Difference between $_ and $- variables in Linux

What is $_ variable in Linux

The $_ in bash is a special variable that holds the last argument of the previous command. This is particularly useful when you want to repeat the last command or argument without having to retype it.

For example, if you were to use the ls command to list files in a directory, and you wanted to use the cd command to navigate into that directory, instead of typing the directory name again, you could use $_:

ls /path/to/directory
cd $_

In this example, $_ would be replaced with /path/to/directory, effectively changing the working directory to the last listed directory.

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It’s worth noting that $_ is a shell built-in variable and it gets updated after every command execution. So, it's always holding the last argument of the most recent command.

What is $- Variable in Linux

The $- in bash is a special variable that holds the current set of options in your current shell. Each character in the string represents a different option. For example, himBH means following options are enabled:

• h - histexpand: when history expansion is enabled
• i - interactive: when current shell is interactive
• m - monitor: when job control is enabled
• B - braceexpand: when brace expansion is enabled
• H - hashall: Locate and remember (hash) commands as they are looked up for execution

You can print the current set of options by simply typing echo $- in your terminal.

Example — A Linux shell script that uses $- variable

In this video, a shell script is written, which shows that each shell script is executed by a non-interactive shell with the help of $- variable and case condition.

Read more about video above: link

Difference between $_ and $-

The $_ and $- are both special variables in bash, but they serve different purposes:

• $_ (underscore): This variable holds the last argument of the previous command. It's particularly useful when you want to reuse the last argument of a command without having to retype it.
• $- (dash): This variable holds the current set of options in your current shell. Each character in the string represents a different option. For example, himBH means following options are enabled: histexpand, interactive, monitor, braceexpand, hashall.

So, the main difference between $_ and $- is that $_ refers to the last argument of the previous command, while $- refers to the current set of shell options.

Read More

What are interactive and non-interactive shells in Linux

How to check if a shell is interactive or not?

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