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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
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April 8, 2024
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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
- First it returns nothing
- Second it returns the last command used
- Last it returns the last parameter used
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)
- First command uses
llto list a python script filename. The filename is saved to_for future use. - Second command
$_runs the python script. - Third command
cat $_lists the contents of the python script.
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.