Bash Tips and Pitfalls

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

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

• To parse option like --value=name ([1])

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...

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)

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 & ) & )

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
Space variable setting - There must be no space between the variable name and the subsequent equal sign. Also the variable name must not be prefixed with a $	srcDir = $1 # WRONG - spaces around = sign $srcDir=$1 # WRONG - variable name must not have $ prefix 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
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
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
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