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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 $a fi exit 0
Output from script:
./ex74.sh: [37: command not found
What's wrong with the above script? Hint: after the if.
Example 32-2. Missing keyword
#!/bin/bash # missing-keyword.sh: What error message will this generate? for a in 1 2 3 do echo "$a" # done # Required keyword 'done' commented out in line 7. exit 0 # === # 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,
Tools for debugging non-working scripts include
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.)
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
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 $?
Specifies an action on receipt of a signal; also useful for debugging.
A signal is 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.
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 echo " Just initialized \$variable to $variable." let "variable *= 3" echo " Just multiplied \$variable by 3." exit # 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
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Version 3 of Bash adds the following internal variables for use by the debugger.
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