin/bash
    # Hunting variables with a trap.

    trap 'echo Variable Listing --- a = $a  b = $b' EXIT
    #  EXIT is the name of the signal generated upon exit from a script.
    #
    #  The command specified by the "trap" doesn't execute until
    #+ the appropriate signal is sent.

    echo "This prints before the \"trap\" --"
    echo "even though the script sees the \"trap\" first."
    echo

    a=39

    b=36

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

Example 32-6. Cleaning up after Control-C

    #!/bin/bash
    # logon.sh: A quick 'n dirty script to check whether you are on-line yet.

    umask 177  # Make sure temp files are not world readable.

    TRUE=1
    LOGFILE=/var/log/messages
    #  Note that $LOGFILE must be readable
    #+ (as root, chmod 644 /var/log/messages).
    TEMPFILE=temp.$$
    #  Create a "unique" temp file name, using process id of the script.
    #     Using 'mktemp' is an alternative.
    #     For example:
    #     TEMPFILE=`mktemp temp.XXXXXX`
    KEYWORD=address
    #  At logon, the line "remote IP address xxx.xxx.xxx.xxx"
    #                      appended to /var/log/messages.
    ONLINE=22
    USER_INTERRUPT=13
    CHECK_LINES=100
    #  How many lines in log file to check.

    trap 'rm -f $TEMPFILE; exit $USER_INTERRUPT' TERM INT
    #  Cleans up the temp file if script interrupted by control-c.

    echo

    while [ $TRUE ]  #Endless loop.
    do
      tail -n $CHECK_LINES $LOGFILE> $TEMPFILE
      #  Saves last 100 lines of system log file as temp file.
      #  Necessary, since newer kernels generate many log messages at log on.
      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.
      then
         echo "On-line"
         rm -f $TEMPFILE    #  Clean up temp file.
         exit $ONLINE
      else
         echo -n "."        #  The -n option to echo suppresses newline,
                            #+ so you get continuous rows of dots.
      fi

      sleep 1
    done

    #  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, per the above note,
    #           and prettify it.

    exit 0

    # Nick Drage suggests an alternate method:

    while true
      do ifconfig ppp0 | grep UP 1> /dev/null && echo "connected" && exit 0
      echo -n "."   # Prints dots (.....) until connected.
      sleep 2
    done

    # Problem: Hitting Control-C to terminate this process may be insufficient.
    #+         (Dots may keep on echoing.)
    # Exercise: Fix this.

    # Stephane Chazelas has yet another alternative:

    CHECK_INTERVAL=1

    while ! tail -n 1 "$LOGFILE" | grep -q "$KEYWORD"
    do echo -n .
       sleep $CHECK_INTERVAL
    done
    echo "On-line"

    # Exercise: Discuss the relative strengths and weaknesses
    #           of each of these various approaches.

Example 32-7. A Simple Implementation of a Progress Bar

    #! /bin/bash
    # progress-bar2.sh
    # Author: Graham Ewart (with reformatting by ABS Guide author).
    # Used in ABS Guide with permission (thanks!).

    # Invoke this script with bash. It doesn't work with sh.

    interval=1
    long_interval=10

    {
         trap "exit" SIGUSR1
         sleep $interval; sleep $interval
         while true
         do
           echo -n '.'     # Use dots.
           sleep $interval
         done; } &         # Start a progress bar as a background process.

    pid=$!
    trap "echo !; kill -USR1 $pid; wait $pid"  EXIT        # To handle ^C.

    echo -n 'Long-running process '
    sleep $long_interval
    echo ' Finished!'

    kill -USR1 $pid
    wait $pid              # Stop the progress bar.
    trap EXIT

    exit $?

| The `DEBUG` argument to trap causes a specified action to execute after every command in a script. This permits tracing variables, for example. Example 32-8. Tracing a variable
    #!/bin/bash

    trap 'echo "VARIABLE-TRACE> \$variable = \"$variable\""' DEBUG
    # Echoes the value of $variable after every command.

    variable=29; line=$LINENO

    echo "  Just initialized \$variable to $variable in line number $line."

    let "variable *= 3"; line=$LINENO
    echo "  Just multiplied \$variable by 3 in line number $line."

    exit 0

    #  The "trap 'command1 . . . command2 . . .' DEBUG" construct is
    #+ more appropriate in the context of a complex script,
    #+ where inserting multiple "echo $variable" statements might be
    #+ awkward and time-consuming.

    # Thanks, Stephane Chazelas for the pointer.

    Output of script:

    VARIABLE-TRACE> $variable = ""
    VARIABLE-TRACE> $variable = "29"
      Just initialized $variable to 29.
    VARIABLE-TRACE> $variable = "29"
    VARIABLE-TRACE> $variable = "87"
      Just multiplied $variable by 3.
    VARIABLE-TRACE> $variable = "87"

Of course, the trap command has other uses aside from debugging, such as disabling certain keystrokes within a script (see Example A-43).

Example 32-9. Running multiple processes (on an SMP box)

    #!/bin/bash
    # parent.sh
    # Running multiple processes on an SMP box.
    # Author: Tedman Eng

    #  This is the first of two scripts,
    #+ both of which must be present in the current working directory.

    LIMIT=$1         # Total number of process to start
    NUMPROC=4        # Number of concurrent threads (forks?)
    PROCID=1         # Starting Process ID
    echo "My PID is $$"

    function start_thread() {
            if [ $PROCID -le $LIMIT ] ; then
                    ./child.sh $PROCID&
                    let "PROCID++"
            else
               echo "Limit reached."
               wait
               exit
            fi
    }

    while [ "$NUMPROC" -gt 0 ]; do
            start_thread;
            let "NUMPROC--"
    done

    while true
    do

    trap "start_thread" SIGRTMIN

    done

    exit 0

    # ======== Second script follows ========

    #!/bin/bash
    # child.sh
    # Running multiple processes on an SMP box.
    # This script is called by parent.sh.
    # Author: Tedman Eng

    temp=$RANDOM
    index=$1
    shift
    let "temp %= 5"
    let "temp += 4"
    echo "Starting $index  Time:$temp" "$@"
    sleep ${temp}
    echo "Ending $index"
    kill -s SIGRTMIN $PPID

    exit 0

    # ======================= SCRIPT AUTHOR'S NOTES ======================= #
    #  It's not completely bug free.
    #  I ran it with limit = 500 and after the first few hundred iterations,
    #+ one of the concurrent threads disappeared!
    #  Not sure if this is collisions from trap signals or something else.
    #  Once the trap is received, there's a brief moment while executing the
    #+ trap handler but before the next trap is set.  During this time, it may
    #+ be possible to miss a trap signal, thus miss spawning a child process.

    #  No doubt someone may spot the bug and will be writing
    #+ . . . in the future.

    # ===================================================================== #

    # ----------------------------------------------------------------------#

    #################################################################
    # The following is the original script written by Vernia Damiano.
    # Unfortunately, it doesn't work properly.
    #################################################################

    #!/bin/bash

    #  Must call script with at least one integer parameter
    #+ (number of concurrent processes).
    #  All other parameters are passed through to the processes started.

    INDICE=8        # Total number of process to start
    TEMPO=5         # Maximum sleep time per process
    E_BADARGS=65    # No arg(s) passed to script.

    if [ $# -eq 0 ] # Check for at least one argument passed to script.
    then
      echo "Usage: `basename $0` number_of_processes [passed params]"
      exit $E_BADARGS
    fi

    NUMPROC=$1              # Number of concurrent process
    shift
    PARAMETRI=( "$@" )      # Parameters of each process

    function avvia() {
             local temp
             local index
             temp=$RANDOM
             index=$1
             shift
             let "temp %= $TEMPO"
             let "temp += 1"
             echo "Starting $index Time:$temp" "$@"
             sleep ${temp}
             echo "Ending $index"
             kill -s SIGRTMIN $$
    }

    function parti() {
             if [ $INDICE -gt 0 ] ; then
                  avvia $INDICE "${PARAMETRI[@]}" &
                    let "INDICE--"
             else
                    trap : SIGRTMIN
             fi
    }

    trap parti SIGRTMIN

    while [ "$NUMPROC" -gt 0 ]; do
             parti;
             let "NUMPROC--"
    done

    wait
    trap - SIGRTMIN

    exit $?

    : <<SCRIPT_AUTHOR_COMMENTS
    I had the need to run a program, with specified options, on a number of
    different files, using a SMP machine. So I thought [I'd] keep running
    a specified number of processes and start a new one each time . . . one
    of these terminates.

    The "wait" instruction does not help, since it waits for a given process
    or *all* process started in background. So I wrote [this] bash script
    that can do the job, using the "trap" instruction.
      --Vernia Damiano
    SCRIPT_AUTHOR_COMMENTS

| `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.
    	command
    	command
    	command
    	trap 2     # Reenables Control-C

### Notes

[1] | By convention, `signal 0` is assigned to exit.

Prev | Home | Next
Of Zeros and Nulls | Up | Options