What does “ — “ (double-dash) mean in the bash shell? And the difference between arguments and options.

Hi Guys, Today we are going to learn about the usage of the double dash “- -” in the shell. But before that let's understand the problem that I faced exactly and how that was resolved by using the “- -”

Problem:

Please take a look at the below screenshot clearly, most of the time Linux commands are failing because of “-” at the beginning of the filename.

problem with — in-shell parameter

To understand more about this problem and the solution for this, let's take a step back and learn about the commands and their options, arguments, and then parameters concepts. Please check the below screenshot for a better understanding.

Difference between arguments, options, and parameters in shell

By observing the above screenshot, everything (which is separated by white space by default) is an argument including the command i.e., $0.

Every argument is classified into 3 types ( intentionally ignoring the Linux sub-commands for making it simple.)

1. $0: This is the first argument. this is nothing but command/program like rm, cp, and some_program.sh
2. Options: Every argument which starts with — (hyphen) considers options to $0. These arguments control the functionality of the command.
   For example:
   *. grep pattern file: search for the pattern in the file
   *. grep -v pattern file: after adding -v options it completely changes the behavior, now it looks for the patterns which won’t match.
3. Parameters: Remaining all the arguments are called parameters.

Now if you check the first screenshot, we will get the actual problem. Hope you spot the issue. Because -file3 starts with -(Hyphen), command (rm, cat..) taking it as an option, not as a parameter.

Finally, we have understood the problem (at least, I am hoping 😉). So we can think of the solutions. A better way to fix this is by explicitly telling the shell to consider -file3 as a parameter, not an option using the double dash.

Man page definition for double dash : A — signals the end of options and disables further option processing. Any arguments afterthe — are treated as filenames and arguments. An argument of — is equivalent to — .

Doubel dash usge in shell

After double dash ( — — ) everything is considered as a parameter type. That is why all the commands are succeded this time.

Note: All Linux commands don’t follow this syntax but those are very few.

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