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Re: unalias works weirdly inside an if-then block


From: Eduardo Bustamante
Subject: Re: unalias works weirdly inside an if-then block
Date: Fri, 12 May 2017 08:11:35 -0500

On Fri, May 12, 2017 at 7:33 AM, Gabor Burjan <address@hidden> wrote:
[...]
> Description:
>         unalias works weirdly inside an if-then block

It's not just an if-then block. It's any kind of block:

     address@hidden:~$ bash alias
     + shopt -s expand_aliases
     + alias 'x=echo x'
     + echo x
     x
     + unalias x
     + echo x
     x
     + x
     alias: line 9: x: command not found

     address@hidden:~$ cat alias
     set -x
     shopt -s expand_aliases
     alias x='echo x'
     x
     {
      unalias x
      x
     }
     x

This is explained in the ALIASES section of the bash manual:

| The rules concerning the definition and use of aliases are somewhat
confusing.  Bash always reads  at  least  one  complete
| line  of input before executing any of the commands on that line.
Aliases are expanded when a command is read, not when it
| is executed.  Therefore, an alias definition appearing on the same
line as another command does not take effect  until  the
| next  line  of  input is read.  The commands following the alias
definition on that line are not affected by the new alias.
| This behavior is also an issue when functions are executed.  Aliases
are expanded when a function definition is  read,  not
| when  the  function is executed, because a function definition is
itself a command.  As a consequence, aliases defined in a
| function are not available until after that function is executed.
To be safe, always put alias definitions on  a  separate
| line, and do not use alias in compound commands.

(the key part here is that aliases are expanded when a command is
read, not when it's executed. Bash needs to parse the whole block, at
which point the alias is expanded. The unalias is not executed but
until after).



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