The structure of Linux commands

The three parts of a command

Linux commands are made up three parts:

• the command itself
• one or more options 
• one or more arguments

By convention, options typically enable or disable certain features, while arguments typically represent a target or object being acted upon.

However, thousands of commands exist, built-in or installable, and they don't always follow the same rules and conventions.

Usually the --help option will tell you how to use the command, and which options and arguments are supported and/or mandatory.

Without options or arguments

Some commands can be used stand-alone - without options or arguments.

Example

On its own, the ls command lists the contents of the current directory:

sarah@laptop: ~$ ls

Passing an argument

• Arguments can be optional or mandatory. 
• Argument containing space or special characters must be wrapped in single or double quotes.
• Some commands accept only a single argument; some accept multiple arguments.

Example

If you pass the name of a directory as an argument to ls, it will list the contents of that directory:

sarah@laptop: ~$ ls /home/sarah

Passing multiple arguments

Some commands accept multiple arguments.

Example

If you pass multiple directories, ls will list the contents of all those directories:

sarah@laptop: ~$ ls /home/sarah /home/hiroshi

Passing an option

Options, also called "optional arguments", are by definition optional. They exist in short and long formats.

Example

If you also want to list hidden files, you can use an option to do so. ls supports a short-option and long-option format to tell it to show hidden files:

sarah@laptop: ~$ ls -a

sarah@laptop: ~$ ls --all

Both options are equivalent.

• Short-options are often used in scripts because they take up less space.
• Long-options are often used when explaining commands, because they are easier to read.

Note that not every option is always available in both a short and a long format.

Passing an option that takes an argument

Some options themselves take (optional) arguments.

Example

The following option tells ls to ignore files with the .pdf extension.:

sarah@laptop: ~$ ls -I "*.pdf"

sarah@laptop: ~$ ls --ignore="*.pdf"

Conventional notation:

• a short-option and its argument are separated by a space
• a long-option and its argument are separated by an equals sign (=)

We advise that you stick to the conventional notation to avoid confusion and complications when using multiple options in different notations.

For completeness:

Here are a few alternative, less-used notations that are also allowed:

sarah@laptop: ~$ ls -I"*.pdf" (no space between short-option and its argument)

sarah@laptop: ~$ ls -I="*.pdf" (no space between short-option and its argument)

sarah@laptop: ~$ ls --ignore "*.pdf" (space instead of equals sign between long-option and its argument)

Again we strongly suggest you don't use these alternatives formats, but stick to the conventional ones mentioned above.
 

Passing multiple options

You can pass multiple short-options and/or long-options to a command, with or without arguments, in the order specified by the command's usage instructions.

You can, but don't have to, combine short-options such as -a -b -c into one short-option: -abc.
You cannot do this with long-options.

If you run ls --help, the first thing it tells you is:

Usage: ls [OPTION] ... [FILE] ...

In other words:

• You can pass one or more options, and they are optional.
• You can pass one or more arguments, and they are optional.

The fact that options and, in this case arguments, are optional is indicated by the square brackets.

Examples

sarah@laptop: ~$ ls --all --ignore="*.pdf"

sarah@laptop: ~$ ls -a --ignore="*.pdf"

sarah@laptop: ~$ ls -a -I "*.pdf"

sarah@laptop: ~$ ls -al "*.pdf"

sarah@laptop: ~$ ls -la "*.pdf"

The above are all equivalent.
 

Passing multiple options and arguments

Bringing it all together, here are a few examples, all of them equivalent:

sarah@laptop: ~$ ls --all --ignore="*.pdf" /home/sarah /home/hiroshi

sarah@laptop: ~$ ls -all -I "*.pdf" /home/sarah /home/hiroshi

As you can see, you are allowed to mix different notations together, though it doesn't always make it easy to read or understand such instructions. Generally, it's a good idea to stick with either short-options or long-options.

Command-line flags and switches

We've discussed command-line options, but the terms command-line flag, and command-line switch also exist.

• Some people use the term option flag.
• Some people use flag or short flag instead of short-option, and long flag or switch instead of long-option.
• Some people use flag when the value can only be true or false; some use the term switch for exactly that purpose.
• Some people have even other reasons for using one or the other.

We have chosen not to use the terms flag or switch, but instead to stick with the conventions used in the GNU Coding Standards for Command-line Interfaces.