How do I parse command line arguments in Bash?

Question

Say, I have a script that gets called with this line:

./myscript -vfd ./foo/bar/someFile -o /fizz/someOtherFile 

or this one:

./myscript -v -f -d -o /fizz/someOtherFile ./foo/bar/someFile  

What's the accepted way of parsing this such that in each case (or some combination of the two) $v, $f, and $d will all be set to true and $outFile will be equal to /fizz/someOtherFile?

Answer 1

Bash Space-Separated (e.g., --option argument)

cat >/tmp/demo-space-separated.sh <<'EOF' #!/bin/bash  POSITIONAL_ARGS=()  while [[ $# -gt 0 ]]; do   case $1 in     -e|--extension)       EXTENSION="$2"       shift # past argument       shift # past value       ;;     -s|--searchpath)       SEARCHPATH="$2"       shift # past argument       shift # past value       ;;     --default)       DEFAULT=YES       shift # past argument       ;;     -*|--*)       echo "Unknown option $1"       exit 1       ;;     *)       POSITIONAL_ARGS+=("$1") # save positional arg       shift # past argument       ;;   esac done  set -- "${POSITIONAL_ARGS[@]}" # restore positional parameters  echo "FILE EXTENSION  = ${EXTENSION}" echo "SEARCH PATH     = ${SEARCHPATH}" echo "DEFAULT         = ${DEFAULT}" echo "Number files in SEARCH PATH with EXTENSION:" $(ls -1 "${SEARCHPATH}"/*."${EXTENSION}" | wc -l)  if [[ -n $1 ]]; then     echo "Last line of file specified as non-opt/last argument:"     tail -1 "$1" fi EOF  chmod +x /tmp/demo-space-separated.sh  /tmp/demo-space-separated.sh -e conf -s /etc /etc/hosts 

Output from copy-pasting the block above

FILE EXTENSION  = conf SEARCH PATH     = /etc DEFAULT         = Number files in SEARCH PATH with EXTENSION: 14 Last line of file specified as non-opt/last argument: #93.184.216.34    example.com 

Usage

demo-space-separated.sh -e conf -s /etc /etc/hosts 

---

Bash Equals-Separated (e.g., --option=argument)

cat >/tmp/demo-equals-separated.sh <<'EOF' #!/bin/bash  for i in "$@"; do   case $i in     -e=*|--extension=*)       EXTENSION="${i#*=}"       shift # past argument=value       ;;     -s=*|--searchpath=*)       SEARCHPATH="${i#*=}"       shift # past argument=value       ;;     --default)       DEFAULT=YES       shift # past argument with no value       ;;     -*|--*)       echo "Unknown option $i"       exit 1       ;;     *)       ;;   esac done  echo "FILE EXTENSION  = ${EXTENSION}" echo "SEARCH PATH     = ${SEARCHPATH}" echo "DEFAULT         = ${DEFAULT}" echo "Number files in SEARCH PATH with EXTENSION:" $(ls -1 "${SEARCHPATH}"/*."${EXTENSION}" | wc -l)  if [[ -n $1 ]]; then     echo "Last line of file specified as non-opt/last argument:"     tail -1 $1 fi EOF  chmod +x /tmp/demo-equals-separated.sh  /tmp/demo-equals-separated.sh -e=conf -s=/etc /etc/hosts 

Output from copy-pasting the block above

FILE EXTENSION  = conf SEARCH PATH     = /etc DEFAULT         = Number files in SEARCH PATH with EXTENSION: 14 Last line of file specified as non-opt/last argument: #93.184.216.34    example.com 

Usage

demo-equals-separated.sh -e=conf -s=/etc /etc/hosts 

---

To better understand ${i#*=} search for "Substring Removal" in this guide. It is functionally equivalent to `sed 's/[^=]*=//' <<< "$i"` which calls a needless subprocess or `echo "$i" | sed 's/[^=]*=//'` which calls two needless subprocesses.

---

Using bash with getopt[s]

getopt(1) limitations (older, relatively-recent getopt versions):

• can't handle arguments that are empty strings
• can't handle arguments with embedded whitespace

More recent getopt versions don't have these limitations. For more information, see these docs.

---

POSIX getopts

Additionally, the POSIX shell and others offer getopts which doen't have these limitations. I've included a simplistic getopts example.

cat >/tmp/demo-getopts.sh <<'EOF' #!/bin/sh  # A POSIX variable OPTIND=1         # Reset in case getopts has been used previously in the shell.  # Initialize our own variables: output_file="" verbose=0  while getopts "h?vf:" opt; do   case "$opt" in     h|\?)       show_help       exit 0       ;;     v)  verbose=1       ;;     f)  output_file=$OPTARG       ;;   esac done  shift $((OPTIND-1))  [ "${1:-}" = "--" ] && shift  echo "verbose=$verbose, output_file='$output_file', Leftovers: $@" EOF  chmod +x /tmp/demo-getopts.sh  /tmp/demo-getopts.sh -vf /etc/hosts foo bar 

Output from copy-pasting the block above

verbose=1, output_file='/etc/hosts', Leftovers: foo bar 

Usage

demo-getopts.sh -vf /etc/hosts foo bar 

The advantages of getopts are:

1. It's more portable, and will work in other shells like dash.
2. It can handle multiple single options like -vf filename in the typical Unix way, automatically.

The disadvantage of getopts is that it can only handle short options (-h, not --help) without additional code.

There is a getopts tutorial which explains what all of the syntax and variables mean. In bash, there is also help getopts, which might be informative.