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For almost every purpose, aliases are superseded by shell functions.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