Command Substitution

Command substitution is a feature in Linux shells that allows you to use the output of a command as an input to another command or to assign it to a variable.

Syntax

The syntax for command substitution is to enclose the command to be executed within $() or backticks . For example:

echo $(date) 

The output of the date command will be substituted in place of $(date), resulting in the current date and time being displayed.

Examples

Here are some examples of how to use command substitution:

Assign output to a variable

dir_list=$(ls) 

This command will assign the output of the ls command to the dir_list variable.

Use output as input to another command

echo "The current date is: $(date +%Y-%m-%d)" 

This command will display the current date in the specified format.

Command substitution with pipes

tail -n $(wc -l file.txt) file.txt 

This command will display the last N lines of the file.txt, where N is the number of lines in the file.

Conclusion

Command substitution is a powerful feature that allows you to use the output of one command as input to another command or to assign it to a variable. It can be a useful tool for streamlining your workflow and automating tasks.