Bash parser for command line arguments

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Regular approach to parsing Bash script cli-arguments only works well for short, one-letter arguments and does not provide important functions, that can help script authors define one-letter synonyms for long --two-dash arguments, printing errors in case arguments are not provided with values when it's needed or when required arguments are necessary in general.

Internal:BashJazz/utest library installed & $BASHJAZZ_PATH env variable set.

External: bash >= v4.2, awk, sed, grep.


To use this library inside your script, you first need to source it with:

  • ...
  • source $BASHJAZZ_PATH/cli-args/
  • ...
Then, there are the following three steps:
  1. Define which arguments are to be accepted and if they require values passed.
  2. Parse what's actually been passed.
  3. Access the values passed with these arguments through the special sub-functions of CliArgs designed for these purposes.


To define which arguments are to be accepted and provide certain rules which these arguments should follow, use the CliArgs define function. The Example below shows all possible options you may use (not all of them are necessary) and is self-explanatory:

  • ...
  • CliArgs define \
  • --no-value-args='a,l:list,u:list-updates,version,b' \
  • --value-args='x,i:input-fn,o:output-fn,log-fn,y' \
  • --required-args='input-fn' \
  • --required-values='input-fn'\
  • --ignore-errors
  • ...

Defining rules doesn't parse anything, it merely prepares the parser for the actual arguments you're about to pass to it. The : separator is for synonyms. That is, we can say that -i is the synonym to --input-fn. CliArgs parser follows the UNIX conventions: the short one-dash argument values are passed as de-facto separate arguments that follow the short one-dash argument (that is, they use space as a separator), while values for the long --two-dash arguments are provided after the = character, with or without single or double quotation marks.

Given the definitions above, your script may now be called like this:

  • ...
  • MyScript -i input.txt --output-fn=output.txt
  • ...

This, of course, is just one possibility. As you can tell from the definitions, some other arguments may have been provided, but, in this case — they weren't, since all, but one of them, are not actually required by the definition above.

If user provided an unknown argument, or forgot to provide --input-fn, or forgot to pass value to it, the script WILL NOT — at this point — print any errors. Firstly, because we used the flag which turns off error reporting and halting script execution — the --ignore-errors flag. Even if omitted it, no errors would appear at this point. So far, we've only defined arguments. It's only when we parse them, an error indicating a particular mistake will be printed and then the program would exit.


This step is easy. Just call the parse() function providing it with all the arguments you had received from the user:

  • CliArgs parse ${@}

At this point, if you have not used the --ignore-errors flag when defining arguments, an error may be printed into the user's terminal and your script will then exit with status 1 — in case the parser finds the that the provided arguments did not follow the expected definitions.


To get access to non-positional arguments, you may use the get_value function and provide it with an argument name. The argument name may be passed in various forms — given the definition we used above, the following lines of code would each produce the same output:

  • CliArgs get_value i
  • CliArgs get_value -i
  • CliArgs get_value --input-fn
  • CliArgs get_value input-fn
  • CliArgs get_value input_fn

This, of course, will print the values into the terminal, so you might want to save the output into a variable or capture its value in a subprocess and use it with the if statement:

  • if [[ -d "$(CliArgs get_value input_fn)" ]]; then
  • >&2 echo "Cannot read the input file - it's a directory."
  • exit 1
  • fi

For arguments that do not require an explicit value, the default value "yes" will be returned.

To access POSITIONAL arguments, use the CliArgs pos_arg function:

  • local description="$(CliArgs pos_arg 0)"

Indexes start with 0, not 1.


It may very often be the case that you would want to use CliArgs multiple times for various functions or inside the sourced scripts. If you do it the usual way, you may bump into issues if you have calls to functions such as CliArgs get_value or CliArgs pos_arg scattered across your program.

In order to clearly separate multiple definitions, calls to the parser and accessing values, you should use the prefix environment variable $CLIARGS_NS ("NS" stands for "namespace"), for instance:

  • ...
  • CLIARGS_NS=main CliArgs define ...
  • CLIARGS_NS=main CliArgs parse ...
  • ...
  • CLIARGS_NS=function1 CliArgs define ...
  • CLIARGS_NS=function1 CliArgs parse ...
  • ...
  • CLIARGS_NS=main CliArgs get_value 'input_fn'
  • CLIARGS_NS=function1 pos_arg 0
  • ...


It's important to differentiate between the things that are simply not yet implemented, but should be, and things that shall not be a part of this library.

First, let's list the things that are simply not implemented yet, but are desirable to have in a cli-parser library:

  1. Combining short arguments: when -xyz means the same as -x -y -z.
  2. Strict separation between arguments that do not require value at all and arguments that require a value to be provided explicitly.
  3. Currently, users may still provide random values to arguments listed as --no-value-args when calling CliArgs define. This isn't too critical, though, because for arguments that don't require an explicit value, you normally only want to check whether they're present or not, so what was provided as the actual value (the default "yes" or something else) shouldn't matter.

Things that will probably never be implemented, as they shall be the responsibility of script authors:

  1. Various validations for argument values, such as length, type (numeric or string or whatnot), or regular expression matching.
  2. Arguments order. In fact positional arguments can easily be mixed with non-positional ones, provided that -a [SPACE] value one-dash argument convention is accounted for.


For more usage examples, see the unit tests in ./




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