Home / Questions / 51213
Accepted
Anonymous Coward
Asked: 2020-02-24 01:19:58 +0800 CST 2020-02-24 01:19:58 +0800 CST

How to make an alias in bash that is evaluated on run?

  • 772

I frequently run this command:

eval $(resize)

In order not to have to write it every time I have added this line to $HOME/.alias

alias r="eval $(resize)"

But when I run rit sets me to 80 columns, not what it corresponds to. If the alias is ready, this appears:

~> alias | grep "alias r="
alias r='eval COLUMNS=80;

That is, $(resize)it is evaluated when the .alias file is loaded but I want it to be evaluated when I execute r.

bash

1 Answers

  1. Best Answer

    Commands that are written inside double quotes are executed automatically. Therefore, when you say alias r="eval $(resize)"this command, it is interpreted and rits expansion becomes valid.

    To make it run every time, just use single quotes:

    alias r='eval $(resize)'

    Let's see an example with a command whose output changes as date. We define the alias and call it three times, once every second:

    I generate aliases with single quotes:

    $ alias d='eval echo $(date)'

    We see that the result changes each time:

    $ for ((i=1;i<3;i++));
> do
>   d
>   sleep 1
> done
Thu Feb 23 14:38:24 CET 2017    # el segundo ahora es 24
Thu Feb 23 14:38:25 CET 2017    # después 25
Thu Feb 23 14:38:26 CET 2017    # ... 26

    With double quotes:

    $ alias d="eval echo $(date)"

    We see that the result is always the same:

    $ for ((i=1;i<3;i++));
> do
>   d
>   sleep 1
> done
Thu Feb 23 14:36:47 CET 2017  # siempre muestra el segundo 47
Thu Feb 23 14:36:47 CET 2017  # ...
Thu Feb 23 14:36:47 CET 2017  # ...

    Let's look at another simpler case with quotes, where we define a text that contains a variable.

    $ v=23

    With single quotes, $vit is not interpreted:

    $ d='hola $v'
$ echo "$d"
hola $v

    With the double quotes, yes:

    $ d="hola $v"
$ echo "$d"
hola 23
    • 6