Bash Tips and Pitfalls: Difference between revisions

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|'''Incorrect variable definition'''
|'''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 <tt>$</tt>
* {{red|'''NO space'''}} between the variable name and the subsequent equal sign
* No dollar <tt>$</tt> prefix!!!
So it is '''<big>{{green|MYVAR&#61;value}}</big>''' and '''not''' '''<big><s>{{red|MYVAR&#61; value}}</s></big>''' !!!
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<source lang="bash" enclose="valid">
<source lang="bash" enclose="valid">
srcDir = $1 # WRONG - spaces around = sign
srcDir = $1 # WRONG - spaces around = sign
$srcDir=$1 # WRONG - variable name must not have $ prefix
$srcDir=$1 # WRONG - $ prefix
maxW= $(sed -rn '/$^/Q' myfile.txt) # WRONG - SPACE!
srcDir=$1 # CORRECT
srcDir="$1" # BEST
srcDir=$1 # CORRECT
srcDir="$1" # BEST
</source>
</source>
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Revision as of 14:07, 5 August 2010

Tips

Parsing Command-Line Option Parameters

#!/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
#!/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 & ) & )

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 [2].

/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'

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 between the variable name and the subsequent equal sign
  • 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
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
Variables assigned by read built-in seems to be empty → use a surrounding parens!
echo "1 2 3" | (read a b c; echo $a $b $c)  # CORRECT
echo "1 2 3" | read a b c; echo $a $b $c    # WRONG
Don't quote tilde in if test block
if [ -a ~/bin/"my file" ]; then echo found; fi # CORRECT
if [ -a "~/bin/my file" ]; then echo found; fi # WRONG
Need quoting when echoing a variable with embedded newlines.
This is because echo takes newlines (like any blanks) as parameter separator
HEADER=$(sed -rn '/$^/Q' myfile.txt)
echo "$HEADER" # CORRECT
echo $HEADER   # WRONG - newline are removed