Bash Scripting Reference

This post is a small summary of the basics of bash scripting. It contains some useful code snippets as well.

Parsing Arguments to a script with getopts

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ARGS=$@
if [[ ${#ARGS} -ne 0 ]]; then
	while getopts ":sT:h" ARG; do
	case "$ARG" in
		s) echo "-s flag set" ;;
		T) echo "-T flag set"
		   echo "The value was $OPTARG" ;;
		h) help ;;
		:) echo "Argument missing for $ARG" ;;
		*) echo "Invalid flag $ARG."
	esac
	done
else
	help
	exit 1
fi
shift "$((OPTIND-1))"

The above script takes a list of required arguments. Should no arguments be provided the script defaults to calling the help function. Adding the : after the T flag in the declaration of the while loop will tell the script that we cant to pass a value with the flag. After parsing is done we reset OPTIND. It is used by getopts to point to the next argument to be processed.

Reference Overview

• $1 bis $9 reference the first 9 arguments passed to a script
• $0 is the script itself
• $# holds the number of arguments passed to the script
• $@ can be used to retrieve the list of command line arguments passed
• $$ process ID of the currently executing process
• $? exit status of the script, is 0 if the script ran successfully and 1 if there was a failure
• variable declarations do not contain white spaces between the name, the assign operator and the value, example: var=val
• all variables are treated as string characters, there are no actual data types
• arrays of values are declared with brackets and delimit the values with white spaces, example myarray=(val1 val2 val3)
• array index starts at 0
• values are retrieved with brackets containing the index, example $myarray[0]
• there are 4 different comparison operators
  1. string operators
  2. integer operators
  3. file operators
  4. boolean operators
• arithmetic operations can be performed on values by wrapping them in double brackets, example: val=$((10 + 10))
• by adding the -x (for xtrace) flag to an invocation of bash we can get verbose output from our scripts containing information about the executed code for debugging purposes
• to get more details while debugging we can also add the -v flag for even more verbose output

String comparison operators

Integer comparison operators

File Operators

Arithmetic operators

Return Codes