Diff for "BashFAQ/035"

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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.

Contents

How can I handle command-line arguments (options) to my script easily?
1. Manual loop
2. getopts

Manual loop

Manually parsing options without the use of a specialized function is the most flexible approach, and is sufficient for most simple scripts.

This example will handle a combination of short (POSIX) and long "GNU style" options with option arguments. Notice how both --file FILE and --file=FILE are handled. Typical scripts may also use functions and local variables, which can greatly improve your code. This example however illustrates a strictly POSIX conforming script.

   1 #!/bin/sh
   2 # POSIX
   3 
   4 # Reset all variables that might be set
   5 file=
   6 verbose=0 # Variables to be evaluated as shell arithmetic should be initialized to a default or validated beforehand.
   7 
   8 while :; do
   9     case $1 in
  10         -h|-\?|--help)   # Call a "show_help" function to display a synopsis, then exit.
  11             show_help
  12             exit
  13             ;;
  14         -f|--file)       # Takes an option argument, ensuring it has been specified.
  15             if [ -n "$2" ]; then
  16                 file=$2
  17                 shift 2
  18                 continue
  19             else
  20                 printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2
  21                 exit 1
  22             fi
  23             ;;
  24         --file=?*)
  25             file=${1#*=} # Delete everything up to "=" and assign the remainder.
  26             ;;
  27         --file=)         # Handle the case of an empty --file=
  28             printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2
  29             exit 1
  30             ;;
  31         -v|--verbose)
  32             verbose=$((verbose + 1)) # Each -v argument adds 1 to verbosity.
  33             ;;
  34         --)              # End of all options.
  35             shift
  36             break
  37             ;;
  38         -?*)
  39             printf 'WARN: Unknown option (ignored): %s\n' "$1" >&2
  40             ;;
  41         *)               # Default case: If no more options then break out of the loop.
  42             break
  43     esac
  44 
  45     shift
  46 done
  47 
  48 # Suppose --file is a required option. Ensure the variable "file" has been set and exit if not.
  49 if [ -z "$file" ]; then
  50     printf 'ERROR: option "--file FILE" not given. See --help.\n' >&2
  51     exit 1
  52 fi
  53 
  54 # Rest of the program here.
  55 # If there are input files (for example) that follow the options, they
  56 # 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.

getopts

Unless it's the version from util-linux, and you use its advanced mode, never use getopt(1). Traditional versions of getopt cannot handle empty argument 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 # Usage info
   4 show_help() {
   5 cat << EOF
   6 Usage: ${0##*/} [-hv] [-f OUTFILE] [FILE]...
   7 Do stuff with FILE and write the result to standard output. With no FILE
   8 or when FILE is -, read standard input.
   9 
  10     -h          display this help and exit
  11     -f OUTFILE  write the result to OUTFILE instead of standard output.
  12     -v          verbose mode. Can be used multiple times for increased
  13                 verbosity.
  14 EOF
  15 }
  16 
  17 # Initialize our own variables:
  18 output_file=""
  19 verbose=0
  20 
  21 OPTIND=1 # Reset is necessary if getopts was used previously in the script.  It is a good idea to make this local in a function.
  22 while getopts "hvf:" opt; do
  23     case "$opt" in
  24         h)
  25             show_help
  26             exit 0
  27             ;;
  28         v)  verbose=$((verbose+1))
  29             ;;
  30         f)  output_file=$OPTARG
  31             ;;
  32         '?')
  33             show_help >&2
  34             exit 1
  35             ;;
  36     esac
  37 done
  38 shift "$((OPTIND-1))" # Shift off the options and optional --.
  39 
  40 printf 'verbose=<%d>\noutput_file=<%s>\nLeftovers:\n' "$verbose" "$output_file"
  41 printf '<%s>\n' "$@"
  42 
  43 # End of file

The advantages of getopts are:

It's portable, and will work in any POSIX shell e.g. dash.
It can handle things like -vf filename in the expected Unix way, automatically.
It understands -- as the option terminator and more generally makes sure, options are parsed like for any standard command.
With some implementations, the error messages will be localised in the language of the user.

The disadvantage of getopts is that (except for ksh93 getopts) it can only handle short options (-h, not --help) without trickery and cannot handle options with optional arguments à la GNU.

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.

For other, more complicated ways of option parsing, see ComplexOptionParsing.

CategoryShell

-  ⇤ ← Revision 72 as of 2012-10-22 20:41:23 → 
  Size: 11126
  Editor: GreyCat
  Comment: Separate getops long option trickery into its own subsection. Also fix the counting in the set -u bit.
+   ← Revision 102 as of 2015-06-18 19:52:05 → ⇥
  Size: 5793
  Editor: GreyCat
  Comment:
-Deletions are marked like this.
+Additions are marked like this.
 Line 1:
-#pragma section-numbers 3
-Line 9:
+Line 8:
+Manually parsing options without the use of a specialized function is the most flexible approach, and is sufficient for most simple scripts.
 Line 10:
-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.
+This example will handle a combination of short (POSIX) and long "GNU style" options with option arguments. Notice how both `--file FILE` and `--file=FILE` are handled. Typical scripts may also use functions and local variables, which can greatly improve your code. This example however illustrates a strictly POSIX conforming script.
-Line 16:
+Line 14:
-# (POSIX shell syntax)
+# POSIX
-Line 19:
+Line 17:
-file=""
verbose=0
+file=
verbose=0 # Variables to be evaluated as shell arithmetic should be initialized to a default or validated beforehand.
-Line 22:
+Line 20:
-while :
do
+while :; do
-Line 25:
+Line 22:
- -h | --help | -\?)
     #  Call your Help() or usage() function here.
     exit 0	# This is not an error, User asked help. Don't do "exit 1"
     ;;
 -f | --file)
     file=$2	# You might want to check if you really got FILE
     shift 2
     ;;
 --file=*)
     file=${1#*=}	# Delete everything up till "="
     shift
         ;;
 -v | --verbose)
     # Each instance of -v adds 1 to verbosity
     verbose=$((verbose+1))
     shift
     ;;
 --) # End of all options
     shift
     break
     ;;
 -*)
     echo "WARN: Unknown option (ignored): $1" >&2
     shift
     ;;
 *)  # no more options. Stop while loop
     break
     ;;
+        -h|-\?|--help)   # Call a "show_help" function to display a synopsis, then exit.
            show_help
            exit
            ;;
        -f|--file)       # Takes an option argument, ensuring it has been specified.
            if [ -n "$2" ]; then
                file=$2
                shift 2
                continue
            else
                printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2
                exit 1
            fi
            ;;
        --file=?*)
            file=${1#*=} # Delete everything up to "=" and assign the remainder.
            ;;
        --file=)         # Handle the case of an empty --file=
            printf 'ERROR: "--file" requires a non-empty option argument.\n' >&2
            exit 1
            ;;
        -v|--verbose)
            verbose=$((verbose + 1)) # Each -v argument adds 1 to verbosity.
            ;;
        --)              # End of all options.
            shift
            break
            ;;
        -?*)
            printf 'WARN: Unknown option (ignored): %s\n' "$1" >&2
            ;;
        *)               # Default case: If no more options then break out of the loop.
            break
-Line 54:
+Line 56:
+    shift
-Line 56:
+Line 60:
-# Suppose some options are required. Check that we got them.

if [ ! "$file" ]; then
    echo "ERROR: option '--file FILE' not given. See --help" >&2
+# Suppose --file is a required option. Ensure the variable "file" has been set and exit if not.
if [ -z "$file" ]; then
    printf 'ERROR: option "--file FILE" not given. See --help.\n' >&2
-Line 67:
+Line 70:
-Line 70:
+Line 72:
-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`.
 1. Replace `case $1 in` with `case ${1+$1} in` (as well as bandaging all the other code that `set -u` breaks).
 1. Replace `case $1 in` with `case ${1-} in` (every potentially undeclared variable could be written as ${variable-} to prevent `set -u` tripping).
 1. Stop using `-u`.
-Line 84:
+Line 73:
-'''Never use getopt(1).''' `getopt` cannot handle empty arguments strings, or arguments with embedded whitespace. Please forget that it ever existed.
+Unless it's the version from util-linux, and you use its advanced mode, '''never use getopt(1).''' Traditional versions of `getopt` cannot handle empty argument strings, or arguments with embedded whitespace.
-Line 92:
+Line 80:
-# A POSIX variable
OPTIND=1         # Reset in case getopts has been used previously in the shell.
+# Usage info
show_help() {
cat << EOF
Usage: ${0##*/} [-hv] [-f OUTFILE] [FILE]...
Do stuff with FILE and write the result to standard output. With no FILE
or when FILE is -, read standard input.

    -h          display this help and exit
    -f OUTFILE  write the result to OUTFILE instead of standard output.
    -v          verbose mode. Can be used multiple times for increased
                verbosity.
EOF
}
-Line 99:
+Line 98:
-while getopts "h?vf:" opt; do
+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.
while getopts "hvf:" opt; do
 Line 101:
- h|\?)
     show_help
     exit 0
     ;;
 v)  verbose=1
     ;;
 f)  output_file=$OPTARG
     ;;
+        h)
            show_help
            exit 0
            ;;
        v)  verbose=$((verbose+1))
            ;;
        f)  output_file=$OPTARG
            ;;
        '?')
            show_help >&2
            exit 1
            ;;
-Line 111:
+Line 115:
+shift "$((OPTIND-1))" # Shift off the options and optional --.
-Line 112:
+Line 117:
-shift $((OPTIND-1))

[ "$1" = "--" ] && shift

echo "verbose=$verbose, output_file='$output_file', Leftovers: $@"
+printf 'verbose=<%d>\noutput_file=<%s>\nLeftovers:\n' "$verbose" "$output_file"
printf '<%s>\n' "$@"
-Line 120:
+Line 122:
+The advantages of `getopts` are:
-Line 121:
+Line 124:
-The advantages of `getopts` are:
 1. It's portable, and will work in e.g. dash.
+. It's portable, and will work in any POSIX shell e.g. dash.
-Line 124:
+Line 126:
+. It understands `--` as the option terminator and more generally makes sure, options are parsed like for any standard command.
 1. With some implementations, the error messages will be localised in the language of the user.
-Line 125:
+Line 129:
-The disadvantage of `getopts` is that it can only handle short options (`-h`, not `--help`) without trickery.
+The disadvantage of `getopts` is that (except for ksh93 `getopts`) it can only handle short options (`-h`, not `--help`) without trickery and cannot handle options with optional arguments à la GNU.
-Line 131:
+Line 135:
-==== getops long option trickery ====

Here is an example which claims to parse long options with `getopts`. The basic idea is quite simple: just put "-:" into the optstring. This trick requires a shell which permits the option-argument (i.e. the filename in "-f filename") to be concatenated to the option (as in "-ffilename"). The [[http://pubs.opengroup.org/onlinepubs/9699919799/utilities/getopts.html|POSIX standard]] says there must be a space between them; bash and dash permit the "-ffilename" variant, but one should not rely on this leniency if attempting to write a portable script.

{{{#!highlight bash
#!/bin/bash
# Uses bash extensions.  Not portable as written.

optspec=":h-:"

while getopts "$optspec" optchar
do
    case "${optchar}" in
 -)
     case "${OPTARG}" in
       loglevel)
    eval val="\$${OPTIND}"; OPTIND=$(( $OPTIND + 1 ))
    echo "Parsing option: '--${OPTARG}', value: '${val}'" >&2
    ;;
       loglevel=*)
    val=${OPTARG#*=}
    opt=${OPTARG%=$val}
    echo "Parsing option: '--${opt}', value: '${val}'" >&2
    ;;
     esac
     ;;
 h)
     echo "usage: $0 [--loglevel[=]<value>]" >&2
     exit 2
     ;;
    esac
done

# End of file
}}}

In practice, this example is so obfuscated that it may be preferable to add concatenated option support (like `-vf filename`) to a manual parsing loop instead, if that was the only reason for using `getopts`.

Here's an improved and more generalized version of above attempt to add support for long options when using `getopts`:
{{{#!highlight bash
#!/bin/bash
# Uses bash extensions.  Not portable as written.

declare -A longoptspec
longoptspec=( [loglevel]=1 ) #use associative array to declare how many arguments a long option expects, in this case we declare that loglevel expects/has one argument, long options that aren't listed in this way will have zero arguments by default
optspec=":h-:"
while getopts "$optspec" opt; do
while true; do
    case "${opt}" in
        -) #OPTARG is name-of-long-option or name-of-long-option=value
            if [[ "${OPTARG}" =~ .*=.* ]] #with this --key=value format only one argument is possible
            then
                opt=${OPTARG/=*/}
                OPTARG=${OPTARG#*=}
                ((OPTIND--))
            else #with this --key value1 value2 format multiple arguments are possible
                opt="$OPTARG"
                OPTARG=(${@:OPTIND:$((longoptspec[$opt]))})
            fi
            ((OPTIND+=longoptspec[$opt]))
            continue #now that opt/OPTARG are set we can process them as if getopts would've given us long options
            ;;
        loglevel)
          loglevel=$OPTARG
            ;;
 h|help)
     echo "usage: $0 [--loglevel[=]<value>]" >&2
     exit 2
     ;;
    esac
break; done
done

# End of file
}}}
With this version you can have long and short options side by side and you shouldn't need to modify the code from line 10 to 22. This solution can also handle multiple arguments for long options, just use ${OPTARG} or ${OPTARG[0]} for the first argument, ${OPTARG[1]} for the second argument, ${OPTARG[2]} for the third argument and so on.
It has the same disadvantage of its predecessor in not being portable and specific to bash.

=== Silly repeated brute-force scanning ===

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

{{{#!highlight bash
#!/bin/bash

HaveOpt ()
{
    local needle=$1
    shift

    while [[ $1 == -* ]]
    do
 # By convention, "--" means end of options.
 case "$1" in
     --)      return 1 ;;
     $needle) return 0 ;;
 esac

 shift
    done

    return 1
}

HaveOpt --quick "$@" && echo "Option quick is set"

# 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 ===

[[http://bhepple.freeshell.org/oddmuse/wiki.cgi/process-getopt|bhepple]] suggests the use of [[http://sourceforge.net/projects/process-getopt/|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 [[BashFAQ/048|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. -[[http://bhepple.freeshell.org/oddmuse/wiki.cgi/process-getopt|bhepple]] ''
+For other, more complicated ways of option parsing, see ComplexOptionParsing.
-Line 300:
+Line 138:
-'' CategoryShell ''
+CategoryShell