Demystifying The Double Dash: Understanding - In Linux Commands

The Meaning of the Double Dash

The double dash (–) is a special syntax in Linux command line utilities used to signify different behaviors depending on the context. At a basic level, the double dash separates options from arguments in a command. However, its meaning goes deeper than that.

The double dash is commonly used for the following purposes in Linux commands:

To signal the end of command options
To allow arguments that resemble options to be passed clearly
To access help content and usage information

In this article, we will demystify the usage of the double dash by explaining what it means in different scenarios. We’ll discuss when and why the double dash is used through plenty of examples. Understanding the double dash allows you to write Linux commands properly and avoids unexpected errors.

Table of Contents

Explaining What the Double Dash Signifies in Linux Command Syntax

The double dash (–) signifies the explicit end of command line options in a utility. All arguments after the double dash will not be interpreted as options, even if they resemble one. This enables arguments that look similar to options, such as filenames starting with a dash (-), to be specified without any confusion or unintended behavior.

Providing Examples of Common Uses of the Double Dash

Here are some common examples of using the double dash when executing Linux utilities from the command line:

```
tar -cvf archive.tar -- ./data
```
– Pack the ./data directory into archive.tar using Tar, where the double dash separates the options like -c, -v, -f from the argument.
```
git checkout -- language.py
```
– Discard changes to the file language.py in Git, where the double dash distinguishes the checkout command from the filename argument.
```
docker run -d -p 80:5000 --name my-app myimage
```
– Launch a Docker container specifying options first, then the double dash to signify that my-app is the container name rather than an option.

Distinguishing Between Options and Arguments

Defining Command Options vs. Arguments

Before going further, it’s important to define what command line options and arguments are in Linux utilities:

Options – Flags or parameters that modify the behavior of the utility. Usually start with a single dash (-) or double dash (–). For example, -a and --all are common options.
Arguments – The inputs supplied to the command, on which the utility acts on. For example, filenames, directories, resources, etc. Arguments follow the options.

The double dash demarcates where options end and arguments begin in the overall command syntax. This enables the program to distinguish between them correctly.

Using Double Dashes to Separate Options from Arguments

The double dash should be used between options and arguments in commands whenever there could be confusion between the two. For example:

mp3rename -r "/Rock Collection/The Police" -- *.mp3

Here, the mp3rename command renames MP3 files using the -r option to specify a new root folder. The double dash separates -r from the asterisk wildcard *.mp3 argument, avoiding any unintended behavior.

Showing Command Examples to Illustrate the Difference

Look at a few more examples showing how options vs. arguments appear in common Linux utilities:

```
du -sh -- ./folders
```
– Compute disk usage summary for the ./folders directory.
```
grep -iRl "error" -- /var/log
```
– Recursively search for "error" ignoring case in /var/log logs.
```
rsync -avP /home/user/docs -- backup@remote:/backups/
```
– Synchronize /home/user/docs to remote host into /backups/ path.

The double dash helps distinguish between options and arguments clearly in all these commands.

Ending Option Parsing

Discussing How Double Dash Signals the End of Options

One of the most common uses of the double dash is to explicitly mark the end of options in a utility. This signals to the program that any further parameters are to be treated as arguments, not options.

The standard Linux convention is that options start with a dash (-), whether a single dash for short flags or double dash for long flags. When the double dash (–) appears, the command stops parsing parameters as options. Everything after is assumed to be an argument instead.

Preventing Arguments from Being Interpreted as Options

Double dashes are very useful to prevent arguments that resemble options from being unintentionally interpreted as options.

For example, filenames or directories often start with one or more dashes. If such an argument appears after options without any separator, the command may mistake it to be an option. Using — avoids this issue.

Showing Examples Like tar, git, docker etc.

Here are some practical examples of using double dash when arguments start with dashes from popular Linux utilities:

```
tar -cvf backup.tar -- -important-folder
```
– Avoid trying to pass “-important-folder” as an option to tar.
```
git add . && git commit -m "--amend" 
```
– Allow a commit message with double dash instead of treating “-amend” as a flag.
```
docker run -d -p 3000:80 --app-name
```
– Prevent “-app-name” from being interpreted as options to docker.

Passing Arguments Looking Like Options

Enabling Arguments Resembling Options to be Passed

A major use case for double dashes is to allow arguments or parameters that start with a dash to be passed clearly to commands. Linux utilities follow the convention that options start with a dash (-). Anything starting with a dash or double dash after options would get confused as an option.

The double dash disambiguates such arguments. By signaling the explicit end of options, it enables arguments resembling options to be specified without any unintended effects.

Avoiding Confusion and Unexpected Behavior

Using the double dash avoids confusion when arguments start with a dash. For example:

ffmpeg -i video.mov -metadata title "-Important Meeting" -- output.mp4

Here, the double dash ensures “-Important Meeting” is interpreted as the video title metadata rather than an ffmpeg option.

Without double dashes, commands may exhibit unexpected behavior, fail with errors or have security issues when ambiguities arise between options and arguments.

Code Examples for Passing Arguments Starting with – or —

Some examples of arguments that start with dashes that can be passed securely using double dashes:

Filenames beginning with – (dash) like -temp.txt
Directory paths like /my/–folder/
Code strings containing — delimiter
Option-style parameters for other programs being executed

cat -- -temp.txt
mkdir -- /my/--folder 
node index.js -- "--input 5"  
python script.py -- "--debug --log-level INFO"

Getting Help on Double Dash

Using –help Option to Understand Command Syntax

Most Linux utilities come with a help content to explain the supported syntax, options, and arguments. The standard way to access this is the --help option itself!

Going through the –help content will clarify if a command accepts a double dash, where it needs to be used, and usage examples that use it. If the double dash is supported, related help content section is usually titled “positional arguments”.

Checking Man Pages for Usage Information

Besides –help built into commands themselves, Linux man pages serve as detailed reference manuals for programs. The man page will contain an explanations of the double dash, such as:

What it signifies in that program
The syntax rules around using it
Examples that apply the double dash

Man pages provide more comprehensive documentation than –help content on utility syntax and semantics.

Understanding When Double Dash is Supported or Required

Based on the –help and man pages, you can understand:

If the double dash is supported in a particular Linux command
When its usage is required vs optional
Where it needs to be placed in the overall syntax

This allows proper usage of the double dash and avoidance of syntax errors. For example, git requires a double dash before file arguments in cases like git checkout, while tar allows it optionally as a separator.

Demystifying The Double Dash: Understanding — In Linux Commands