Bash debugging tips

Use a unique name for your script

Don't laugh! This is a classic mistake

Read the error messages

Reading and interpreting error messages is 50% of your job as debugger! Error messages actually mean something. At the very least, they can give you hints as to where to start debugging. READ YOUR ERROR MESSAGES!

You may ask yourself why is this mentioned as debugging tip? Well, you would be surprised how many shell users ignore the text of error messages! When I find some time, I'll paste 2 or 3 IRC log-snips here, just to show you that annoying fact.

Use a good editor

From my personal experience, I can suggest vim or GNU emacs.

Write logfiles

An available syslog interface is logger ( online manpage).

Inject debugging code

echo "DEBUG: current i=$i" >&2

If you read input from anywhere, such as a file or command substitution, print the debug output with literal quotes, to see leading and trailing spaces!

pid=$(< fooservice.pid)
echo "DEBUG: read from file: pid=\"$pid\"" >&2

Bash's printf command has the %q format, which is handy for verifying whether strings are what they appear to be.

foo=$(< inputfile)
printf "DEBUG: foo is |%q|\n" "$foo" >&2
# exposes whitespace (such as CRs, see below) and non-printing characters

Use shell debug output

• set -v mode (set -o verbose)
  • print commands to be executed to stderr as if they were read from input (script file or keyboard)
  • print everything before any ( substitution and expansion, …) is applied
• set -x mode (set -o xtrace)
  • print everything as if it were executed, after substitution and expansion is applied
  • indicate the depth-level of the subshell (by default by prefixing a + (plus) sign to the displayed command)
  • indicate the recognized words after word splitting by marking them like 'x y'
  • in shell version 4.1, this debug output can be printed to a configurable file descriptor, rather than sdtout by setting the BASH_XTRACEFD variable.

Hint: These modes can be entered when calling Bash:

• from commandline: bash -vx ./myscript
• from shebang (OS dependant): #!/bin/bash -vx

Simple example of how to interpret xtrace output

set -x
foo="bar baz"
[ $foo = test ]

That fails. Why? Let's see the xtrace output:

+ '[' bar baz = test ']'

And now you see that it's ("bar" and "baz") recognized as two separate words (which you would have realized if you READ THE ERROR MESSAGES ;) ). Let's check it…

# next try
[ "$foo" = test ]

xtrace now gives

+ '[' 'bar baz' = test ']'
      ^       ^
    word markers!

Making xtrace more useful

xtrace output would be more useful if it contained source file and line number. Add this assignment PS4 at the beginning of your script to enable the inclusion of that information:

export PS4='+(${BASH_SOURCE}:${LINENO}): ${FUNCNAME[0]:+${FUNCNAME[0]}(): }'

The output would look like this when you trace code outside a function:

+(somefile.bash:412): echo 'Hello world'

+(somefile.bash:412): myfunc(): echo 'Hello world'

That helps a lot when the script is long, or when the main script sources many other files.

Set flag variables with descriptive words

Debugging commands depending on a set variable

debugme() {
 [[ $script_debug = 1 ]] && "$@" || :
 # be sure to append || : or || true here or use return 0, since the return code
 # of this function should always be 0 to not influence anything else with an unwanted
 # "false" return code (for example the script's exit code if this function is used
 # as the very last command in the script)
}

This function does nothing when script_debug is unset or empty, but it executes the given parameters as commands when script_debug is set. Use it like this:

script_debug=1
# to turn it off, set script_debug=0

debugme logger "Sorting the database"
database_sort
debugme logger "Finished sorting the database, exit code $?"

Of course this can be used to execute something other than echo during debugging:

debugme set -x
# ... some code ...
debugme set +x

Dry-run STDIN driven commands

ftp user@host <<FTP
cd /data
get current.log
dele current.log
FTP

A method to dry-run this with debug output is:

if [[ $DRY_RUN = yes ]]; then
  sed 's/^/DRY_RUN FTP: /'
else
  ftp user@host
fi <<FTP
cd /data
get current.log
dele current.log
FTP

This can be wrapped in a shell function for more readable code.

Common error messages

Unexpected end of file

script.sh: line 100: syntax error: unexpected end of file

Usually indicates exactly what it says: An unexpected end of file. It's unexpected because Bash waits for the closing of a compound command:

• did you close your do with a done?
• did you close your if with a fi?
• did you close your case with a esac?
• did you close your { with a }?
• did you close your ( with a )?

Note: It seems that here-documents (tested on versions 1.14.7, 2.05b, 3.1.17 and 4.0) are correctly terminated when there is an EOF before the end-of-here-document tag (see redirection). The reason is unknown, but it seems to be deliberate. Bash 4.0 added an extra message for this: warning: here-document at line <N> delimited by end-of-file (wanted `<MARKER>')

Unexpected end of file while looking for matching ...

script.sh: line 50: unexpected EOF while looking for matching `"'
script.sh: line 100: syntax error: unexpected end of file

This one indicates the double-quote opened in line 50 does not have a matching closing quote.

These unmatched errors occur with:

• double-quote pairs
• single-quote pairs (also $'string'!)
• missing a closing } with parameter expansion syntax

Too many arguments

bash: test: too many arguments

You most likely forgot to quote a variable expansion somewhere. See the example for xtrace output from above. External commands may display such an error message though in our example, it was the internal test-command that yielded the error.

!": event not found

$ echo "Hello world!"
bash: !": event not found

This is not an error per se. It happens in interactive shells, when the C-Shell-styled history expansion ("!searchword") is enabled. This is the default. Disable it like this:

set +H
# or
set +o histexpand

syntax error near unexpected token `('

$ foo () { echo "Hello world"; }
bash: syntax error near unexpected token `('

you most likely have an alias defined with the same name as the function (here: foo). Alias expansion happens before the real language interpretion, thus the alias is expanded and makes your function definition invalid.

The CRLF issue

What is the CRLF issue?

• MSDOS uses: \r\n (ASCII CR #13 ^M, ASCII LF #10)
• UNIX® uses: \n (ASCII LF #10)

Keep in mind your script is a plain text file, and the CR character means nothing special to UNIX® - it is treated like any other character. If it's printed to your terminal, a carriage return will effectively place the cursor at the beginning of the current line. This can cause much confusion and many headaches, since lines containing CRs are not what they appear to be when printed. In summary, CRs are a pain.

How did a CR end up in my file?

• a DOS/Windows text editor
• a UNIX® text editor that is "too smart" when determining the file content type (and thinks "it's a DOS text file")
• a direct copy and paste from certain webpages (some pastebins are known for this)

Why do CRs hurt?

#!/bin/bash^M
^M
echo "Hello world"^M
...

Here's what happens because of the #!/bin/bash^M in our shebang:

• the file /bin/bash^M doesn't exist (hopefully)
• So Bash prints an error message which (depending on the terminal, the Bash version, or custom patches!) may or may not expose the problem.
• the script can't be executed

The error message can vary. If you're lucky, you'll get:

bash: ./testing.sh: /bin/bash^M: bad interpreter: No such file or directory

: bad interpreter: No such file or directory

How can I find and eliminate them?

• in VI/VIM: :set list
• with cat(1): cat -v FILE

To eliminate them (only a few examples)

• blindly with tr(1): tr -d '\r' <FILE >FILE.new
• controlled with recode(1): recode MSDOS..latin1 FILE
• controlled with dos2unix(1): dos2unix FILE

See also

• the set builtin command (for -v and -x)

• DEBUG trap
• BASH Debugger http://bashdb.sourceforge.net/

Discussion

Debugger for Bash version 3(Bourne again shell). Plugin for Eclipse. http://sourceforge.net/projects/basheclipse/

Paolo Supino, 2012/01/02 10:20

#!/bin/bash
 
PS4='+(${BASH_SOURCE}:${LINENO}): ${FUNCNAME[0]:+${FUNCNAME[0]}(): }'
 
export PS4
 
usage()
{
cat <<'EOF'
usage: debug [option] script arguments
possible options are:
 
- help|usage: print this screen
- verbose: sets -xv flags
- noexec: sets -xvn flags
- no parameter sets -x flags
EOF
 
fmt << 'EOF'
 
 if the script takes arguments remember to enclose the script and arugments
 in ""
EOF
}
 
debug_cmd()
{
   /bin/bash $FLAGS $SCRIPT
}
 
if [ $# -gt 0 ]; then
   case "$1" in
         "verbose")
            FLAGS=-xv
            SCRIPT=$2
         ;;
         "noexec")
            FLAGS=-xvn
            SCRIPT=$2
         ;;
         "help"|"usage")
            usage
            exit 3
         ;;
         *)
            FLAGS=-x
            SCRIPT=$1
         ;;
   esac
   debug_cmd
else
   usage
fi

I've updated the debug.sh posted above to display colors in the ouput when using only +x (makes everything more readable). the new script is:

#!/bin/bash
 
color_def="~/.colorrc"
 
if [[ -f $color_def ]]; then
   . $color_def
else
   # color definitions
   black="$(tput setaf 0)"
   darkgrey="$(tput bold ; tput setaf 0)"
   lightgrey="$(tput setaf 7)"
   white="$(tput bold ; tput setaf 7)"
   red="$(tput setaf 1)"
   lightred="$(tput bold ; tput setaf 1)"
   green="$(tput setaf 2)"
   lightgreen="$(tput bold ; tput setaf 2)"
   yellow="$(tput setaf 3)"
   blue="$(tput setaf 4)"
   lightblue="$(tput bold ; tput setaf 4)"
   purple="$(tput setaf 5)"
   pink="$(tput bold ; tput setaf 5)"
   cyan="$(tput setaf 6)"
   lightcyan="$(tput bold ; tput setaf 6)"
   nc="$(tput sgr0)" # no color
fi
export darkgrey lightgreywhite red lightred green lightgreen yellow blue
export lightblue purple pink cyan lightcyan nc
if [[ ! $level_color ]]; then
   level_color=$cyan
fi
if [[ ! $script_color ]]; then
   script_color=$yellow
fi
if [[ ! $linenum_color ]]; then
   linenum_color=$red
fi
if [[ ! $funcname_color ]]; then
   funcname_color=$green
fi
if [[ ! $command_color ]]; then
   command_color=$white
fi
export script_color linenum_color funcname_color
 
reset_screen() {
 
   echo $nc
}
reset_screen
 
usage()
{
cat <<'EOF'
 
usage: debug [option] script arguments
 
possilbe options are:
- help|usage: print this screen
- verbose: sets -xv flags
- noexec: sets -xvn flags
- no parameter sets -x flags
 
EOF
fmt <<EOF
if the script takes arguments remember to enclose the script and arugments
in ""
EOF
 
fmt <<EOF
 
The script prints the script name, script line number and function name as it
executes the script. The various parts of the script prompt are printed in
color. If the default colors are not suitable than you can set the environment
varialbes script_color linenum_color funcname_color to any of the following
colors: ${darkgrey}darkgrey$nc, ${lightgrey}light grey$nc, ${white}white,
${red}red, ${lightred}light red, ${green}green, ${lightgreen}light green,
${yellow}yellow, ${blue}blue, ${lightblue}light blue, ${purple}purple,
${pink}pink, ${cyan}cyan, ${lightcyan}light cyan$nc.
EOF
 
cat <<EOF
 
default colors are:
${level_color}- shell level color:cyan$nc
${script_color}- script name: yellow$nc
${linenum_color}- line number: red$nc
${funcname_color}- function name: green$nc
${command_color}- command executed: white'$nc
EOF
}
 
 
debug_cmd()
{
   trap reset_screen INT
   /bin/bash $FLAGS $SCRIPT
}
 
if [ $# -gt 0 ]; then
   case "$1" in
         "verbose")
            FLAGS=-xv
            SCRIPT=$2
         ;;
         "noexec")
            FLAGS=-xvn
            SCRIPT=$2
         ;;
         "help"|"usage")
            usage
            exit 3
         ;;
         *)
            FLAGS=-x
            PS4="${level_color}+${script_color}"'(${BASH_SOURCE##*/}:${linenum_color}${LINENO}${script_color}):'" ${funcname_color}"
            export PS4
            SCRIPT=$1
         ;;
   esac
   debug_cmd
else
   usage
fi
 
reset_screen

My last debug.sh version had a couple of bugs: 1: it didn't print the function name when run with only -x flag. 2: The first line of prompt had a different coloring scheme than the rest of the lines… In the version below I fixed those problems. Please delete my previous version and post this one instead. Thanx.

#!/bin/bash

color_def="~/.colorrc"

if -f $color_def; then

 . $color_def

else

 # color definitions
 black="$(tput setaf 0)"
 darkgrey="$(tput bold ; tput setaf 0)"
 lightgrey="$(tput setaf 7)"
 white="$(tput bold ; tput setaf 7)"
 red="$(tput setaf 1)"
 lightred="$(tput bold ; tput setaf 1)"
 green="$(tput setaf 2)"
 lightgreen="$(tput bold ; tput setaf 2)"
 yellow="$(tput setaf 3)"
 blue="$(tput setaf 4)"
 lightblue="$(tput bold ; tput setaf 4)"
 purple="$(tput setaf 5)"
 pink="$(tput bold ; tput setaf 5)"
 cyan="$(tput setaf 6)"
 lightcyan="$(tput bold ; tput setaf 6)"
 nc="$(tput sgr0)" # no color

fi export darkgrey lightgreywhite red lightred green lightgreen yellow blue export lightblue purple pink cyan lightcyan nc if ! $lc; then

 lc=$cyan

fi if ! $sc; then

 sc=$yellow

fi if ! $lnc; then

 lnc=$red

fi if ! $fc; then

 fc=$green

fi if ! $cc; then

 cc=$white

fi export sc lnc fc

reset_screen() {

 echo $nc

} reset_screen

usage() { cat «'EOF'

usage: debug [option] script arguments

possilbe options are: - help|usage: print this screen - test|compile: sets -n flag - verbose: sets -xv flags - noexec: sets -xvn flags - no parameter sets -x flag

EOF fmt «EOF if the script takes arguments remember to enclose the script and arugments in "" EOF

fmt «EOF

The script prints the script name, script line number and function name as it executes the script. The various parts of the script prompt are printed in color. If the default colors are not suitable than you can set the environment varialbes sc lnc fc to any of the following colors: ${darkgrey}darkgrey$nc, ${lightgrey}light grey$nc, ${white}white, ${red}red, ${lightred}light red, ${green}green, ${lightgreen}light green, ${yellow}yellow, ${blue}blue, ${lightblue}light blue, ${purple}purple, ${pink}pink, ${cyan}cyan, ${lightcyan}light cyan$nc. EOF

cat «EOF

default colors are: ${lc}- shell level color: cyan ${sc}- script name: yellow ${lnc}- line number: red ${fc}- function name: green ${cc}- command executed: white EOF }

debug_cmd() {

 trap reset_screen INT
 /bin/bash $FLAGS $SCRIPT

}

if [ $# -gt 0 ]; then

 case "$1" in
       "test"|"compile")
          FLAGS=-n
          SCRIPT=$2
       ;;
       "verbose")
          FLAGS=-xv
          SCRIPT=$2
       ;;
       "noexec")
          FLAGS=-xvn
          SCRIPT=$2
       ;;
       "help"|"usage")
          usage
          exit 3
       ;;
       *)
          FLAGS=-x
          PS4="${white}${lc}+${sc}"'(${BASH_SOURCE##*/}'":${lnc}"'${LINENO}'"${sc}): ${fc}"'${FUNCNAME[0]}'"(): ${cc}"
          export PS4
          SCRIPT=$1
       ;;
 esac
 debug_cmd

else

 usage

fi

reset_screen

Robert Wlaschin, 2012/09/27 08:08

Putting a line like the following:

# debug switch [ -z "$DEBUG" ] && DEBUG=0 || :

…

[ $DEBUG = 0 ] || echo "Debug output"

Will allow passing in a value through environment variables, meaning that code does not need to be changed or put in unnecessary command-line arguments to enable debugging

This is how it could be used

# turn on debugging DEBUG=1 ./runtest

# regular no debug ./runtest

Thoughts?

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