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Pythonic command line arguments parser

Pythonic command line arguments parser

docopts

Shell interpreter for docopt, the command-line interface description language.

docopts parses the command line argument vector argv according to the docopt string msg and echoes the results to standard output as a snippet of Bash source code. Passing this snippet as an argument to eval(1) is sufficient for handling the CLI needs of most scripts.

If argv matches one of the usage patterns defined in msg, docopts
generates code for storing the parsed arguments as Bash variables. As most
command line argument names are not valid Bash identifiers, some name mangling
will take place:

  • <Angle_Brackets>: Angle_Brackets
  • UPPER-CASE: UPPER_CASE
  • --Long-Option: Long_Option
  • -S: S

If one of the argument names cannot be mangled into a valid Bash identifier,
or two argument names map to the same variable name, docopt will exit with
an error, and you should really rethink your CLI. The -- and -
commands will not be stored.

Alternatively, docopts can be invoked with the -A <name> option, which
stores the parsed arguments as fields of a Bash 4 associative array called
<name> instead. However, as Bash does not natively support nested arrays,
they are faked for repeatable arguments with the following access syntax::

${args[ARG,#]} # the number of arguments to ARG
${args[ARG,0]} # the first argument to ARG
${args[ARG,1]} # the second argument to ARG, etc.

The arguments are stored as follows:

  • Non-repeatable, valueless arguments: true if found, false if not
  • Repeatable valueless arguments: the count of their instances in argv
  • Non-repeatable arguments with values: the value as a string if found,
    the empty string if not
  • Repeatable arguments with values: a Bash array of the parsed values

Unless the --no-help option is given, docopts handles the --help
and --version options and their possible aliases specially,
generating code for printing the relevant message to standard output and
terminating successfully if either option is encountered when parsing argv.
Note however that this also requires listing the relevant option in
msg and, in --version's case, invoking docopts with the --version
option.

If argv does not match any usage pattern in msg, docopts will generate
code for exiting the program with status 64 (EX_USAGE in
sysexits(3) <http://man.cx/sysexits(3)>_) and printing a diagnostic error
message.

Note that due to the above, docopts can't be used to parse shell function
arguments: exit(1) <http://man.cx/exit(1)>_ quits the entire interpreter,
not just the current function.

OPTIONS

-h , --help= The help message in docopt format.
If - is given, read the help message from
standard input.
If no argument is given, print docopts's own
help message and quit.
-V , --version= A version message.
If - is given, read the version message from
standard input. If the help message is also
read from standard input, read it first.
If no argument is given, print docopts's own
version message and quit.
-O, --options-first Disallow interspersing options and positional
arguments: all arguments starting from the
first one that does not begin with a dash will
be treated as positional arguments.
-H, --no-help Don't handle --help and --version specially.
-A Export the arguments as a Bash 4.x associative
array called .
-s , --separator= The string to use to separate the help message
from the version message when both are given
via standard input. [default: ----]

EXAMPLES

Read the help and version messages from standard input::

eval "$(docopts -V - -h - : "[email protected]" <<EOF
Usage: rock [options] <argv>...

      --verbose  Generate verbose messages.
      --help     Show help options.
      --version  Print program version.
----
rock 0.1.0
Copyright (C) 200X Thomas Light
License RIT (Robot Institute of Technology)
This is free software: you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.
EOF
)"

if $verbose ; then
    echo "Hello, world!"
fi

Parse the help and version messages from script comments and pass them as
command line arguments::

#? rock 0.1.0
#? Copyright (C) 200X Thomas Light
#? License RIT (Robot Institute of Technology)
#? This is free software: you are free to change and redistribute it.
#? There is NO WARRANTY, to the extent permitted by law.

##? Usage: rock [options] <argv>...
##? 
##?       --help     Show help options.
##?       --version  Print program version.

help=$(grep "^##?" "$0" | cut -c 5-)
version=$(grep "^#?"  "$0" | cut -c 4-)
eval "$(docopts -h "$help" -V "$version" : "[email protected]")"

for arg in "${argv[@]}"; do
    echo "$arg"
done

Using the associative array::

eval "$(docopts -A args -h "$help" : "[email protected]")"

if ${args[subcommand]} ; then
    echo "subcommand was given"
fi

if [ -n "${args[--long-option-with-argument]}" ] ; then
    echo "${args[--long-option-with-argument]}"
else
    echo "--long-option-with-argument was not given"
fi

i=0
while [[ $i -lt ${args[<argument-with-multiple-values>,#]} ]] ; do
    echo "${args[<argument-with-multiple-values>,$i]}"
    i=$[$i+1]
done

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