Bash – Why doesn’t alias work inside if

aliasbash

My .bashrc sets up a bunch of aliases for me to use as needed, and then runs one of them automatically.

Turns out this caused some problems with automated scripts ssh-ing into my machine, as opposed to using an interactive shell. So, to fix this, I put them inside an if block so they wouldn't be defined or run for those automated scripts…

if [ -n "$TERM" ] && [ "$TERM" != "dumb" ] ; then
    alias short='long command here'
    alias another='very long command here'
    # ...

    short
fi

Only to see short: command not found!

Let's reduce this to the minimum…

$ cat alias.sh
alias foo='echo hi'
foo

$ sh alias.sh
hi

cat alias-in-if.sh
if true ; then
        alias foo='echo hi'
        foo
fi

sh alias-in-if.sh
alias-in-if.sh: line 3: foo: command not found

Why does the first script work, and not the second?

(I've answered my own question.)

Best Answer

I searched around and didn't find anything to do with aliases inside ifs, only aliases inside functions, which seemed a different issue at first.

But actually, this answer solves it:

Bash always reads at least one complete line of input before executing any of the commands on that line. [...] The commands following the alias definition on that line are not affected by the new alias. [...] To be safe, always put alias definitions on a separate line, and do not use alias in compound commands.

Seems this isn't just limited to functions, but if statements as well. The entire if block is treated as a single line. The solution, to ensure the alias is only defined and executed if the condition is true, yet ensure it isn't even defined if the condition is false, is to evaluate the condition twice like this:

cat alias-before-if.sh
if true ; then
        alias foo='echo hi'
fi

if true; then
        foo
fi

$ sh alias-before-if.sh
hi

So my original script would be:

if [ -n "$TERM" ] && [ "$TERM" != "dumb" ] ; then
    alias short='long command here'
    alias another='very long command here'
    # ...
fi

if [ -n "$TERM" ] && [ "$TERM" != "dumb" ] ; then
    short
fi

Alternate solution

The best way I can think of doing this is the following:

alias root='sudo $(fc -ln -1)'

This will work for simple commands, but for more complex ones we need to tweak it.

alias root='sudo sh -c "$(fc -ln -1)"'

The reason for the change is because of something like this:

# alias root='sudo $(fc -ln -1)'
# echo "I AM $(whoami)"
I AM patrick
# root
"I AM $(whoami)"

As you can see no shell parsing is done. By changing it to use sh -c "..." instead, it does shell interpretation.

# alias root='sudo sh -c "$(fc -ln -1)"'
# echo "I AM $(whoami)"
I AM patrick
# root
I AM root

Another way (I thought of this one first, so keeping it in the answer, but it's not as nice as the one above):

alias root='fc -e "sed -i -e \"s/^/sudo /\""'

The fc -e command will run the specified command passing it a file containing the previously executed command. We just run sed on that file to prefix the command with sudo.

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

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 "$@"; }

Best Answer

Related Solutions

Bash – Why does !! inside an alias not work

Alternate solution

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

Related Question