Cross-platform getopt for a shell script

问题

I've just found out that getopt is not cross-platform (in particular for FreeBSD and Linux). What is the best workaround for this issue?

回答1:

Use getopts (with an "s").

According to Bash FAQ 35:

Unless it's the version from util-linux, and you use its advanced mode, never use getopt(1). getopt cannot handle empty arguments strings, or arguments with embedded whitespace. Please forget that it ever existed.

The POSIX shell (and others) offer getopts which is safe to use instead.

回答2:

There are essentially two versions of the getopt command: the original version and the GNU enhanced version. The GNU enhanced version is backward compatible with the original version, so if you only use the features of the original version it will work with both.

Detect which version of getopt is available

You can detect which version is available and use the enhanced features if the GNU enhanced version is available, and limit yourself to the original features if the GNU enhanced version is not available. The enhanced version has a -T option for testing which version is available.

getopt -T > /dev/null
if [ $? -eq 4 ]; then
    # GNU enhanced getopt is available
    set -- `getopt --long help,output:,version --options ho:v -- "$@"`
else
    # Original getopt is available
    set -- `getopt ho:v "$@"`
fi

Consider using built-in shell command getopts (with an "s") instead, because it is more portable. However, getopts does not support long options (e.g. --help).

If you like long options, use getopt and use the above test to see if the GNU enhanced version of getopt is available or not. If the enhanced version is not available, the script can gracefully degrade to either using the original version of getopt (with no support for long option names and no whitespace support) or using getopts (with no support for long option names).

Using GNU enhanced getopt properly

Getting the GNU enhanced version to process arguments with whitespace properly is tricky. Here's how it is done:

ARGS=`getopt --long help,output:,verbose --options ho:v -- "$@"`
if [ $? -ne 0 ]; then
  echo "Usage error (use -h for help)" >&2
  exit 2
fi
eval set -- $ARGS

# Parameters are now sorted: options appear first, followed by --, then arguments
# e.g. entering: "foo bar" -o abc baz -v
#      produces: -o 'abc' -v -- 'foo bar' 'baz'

The secret is to use "$@" where the double quotes are very important (in line 1), and to eval the set command (in line 6).

So errors raised by getopt can be detected and handled, the call to getopt is done separately from the eval with the two linked by the ARGS variable.

Complete working example

PROG=`basename $0`

getopt -T > /dev/null
if [ $? -eq 4 ]; then
  # GNU enhanced getopt is available
  ARGS=`getopt --name "$PROG" --long help,output:,verbose --options ho:v -- "$@"`
else
  # Original getopt is available (no long option names, no whitespace, no sorting)
  ARGS=`getopt ho:v "$@"`
fi
if [ $? -ne 0 ]; then
  echo "$PROG: usage error (use -h for help)" >&2
  exit 2
fi
eval set -- $ARGS

while [ $# -gt 0 ]; do
    case "$1" in
        -h | --help)     HELP=yes;;
        -o | --output)   OUTFILE="$2"; shift;;
        -v | --verbose)  VERBOSE=yes;;
        --)              shift; break;; # end of options
    esac
    shift
done

if [ $# -gt 0 ]; then
  # Remaining parameters can be processed
  for ARG in "$@"; do
    echo "$PROG: argument: $ARG"
  done
fi

echo "$PROG: verbose: $VERBOSE"
echo "$PROG: output: $OUTFILE"
echo "$PROG: help: $HELP"

This example can be downloaded from https://gist.github.com/hoylen/6607180

The comparison table on Wikipedia's entry on getopts compares the different features.