Bash Tips and Pitfalls

Tips

Parsing Command-Line Option Parameters

• To ease parsing, pre-parse with executable getopt or Bash/sh built-in getopts, see here for more information and examples.

• 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

Miscellaneous

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

>filename

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...

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

Pits

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