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Tags: command-line   bash  
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URL: https://askubuntu.com/q/1198935
Title: What is the purpose of the special parameter "_" (single underscore) in environment?
ID: /2019/12/27/What-is-the-purpose-of-the-special-parameter-___-_single-underscore_-in-environment_
Created: December 27, 2019    Edited:  June 12, 2020
Upload: November 24, 2022    Layout:  post
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I’m trying to understand how the environment variable _ can be used. Below is an example of using it:

$ echo $_

$ echo $_
echo

$ ls non-existant-filename
ls: cannot access 'non-existant-filename': No such file or directory

$ echo $_
non-existant-filename

This might be a handy variable for bash scripts but only if it’s function is fully understood.


Some useful applications of _

I found some useful applications of _.

_ contains the last filename you can recycle

In this example _ is used to keep the last filename which you can reuse in subsequent commands without retying it.

$ ll ~/python/scroll1.py
-rwxrwxrwx 1 rick rick 2384 Dec 27 09:15 /home/rick/python/scroll1.py*

$ $_
# The python program ~/python/scroll1.py is executed

$ cat $_
#!/usr/bin/env python
# -*- coding: utf-8 -*-
   (... SNIP ... remaining contents of ~/python/scroll1.py appears on screen)

So the $_ variable/parameter can save some typing.

_ contains the last program run

Here’s an example of differences between env and printenv updating the _ variable/parameter:

$ env > env.txt

$ printenv > printenv.txt

$ diff env.txt printenv.txt
66c66
< _=/usr/bin/env
---
> _=/usr/bin/printenv

Because a parameter wasn’t passed to either command, the _ isn’t updated with the last used parameter as in the previous example but, it is updated with the last command used.

Also noticed how _ is updated before the commands env and printenv are executed because _ it appears in the output.

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