Bash reading arguments, most effective way

JeffreyV · 08-17-2010, 08:05 PM

I've been reading about getopt and getopts but it doesn't seem like it's possible to parse arguments like --foo or even -foo. I've started my own script trying to achieve this, but I'm still wondering if I'm losing performance and if there is a better way to do this task.

Also I'm using the [[ =~ ]] regex syntax which seems to be available only in newer bash versions, should it be a big issue?

My bash version: GNU bash, version 4.1.7(2)-release (x86_64-unknown-linux-gnu)

My scripts goes as follows

Code:

#!/bin/bash
if [ $# -eq 0 ]; then
    printf "You need to give arguments!!!\n" >&2
    exit 1
fi

SKIP=0
for arg in "$@"; do
    [ $SKIP -eq 1 ] && SKIP=0 && continue

    if [[ $arg =~ ^--foo=.*$ ]]; then
        printf "foo = %s\n" "${arg/#*=/}"
        shift
    elif [[ $arg =~ ^--config=.*$ ]]; then
        printf "config = %s\n" "${arg/#*=/}"
        shift
    elif [[ $arg =~ ^--bar$ ]]; then
        shift
        [ -z "$1" -o "${1/%[^-]*/}" = "--" ] && printf "No argument for bar\n" >&2 && break
        printf "bar = %s\n" "$1"
        shift
        
        # Skip next one
        SKIP=1
    elif [[ $arg =~ ^-a$ ]]; then
        shift 
        [ -z "$1" -o "${1/%[^-]*/}" = "--" ] && printf "No argument for -a\n" >&2 && break

        printf "a = %s\n" "$1"
        shift
        SKIP=1
    elif [[ $arg =~ ^--playlist=.+$ ]]; then
        printf "playlist = %s\n" "${arg/#*=/}"
        shift
    else
        if [[ $arg =~ ^--.*$ ]]; then
            printf "Unknown option\n" >&2
        else
            printf "Regular argument = %s\n" "$arg"
        fi

        shift
    fi
done

When running it like this

./argument_test.sh dontuse --foo="test" normal --config="s.xml" --bar "something" --playlist="playlist.txt" -a

This is what I get as output, which I think should be the correct thing.

Regular argument = dontuse
foo = test
Regular argument = normal
config = s.xml
bar = something
playlist = playlist.txt
No argument for -a

Edit: note that --bar doesn't use the equals sign, which means it has to use the next argument in line

David the H. · 08-17-2010, 08:23 PM

The bash built-in getopts can't handle long options, but the external getopt can, assuming it's the standard gnu version found in the util-linux package.

This thread appears to have some good suggestions as well.

http://stackoverflow.com/questions/4...d-line-options

konsolebox · 08-17-2010, 08:26 PM

You can do something similar to this:

Code:

#!/bin/bash

A=false
F=''

while [[ $# -ge 1 ]]; do
	case "$1" in
	-a)
		A=true
		;;
	--foo)
		F=$2
		shift
		;;
	--foo=*)
		F=${1#--foo=}
		;;
	*)
		# ... invalid argument?
		;;
	esac
	shift
done

JeffreyV · 08-18-2010, 03:31 AM

Thanks for the replays. Seems that combining custom code and case statements with the getopt command is a good way to do the job. But still there is always errors that can occur. I found the next page, and it seems very useful.

http://code.google.com/p/shflags/

konsolebox · 08-18-2010, 04:20 AM

Quote:

Originally Posted by JeffreyV

Thanks for the replays. Seems that combining custom code and case statements with the getopt command is a good way to do the job. But still there is always errors that can occur.

The one I presented doesn't use getopts... So what do you think about it?

JeffreyV · 08-19-2010, 05:23 AM

Quote:

Originally Posted by konsolebox

The one I presented doesn't use getopts... So what do you think about it?

It looks good, but like my version, you need to program something that checks for errors, if you don't specify a value.

well, getopts "abc:", will report an error if you use -c without passing a value. Which is quite useful since the user would know that he made a mistake. I guess you can wrap it all up in a function, which I will try.

There's a part that is fail proof though

Code:

	--foo=*)
		F=${1#--foo=}
		;;

Since you don't have to go mess with the argument following. A little more safe might be
--foo=[A-Za-z_-.]+

but I'm quite vague about the regex patterns that the case statement supports, so I'm going to try it out

konsolebox · 08-19-2010, 05:38 AM

You can use extended pattern matching in bash.

For example:

Code:

#!/bin/bash

shopt -s extglob

A=false
F=''

while [[ $# -ge 1 ]]; do
	case "$1" in
...
	--foo)
		if [[ $# -ne 2 ]]; then
			# tell the user that it needs an argument
		else
			F=$2
			shift
		fi
		;;
	--foo=+([[:alpha:]_-.]))
		;;
...
	*)
		# ... invalid argument?
		;;
	esac
	shift
done

Notice that there's always a way to check if an argument is passed to an option.