arg - A simple command line argument parser

This is a simple tool with a simple job: figure out the value of a single command line argument. Tools like getopt and getopts are unnecessarily complex and difficult to use, while also not supporting common use cases well. Parsing an argument in a shell script shouldn't require a tutorial or dozen lines of shell code!

To quote the getopt manpage:

Each shellscript has to carry complex code to parse arguments halfway correcty (like the example presented here). A better getopt-like tool would move much of the complexity into the tool and keep the client shell scripts simpler.

Usage

Simply run: arg <flag> <extra args...>. <flag> can be anything, like -f, -flag, --flag, or flag. If the flag occurs among the rest of the command line arguments, arg will print the flag's value (if any) and exit successfully, otherwise, it will fail (exit status 1). In a shell script, this would look like:

#!/bin/sh
dir=`arg --dir "$@" || arg -d "$@" || echo "$HOME/Downloads"`
if ! server=`arg --server "$@" || arg -s "$@"`; then
    echo "No server provided :(" && exit 1
fi
if arg --verbose "$@" || arg -v "$@"; then
    echo "Downloading to $dir"
fi
curl "$server/file.zip" > "$dir/file.zip"

The flag's value may be of the form <flag>=<value> or <flag> <value>. Single-letter flags will also match when grouped with other single letter flags, but will not have a value (e.g. arg -b -abc foo will succeed without printing anything).

When using arg in a shell script, it is best to use quotes around $@, as in arg --foo "$@", so arguments with spaces will be forwarded correctly.

1 # arg - A simple command line argument parser

2 This is a simple tool with a simple job: figure out the value of a single

3 command line argument. Tools like `getopt` and `getopts` are unnecessarily

4 complex and difficult to use, while also not supporting common use cases well.

5 Parsing an argument in a shell script shouldn't require a tutorial or dozen

6 lines of shell code!

8 To quote the `getopt` manpage:

10 > Each shellscript has to carry complex code to parse arguments halfway

11 > correcty (like the example presented here). A better getopt-like tool would

12 > move much of the complexity into the tool and keep the client shell scripts

13 > simpler.

15 ## Usage

16 Simply run: `arg <flag> <extra args...>`. `<flag>` can be anything, like `-f`,

17 `-flag`, `--flag`, or `flag`. If the flag occurs among the rest of the

18 command line arguments, `arg` will print the flag's value (if any) and exit

19 successfully, otherwise, it will fail (exit status 1). In a shell script, this

20 would look like:

22 ```

23 #!/bin/sh

24 dir=`arg --dir "$@" || arg -d "$@" || echo "$HOME/Downloads"`

25 if ! server=`arg --server "$@" || arg -s "$@"`; then

26 echo "No server provided :(" && exit 1

27 fi

28 if arg --verbose "$@" || arg -v "$@"; then

29 echo "Downloading to $dir"

30 fi

31 curl "$server/file.zip" > "$dir/file.zip"

32 ```

34 The flag's value may be of the form `<flag>=<value>` or `<flag> <value>`.

35 Single-letter flags will also match when grouped with other single letter

36 flags, but will not have a value (e.g. `arg -b -abc foo` will succeed without

37 printing anything).

39 When using `arg` in a shell script, it is best to use quotes around `$@`, as in

40 `arg --foo "$@"`, so arguments with spaces will be forwarded correctly.