Diff for "BashFAQ/035"

How can I handle command-line arguments (options) to my script easily?

Well, that depends a great deal on what you want to do with them. There are several approaches, each with its strengths and weaknesses.

Manual loop

This approach handles any arbitrary set of options, because you're writing the parser yourself. For 90% of programs, this is the simplest approach (because you rarely need fancy stuff).

This example will handle a combination of short and long options. Notice how both "--file" and "--file=FILE" are handled.

   1 #!/bin/sh
   2 # (POSIX shell syntax)
   3 
   4 # Reset all variables that might be set
   5 file=""
   6 verbose=0
   7 
   8 while :
   9 do
  10     case $1 in
  11         -h | --help | -\?)
  12             #  Call your Help() or usage() function here.
  13             exit 0      # This is not an error, User asked help. Don't do "exit 1"
  14             ;;
  15         -f | --file)
  16             file=$2     # You might want to check if you really got FILE
  17             shift 2
  18             ;;
  19         --file=*)
  20             file=${1#*=}        # Delete everything up till "="
  21             shift
  22             ;;
  23         -v | --verbose)
  24             # Each instance of -v adds 1 to verbosity
  25             verbose=$((verbose+1))
  26             shift
  27             ;;
  28         --) # End of all options
  29             shift
  30             break
  31             ;;
  32         -*)
  33             echo "WARN: Unknown option (ignored): $1" >&2
  34             shift
  35             ;;
  36         *)  # no more options. Stop while loop
  37             break
  38             ;;
  39     esac
  40 done
  41 
  42 # Suppose some options are required. Check that we got them.
  43 
  44 if [ ! "$file" ]; then
  45     echo "ERROR: option '--file FILE' not given. See --help" >&2
  46     exit 1
  47 fi
  48 
  49 # Rest of the program here.
  50 # If there are input files (for example) that follow the options, they
  51 # will remain in the "$@" positional parameters.

This parser does not handle separate options concatenated together (like -xvf being understood as -x -v -f). This could be added with effort, but this is left as an exercise for the reader.

Some Bash programmers like to write this at the beginning of their scripts to guard against unused variables:

    set -u     # or, set -o nounset

The use of this breaks the loop above, as "$1" may not be set upon entering the loop. There are four solutions to this issue:

1. Stop using -u.

2. Replace case $1 in with case ${1+$1} in (as well as bandaging all the other code that set -u breaks).

3. Replace case $1 in with case ${1-} in (every potentially undeclared variable could be written as ${variable-} to prevent set -u tripping).

4. Stop using -u.

getopts

Unless it's the version from util-linux, and use its advanced mode, never use getopt(1). Traditional versions of getopt cannot handle empty arguments strings, or arguments with embedded whitespace.

The POSIX shell (and others) offer getopts which is safe to use instead. Here is a simplistic getopts example:

   1 #!/bin/sh
   2 
   3 # Initialize our own variables:
   4 output_file=""
   5 verbose=0
   6 
   7 OPTIND=1 # Reset is necessary if getopts was used previously in the script.  It is a good idea to make this local in a function.
   8 while getopts "hvf:" opt; do
   9     case "$opt" in
  10         h)
  11             show_help
  12             exit 0
  13             ;;
  14         v)  verbose=1
  15             ;;
  16         f)  output_file=$OPTARG
  17             ;;
  18     esac
  19 done
  20 shift $((OPTIND-1)) # Shift off the options and optional --.
  21 
  22 echo "verbose=$verbose, output_file='$output_file', Leftovers: $@"
  23 
  24 # End of file

The advantages of getopts are:

1. It's portable, and will work in e.g. dash.

2. It can handle things like -vf filename in the expected Unix way, automatically.

The disadvantage of getopts is that (except on ksh93), it can only handle short options (-h, not --help) without trickery.

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.

There is also still the disadvantage that options are coded in at least 2, probably 3 places - in the call to getopts, in the case statement that processes them and presumably in the help message that you are going to get around to writing one of these days. This is a classic opportunity for errors to creep in as the code is written and maintained - often not discovered till much, much later. This can be avoided by using callback functions, but this approach kind of defeats the purpose of using getopts at all.

?, :, and - are the 3 characters that are not allowed as option names by getopts.

Silly repeated brute-force scanning

Another approach is to check options with if statements "on demand". A function like this one may be useful:

   1 #!/bin/bash
   2 
   3 HaveOpt ()
   4 {
   5     local needle=$1
   6     shift
   7 
   8     while [[ $1 == -* ]]
   9     do
  10         # By convention, "--" means end of options.
  11         case "$1" in
  12             --)      return 1 ;;
  13             $needle) return 0 ;;
  14         esac
  15 
  16         shift
  17     done
  18 
  19     return 1
  20 }
  21 
  22 HaveOpt --quick "$@" && echo "Option quick is set"
  23 
  24 # End of file

and it will work if script is run as:

• YES: ./script --quick
• YES: ./script -other --quick

but will stop on first argument with no "-" in front (or on --):

• NO: ./script -bar foo --quick
• NO: ./script -bar -- --quick

Of course, this approach (iterating over the argument list every time you want to check for one) is far less efficient than just iterating once and setting flag variables.

It also spreads the options throughout the program. The literal option --quick may appear a hundred lines down inside the main body of the program, nowhere near any other option name. This is a nightmare for maintenance.

Complex nonstandard add-on utilities

bhepple suggests the use of process-getopt (GPL licensed) and offers this example code:

PROG=$(basename $0)
VERSION='1.2'
USAGE="A tiny example using process-getopt(1)"

# call process-getopt functions to define some options:
source process-getopt

SLOT=""
SLOT_func()   { [ "${1:-""}" ] && SLOT="yes"; }      # callback for SLOT option
add_opt SLOT "boolean option" s "" slot

TOKEN=""
TOKEN_func()  { [ "${1:-""}" ] && TOKEN="$2"; }      # callback for TOKEN option
add_opt TOKEN "this option takes a value" t n token number

add_std_opts     # define the standard options --help etc:

TEMP=$(call_getopt "$@") || exit 1
eval set -- "$TEMP" # just as with getopt(1)

# remove the options from the command line
process_opts "$@" || shift "$?"

echo "SLOT=$SLOT"
echo "TOKEN=$TOKEN"
echo "args=$@"

Here, all information about each option is defined in one place making for much easier authoring and maintenance. A lot of the dirty work is handled automatically and standards are obeyed as in getopt(1) - because it calls getopt for you.

• Actually, what the author forgot to say was that it's actually using getopts semantics, rather than getopt. I ran this test:

   ~/process-getopt-1.6$ set -- one 'rm -rf /' 'foo;bar' "'"
   ~/process-getopt-1.6$ call_getopt "$@"
    -- 'rm -rf /' 'foo;bar' ''\'''

• It appears to be intelligent enough to handle null options, whitespace-containing options, and single-quote-containing options in a manner that makes the eval not blow up in your face. But this is not an endorsement of the process-getopt software overall; I don't know it well enough. -GreyCat

It's written and tested on Linux where getopt(1) supports long options. For portability, it tests the local getopt(1) at runtime and if it finds an non-GNU one (ie one that does not return 4 for getopt --test) it only processes short options. It does not use the bash builtin getopts(1) command. -bhepple

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