Re: Alias and if

[Chris F.A. Johnson]

On Sun, 4 Dec 2005, XIAO Gang wrote:

> Version: GNU bash, version 3.00.16(1)-release (i386-pc-linux-gnu)
>
> Hardware: AMD Athlon 3GHz+, 1G RAM.
>
> OS: Linux (Knoppix-3.9).
>
> Compilation: Distributed by Debian.
>
> Description: An alias defined within an "if" branch has no effect.
>
> Recipe: in the following script, the alias "mycp" is unknown command. The 
> problem disappears when
> the line "alias myls=ls" is moved before the "if" line.
>
> Note that the definition is however recorgnized by "alias -p".

    In a script, you need to turn on alias expansion.

    However, note this section from the bash man page:

      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 com-
      pound 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  com-
      pound commands.

       For almost every purpose, aliases are superseded by shell functions.


> The same phenomenon is reported on slightly different recent builds of Linux.
>
> ####################################
>
> #! /bin/bash

   Add:

shopt -s expand_aliases


> if true; then
> alias myls=ls
> alias -p
> myls
> fi
>
> myls
>
> ####################################

--
   Chris F.A. Johnson                      <http://cfaj.freeshell.org>
   ===================================================================
   Author:
   Shell Scripting Recipes: A Problem-Solution Approach (2005, Apress)