Bash – Is there some way to make alias expansion work inside if…fi on Bash

aliasbash

Check this script:

#!/bin/bash
if true;then
    alias WeirdTest='uptime';shopt -s expand_aliases
    WeirdTest
fi
WeirdTest

The first time WeirdTest is executed it says "command not found".

Best Answer

This is a limitation of bash. Quoting the manual:

The rules concerning the definition and use of aliases are somewhat confusing.

Bash expands aliases when it reads a command. A command, in this sense, consists of complete commands (the whole if … fi block is one compound command) and complete lines (so if you wrote … fi; WeirdTest rather than put a newline after fi, the second occurrence of WierdTest wouldn't be expanded either). In your script, when the if command is being read, the WeirdTest alias doesn't exist yet.

A possible workaround is to define a function:

if …; then
  WeirdTest () { uptime; }
  WeirdTest
fi
WeirdTest

If you wanted to use an alias so that it could call an external command by the same name, you can do that with a function by adding command before it.

WeirdTest () { command WeirdTest --extra-option "$@"; }

Related Solutions

Bash – watch command alias expansion

Aliases are only expanded as the first argument, or after another alias with a trailing space on the end of the command.

From bash's help alias:

A trailing space in VALUE causes the next word to be checked for alias substitution when the alias is expanded.

To do this, try the following:

alias watch='watch '
alias ll='ls -l --color=tty'
watch ll

Bear in mind that some versions of watch strip colours by default, on some versions this can be stopped by using --color or -G.

Bash: Use an alias in a variable

After some testing, I have concluded the following:

Aliases only work in interactive mode (add -i to the shebang).
Aliases are not evaluated when they come from an interpreted source (in this case, the variable.
You can get bash to use the alias with eval $1. Note that evaling anything created with a variable is dangerous, but since the whole point of the script requires arbitrary execution, I won't make too big a deal out of that.

From the bash man page:

Aliases are not expanded when the shell is not interactive, unless the expand_aliases shell option is set using shopt (see the description of shopt under SHELL BUILTIN COMMANDS below).

So you could add shopt -s expand_aliases instead of -i.

Also,

Aliases are expanded when a command is read, not when it is executed.

Since variables are not expanded before the command is read, they will not be expanded further using the alias.

Best Answer

Related Solutions

Bash – watch command alias expansion

Bash: Use an alias in a variable

Related Question