Bash Tips and Pitfalls

Reference

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• Bash

External links:

• Writing Robust Bash Shell Scripts
• Advanced Bash-Shell Scripting - Gotchas
• Command Line Fu

Tips for Robust Scripts

Reference: [1], [2], [3].

Use set -u

This will detect uninitialized variable, the king of all evils!

#! /bin/bash
set -o nounset                          # Or "set -u"

chroot=$1
rm -r $chroot/etc                       # Will delete /etc if $1 is not given!!!

Use set -e

Script will exit if any command fails.

#! /bin/bash
set -o errexit                                               # Or "set -e"

# Don't do
command                                                      # Will fail and exit!
if [ "$?"-ne 0]; then echo "command failed"; exit 1; fi 
# But do instead:
command || { echo "command failed"; exit 1; }                # Ok

# Temporarily disable the check for some code section
set +e
command1
command2
set -e

Expect space in filenames

if [ $filename = "foo" ];                      # WRONG
if [ "$filename" = "foo" ];                    # Correct

for i in $@; do echo $i; done }                # WRONG
{ for i in "$@"; do echo $i; done }            # Correct

find | xargs ls                                # WRONG
find -print0 | xargs -0 ls                     # Correct

for i in $(locate .pdf); do basename $i; done  # WRONG
locate .pdf | xargs -d '\n' -n 1 basemane      # Correct

Use signals to fail cleanly

if [ ! -e $lockfile ]; then
   trap "rm -f $lockfile; exit" INT TERM EXIT
   touch $lockfile
   critical-section
   rm $lockfile
   trap - INT TERM EXIT
else
   echo "critical-section is already running"
fi

Beware of Race conditions

There is race condition between the test of file and its creation. If 2 processes run simultaneously, they might both pass the test successfully and think that they are running alone. To solve it, we need an operation that tests & create the file in an atomic way. An example in [4] and [5], is to use IO redirection and bash's noclobber mode, which won't redirect to an existing file:

if ( set -o noclobber; echo "$$" > "$lockfile") 2> /dev/null; 
then
   trap 'rm -f "$lockfile"; exit $?' INT TERM EXIT

   critical-section
   
   rm -f "$lockfile"
   trap - INT TERM EXIT
else
   echo "Failed to acquire lockfile: $lockfile." 
   echo "Held by $(cat $lockfile)"
fi

Another solution is to use mkdir (see [6]). mkdir is atomic, it will fail if directory already exists, or create it otherwise, both atomically.

LOCKDIR="~/.$(basename $0).lock"
if (mkdir “$LOCKDIR”); then echo “Could not lock…”; exit 1; fi
# “locking” succesful
do_stuff()
rmdir -f “$LOCKDIR”

A more thorough example below from [7]:

~/test-locking.sh (<file name="test-locking.sh" tag="source">download</file>)

{{{content}}}

Tips

Parsing Command-Line Option Parameters

• To ease parsing, pre-parse with executable getopt (see here for more information and examples).

#!/bin/bash
# (old version)
args=`getopt abc: $*`
if test $? != 0
  then
    echo 'Usage: -a -b -c file'
    exit 1
fi
set -- $args
for i
do
  case "$i" in
    -c) shift;echo "flag c set to $1";shift;;
    -a) shift;echo "flag a set";;
    -b) shift;echo "flag b set";;
  esac
done

$ ./g -abc "foo"
flag a set
flag b set
flag c set to foo

• Better yet, parse using Bash/sh built-in getopts (see here for more information and examples).

#!/bin/bash
while getopts  "abc:" flag
do
  echo "$flag" $OPTIND $OPTARG
done
shift $((OPTIND-1))
echo $@

$ ./g -abc "foo" "bar"
a 1
b 1
c 3 foo
bar

• To parse option like --value=name ([8])

until [[ ! "$*" ]]; do
  if [[ ${1:0:2} = '--' ]]; then
    PAIR=${1:2}
    PARAMETER=$(echo ${PAIR%=*} | tr [:lower:]- [:upper:]_)
    eval P_$PARAMETER=${PAIR##*=}
  fi
  shift
done

Empty a file keeping permissions

Empty a file named filename, keeping the same permission and user/group:

>filename

Print multi-lines with echo

Print multi-lines text with echo:

$ echo -e "Some text\n...on 2 lines..."                    # Enable interpretation of backslash escapes (must be quoted!)
Some text
...on 2 lines...

Print multi-line variables with echo

One can save in a variable the multi-line output of a command. Later this variable can echoed while preserving the linefeeds if the variable is enclosed in quotes "...":

$ mymultilinevar=$(<myfile.txt sed -e'/first line/,/last line/')
$ echo "$mymultilinevar"
first line
second line
...
last line

Echo with colors

The command echo can display colors thanks to escape sequence commands [9]:

echo -e "\033[35;1m Shocking \033[0m"       #Display "shocking" in bright purple

The first character is the escape character 27 (033 in octal). One can also type directly ^[ (i.e. Ctrl-AltGr-[). The syntax is (where spaces were added for clarity)

\033 [ <command> m
\033 [ <command> ; <command> m

Note that commands can be chained. The set of commands is given in the color table below:

code	style	code	foreground	code	foreground	code	background	code	background
0	default colour			90	dark grey	40	black	100	dark grey
1	bold	31	red	91	light red	41	red	101	light red
4	underlined	32	green	92	light green	42	green	102	light green
5	flashing text	33	orange	93	yellow	43	orange	103	yellow
7	reverse field	34	blue	94	light blue	44	blue	104	light blue
		35	purple	95	light purple	45	purple	105	light purple
		36	cyan	96	turquoise	46	cyan	106	turquoise
		37	grey			47	grey

Get file size

The different ways to extract file size in a Bash script:

SIZE=$(stat -c%s "$FILENAME")                              # Using stat
SIZE=$(ls -l $FILENAME | awk -F" "'{ print $5 }')          # Using ls / awk
SIZE=$(du -b $FILENAME | sed 's/\([0-9]*\)\(.*\)/\1/')     # Using du
SIZE=$(cat $FILENAME | wc -c)                              # Using cat / wc
SIZE=$(ls -l $FILENAME | cut -d " " -f 6)                  # Using ls / cut

Read file content into env variable

Read the content of a file into an environment variable:

PID=`cat $PIDFILE`
read PID < $PIDFILE

Get the PID of a new process

Getting the pid of a new process (when other processes with same name are already running)

oldPID=`pidofproc /usr/bin/ssh`
/usr/bin/ssh -f -N -n -q -D 1080 noekeon
RETVAL=$?
newPID=`pidofproc /usr/bin/ssh`
uniqPID=`echo $oldPID $newPID|sed -e 's/ /\n/g'|sort|uniq -u`
echo $uniqPID

Get the PID of a running process

Getting the pid of a running process

pid=$(pidof -o $$ -o $PPID - o %PPID -x /bin/ssh)

Detect if a given process is running

This is actually a tricky one. Some good solutions, all giving answer in $?:

[ -e /proc/$pid ]               # PID  - nice, but is it portable?
ps -p $pid >/dev/null           # PID  - need redirect, otherwise ps will print the process found
pgrep "^$name$"                 # NAME - probably the best using command-name
pkill -0 $name                  # NAME - ... similar & less robust (fail if process can't accept signal)
/bin/kill -0 $pid 2>/dev/null   # PID  - need redirect, otherwise kill will complain if no process found
                                #        ... also works with bash built-in kill

Some wrong / bad solutions:

ps -aef | grep $pid                   # --== FAIL ==-- Will match grep process itself + $pid as ppid
ps -aef | grep $name                  # --== FAIL ==-- Will match grep process itself
ps -aef | grep -v grep | grep $pid    # --== UGLY ==-- ... and slow. Better use ps -fp $(pgrep $pid)
ps -p $pid | grep $pid                # --== SLOW ==-- better test $? immediately

Don't use this method for locking in startup scripts. Be careful with race condition. The best solution is to use a mutex, or use an atomic command (like mkdir). See for example:

• http://flabdablet.nfshost.com/linux-scripts/test-locking.sh
• http://www.davidpashley.com/articles/writing-robust-shell-scripts.html#id2326620

Launch a process in the background

Different ways to launch process in the background (unordered - might be useful one day...). The double ampersand trick comes from here.

myprocess.exe &
exec myprocess.exe
exec myprocess.exe &
( ( exec myprocess.exe & ) & )
nohup myprocess.exe &
( ( nohup myprocess.exe & ) & )

Display the name / body of functions

To list the functions declared in the current environment, or to list the body of a function:

declare -f                    # List all defined functions and their bodies
declare -f name               # List the body of function "name"
declare -F                    # List name of all defined functions

Return the subnet address

Solution from [10].

/sbin/ifconfig eth0 |
grep 'inet addr' | tr .: '  ' |
(read inet addr a b c d Bcast e f g h Mask i j k l;
echo $(( $a & $i )).$(( $b & $j )).$(( $c & $k )).$(( $d & $l )) )

Remove file name extensions

FILENAME="myfile.pdf"
echo ${FILENAME%%.pdf}          # only matches '.pdf', not '.PDF'
echo ${FILENAME%%.???}          # only matches 3-letter extension

Formatted output / printing using printf

printf is a Bash built-in function that allows printing formatted output much like the standard C printf instructions.

printf "%02d" 1                  # outputs '01'

Delete files with special characters

find . -inum [inode] -exec rm -i {} \;     # Use inode
rm -- -foo                                 # Special case for name with a heading dash
rm ./-foo

Remove useless invocation of 'cat'

There are basically only 3 valid uses of cat:

• Show the content of a file in a terminal
• Write a "here" document or standard input to a file in a terminal
• Concatenating several files together (hence the name of cat)

However cat is frequently used for other purposes like piping a file in a process. This is a bad habit. It is slow and add an unnecessary process. A better alternative is to use the file redirection feature of the shell:

Correct use cat file # Correct cat <<EOF >file # Correct cat file1 file2 # Correct

Bad use (and fix) cat file | myprocess # Bad $(cat file) # Bad <file myprocess # Correct $(< file) # Correct

Using Process Substitution

The process substitution feature of Bash takes the form <(list) or >(list). The process list is run with its input or output connected to a FIFO (named pipe) or a file in /dev/fd. The name of this file is then passed as an argument to the current command (as a result of the expansion). We can see this explicitly with the following examples:

echo >(true)
# /dev/fd/63
echo <(true)
# /dev/fd/63

This feature can be used to build some very advanced redirection [11]:

diff <(ls dir1) <(ls dir2)                                         # Compare the content of 2 directories
sort -k 9 <(ls -l /bin) <(ls -l /usr/bin) <(ls -l /usr/X11R6/bin)  # Sort content of 3 directories
tar cf >(gzip -c > file.tar.gz) $directory                         # Equivalent of tar czf file.tar.gz $directory

It can also be used to use variables that would otherwise be limited to some subprocess, like:

: | ((x++))           # This actually starts a subprocess
: | ( ((x++)) )       # ... like this. 
echo x                # ... so 'x' is undefined here

((x++)) < <(:)        # now variable 'x' remains in the main process
echo $x               # x is defined

Redirecting stdout and stderr with tee and a pipe

Using tee and the standard piping mechanism, it is easy to redirect the content of stdout to a file and stdout:

command | tee stdout.log           # Keep a copy of 'command' output in file 'stdout.log'

What if we also want to do the same with stderr? In other words, can we also pipe stderr?
Yes, in Bash this is easy! We only need to use the process substitution feature (reference [12])!

command 2> >(tee stderr.log) >&2                     # Keep a copy of 'command' stderr in file 'stderr.log'
command > >(tee stdout.log) 2> >(tee stderr.log >&2) # Keep both a copy of stdout and stderr in separate files

Note that tee always print the content of stdin to stdout. That's why we need the redirection >&2 to redirect it back to stderr.

Pits

A list of frequent gotcha's !

Description	Example
Space! - Don't forget to add spaces whenever necessary, in particular around brace in function definition, or in test conditions for ifs.	if -space- [ -space- -f /etc/foo -space- ]; then ... function myfunc() { -space- echo Hello, World!; }
Quote - Always quote parameters, variables passed to test in if ... then ... else:	if [ "$name" -eq 5 ]; then ...
For loops with file - Use simply * to list files in for loops, not `ls *`:	for file in *; cat "$file"; done # SUCCEEDS, even if white space for file in `ls *`; cat "$file"; done # FAILS miserably
Incorrect variable definition NO space around equal sign var = val is interpreted as command var with param = val No dollar $ prefix!!! So it is MYVAR=value and not MYVAR= value !!!	srcDir = $1 # WRONG - spaces around = sign $srcDir=$1 # WRONG - $ prefix maxW= $(sed -rn '/$^/Q' myfile.txt) # WRONG - SPACE! srcDir=$1 # CORRECT srcDir="$1" # BEST
Semi-colon in find - Semi-colon in find commands must be escaped !	find . -exec echo {} ; # WRONG - semi-colon not escaped find . -exec echo {} \; # CORRECT
Using a bash built-in instead of external program Bash built-in commands override external commands with same name (eg. kill and echo)	$ type kill # kill is a shell builtin $ type /bin/kill # /bin/kill is /bin/kill $ /bin/kill -v # kill (cygwin) 1.14
Wrong redirection order	read pid < $PID_FILE 2> /dev/null # WRONG - error msg if $PID_FILE # doesn't exist read pid 2> /dev/null < $PID_FILE # CORRECT
Variable not exported outside parens	( read pid < $PID_FILE ) 2> /dev/null # WRONG - var pid not kept read pid 2> /dev/null < $PID_FILE # CORRECT
Read and piping Don't pipe to read command, or use parens to preserve subshell! Better yet, use set	echo "1 2 3" | read a b c; echo $a $b $c # WRONG - subshell echo "1 2 3" | (read a b c; echo $a $b $c) # CORRECT - same subshell set -- $(echo "1 2 3"); echo $1, $2, $3 # BETTER
Don't quote tilde in if test block	if [ -a ~/bin/"my file" ]; then echo found; fi # CORRECT if [ -a "~/bin/my file" ]; then echo found; fi # WRONG
Need quoting when echoing a variable with embedded newlines. This is because echo takes newlines (like any blanks) as parameter separator	HEADER=$(sed -rn '/$^/Q' myfile.txt) echo "$HEADER" # CORRECT echo $HEADER # WRONG - newline are removed