Ubuntu – Difference between | and ||

bashcommand line

What is the difference between | and || ?

When I run this command :

ls -la | id

I get the result of id

When I run this command
:

ls -la || id

I get the result of ls -la

So what is the difference between them ?

Best Answer

| is the pipe operator, which passes the output of the first command to the one that follows.

from man bash:

A pipeline is a sequence of one or more commands separated by the character |. The format for a pipeline is:
[time [-p]] [ ! ] command [ | command2 ... ]
The standard output of command is connected via a pipe to the standard input of command2.

In the example you provide, id doesn't seem to do anything with the output of ls so it just returns the same output as running id alone.

|| is the logical OR operator, and specifies what to do if the first command returns false or fails (is non-zero).

from man bash:

The control operators && and || denote AND lists and OR lists, respectively. An AND list has the form
command1 && command2
command2 is executed if, and only if, command1 returns an exit status of zero.

An OR list has the form
command1 || command2
command2 is executed if and only if command1 returns a non-zero exit status. The return status of AND and OR lists is the exit status of the last command executed in the list.

In your example, ls -la runs successfully so the id command isn't run. If you did the following:

ls -z || id

and try to pass an invalid option z to ls, then it fails and the id command gets run.

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Ubuntu – What’s the difference between “purge” and “remove –purge”

Whether they should or shouldn't both exist is perhaps a debating point.

However - they are absolutely identical.

From the man page of apt-get

--purge
           Use purge instead of remove for anything that would be removed. An
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They key part is --purge is equivalent to the purge command

As to why - I would surmise this is historical -

apt-get --purge remove came before apt-get purge

Looking at the old apt documentation it make reference to the older version of the command. The newer documentation gives the aptitude & apt-get purge example.

For the sake of consistency - its a good idea to not remove old interfaces - if you have an old script - it will still work today because the interface commands still exist.

Mind you that doesnt stop Gnome from deprecating api's - but that's another story...

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The script probably knows which shell it should be running in. The first line might be something like

#!/bin/bash

If you run it with sh, the first line (a.k.a. shebang) is ignored. A different shell tries to run the script, but does not understand it - it is like running Java code in Pascal. If you run it with the right shell, it should behave identically:

/bin/bash script.sh

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