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Unix

CLI Syntax

xxd - usage - cool-retro-term

This blog article is essentially a repost of a small summary about CLI syntax on Unix and Windows systems. Originally intended it as a chapter for the cli-book I wanted to write. Unfortunately, I never found enough time to keep working on it. However, I still aim to gather more information and knowledge regarding the Terminal, Shells, and various command line tools available. Therefore, I decided to try sharing this information one blog post at a time. Since CLI syntax is crucial for understanding and developing CLI tools, I thought it would be beneficial to share and update the content I had initially written for the book.

Before you start reading this article you may consider reading the POSIX Utility Conventions. It is approximately a 10-20 minute read which will provide a solid base when it comes to CLIs and their syntax.

Let's get some content going

Terminology

If you read about CLIs, you will usually encounter specific terminology: 

utility, command, subcommand, argument, option, flag, switch, operand
Still, the meanings may slightly differ depending on the source or context in which a term is mentioned. To ensure we are all on the same page regarding these terms, the table below describes the terms and their meanings in the context of this article and on this blog in general.

Term Meaning on this Blog Names in other Standards/Definitions
command The CLI tool itself utility (Posix)
subcommand A subcommand of a command or of a command group
argument (required/positional) Required or Positional Argument operand (Posix)
option A setting with an additional argument/value (key-value pair) option (Posix)
switch Like an option but with a set of mutually exclusive arguments/values option (Posix)
options Superset of flags + switches + options options (Posix)

Note

With the specification above, there is ambiguity regarding the term options. Here, options could mean either:

  • The plural of an option, a setting with an additional argument
  • options, the superset of flags, switches, and option

Therefore, when referring to the plural of option, one should say "options of type option," and when referring to the superset of all options, switches, and flags, one should simply say "options."

Below, you will find an annotated help example of a CLI, demonstrating where and how the different terms and types find their place.

Unix Syntax

Symbol Element Description
Text text Items you must type as shown
TEXT upper case text Placeholder for which you must supply a value
<name> text in angle brackets Placeholder for which you must supply a value
[] bracket Optional items
{} curly braces Set of mutually exclusive but required items, choose one
() braces Set of mutually exclusive but required items, choose one
| vertical bar Separates mutually exclusive items
… ellipsis Indicates that item (flag, option, argument, ...) can be repeated multiple times
- dash Indicates start of short option(s), if it stands alone it is often used as file argument parameter to specify stdin
-- double dash Indicates the start of a long option or if not followed by an option name it indicates the end of the CLI arguments for the command

Windows Syntax

Symbol Element Description
Text Text Items you must type as shown
<Text> Text inside angle brackets Placeholder for which you must supply a value
[Text] Text inside square brackets Optional items
{Text} Text inside braces Set of required items. You must choose one
| Vertical bar Separator for mutually exclusive items. You must choose one
… Ellipsis Items that can be repeated and used multiple times

The original source for the table above can be found here.

Examples

Usage Examples

Element Example(s)
Required/Positional Arguments ARGUMENT, <argument>, <ARGUMENT>
Option with value --option=value, --option=VALUE, --option <value>, --option=<value>, --option value, --option VALUE
Switch --light={on | off}, --power {up | down}
Flag --track, --verbose, -V
Optional Items --track, --verbose, -V, [<argument>]
Mutually-Exclusive & Required (--track | --no-track), <FILE> | <PATH>
End of passing CLI arguments user@host ~$ cli foo -b --this -- other args
Repeating Elements <argument>...

Annotated Example Help

Note: Everything following # is a comment

Short description what this command does

usage:

cmd [flags] [options] <username> <password>

arguments:
    <username>  which shall be used for the login             # 1. required/positional-argument
    <password>  associated with the specified username        # 2. required/positional-argument

flags:
    -f, --force                 execute action anyway         # 5. flag
    -h, --help                  show this help message        # 6. flag

options:
    -i <file>, --input <file>   file which shall be processed # 3. option
    -s <unit>, --speed <unit>   mph, kmh  [default: mph]      # 4. switch

References & Resources

Note

Git is an excellent resource for finding usage examples, seeing the CLI syntax in action, and understanding the nuances (it is well-documented, and there are plenty of answers available on Stack Overflow & Co.). If you ever get stuck figuring out how to structure your CLI, I recommend browsing through some Git commands.