How can I test if a particular alias is defined?

Question

I have a bash script that needs to behave differently if a particular alias is defined. Is there a way to test if a particular command is an alias in bash?

Answer 1

If alias is passed an alias name without =value, it just prints the alias definition if that alias is defined, or fails with an error if there's no such alias.

So you can just do:

if alias your_alias_name >/dev/null 2>&1; then 
  do_something
else 
  do_another_thing; 
fi

(replace your_alias_name as required)

Answer 2

I’m gonna make @jimmij’s comment from 5 years ago into an answer:

type -t is designed for exactly that purpose. It will output alias if the given command is an alias. If the command does not exist, it won’t output anything, which means you don’t need to redirect its stderr or something.

Example:

if [ "$(type -t foo)" = 'alias' ]; then
    echo 'foo is an alias'
else
    echo 'foo is not an alias'
fi

Note that the -t switch is not defined in POSIX. Shells other than bash might not implement it.

Answer 3

Word of warning:

Shell functions and aliases are limited to the shell and do not work in executed shell scripts.

Found in https://askubuntu.com/a/98796 , confirmed by my own bitter experience.

Somewhat annoyingly this is not mentioned explicitly in the relevant section of the Bash user manual, the only hint is that

Alias expansion (see Aliases) is performed by default.

which was not intuitive enough for me; YMMV.

In short, all the technologies mentioned in the other answers work only if you check aliases defined in the same script (or in another file you source-d into your script). You cannot check from within a (non-interactive) script whether an alias was defined in the shell where you invoked said script.

Here is a little script aliastest.sh that demonstrates the issue:

#!/usr/bin/env bash

shopt -s expand_aliases

is_alias() {
    if [[ "$(type -t $1)" == "alias" ]]; then
        echo "$1 is an alias"
    else
        echo "Cannot find $1"
    fi
}

# == MAIN ==

if [[ $# -lt 1 ]]; then
    echo "Usage: $0 <command> [ <command> ...]"
    exit 1
fi

for cmd in $@; do
    is_alias $cmd
done

Let's assume you defined the popular ll alias in your shell with the command alias ll='ls -alF' (you may have it already). If you run ./aliastest.sh ll it will tell you Cannot find ll.

But despair not, there is a workaround:

If you tell Bash to run your script as "interactive", then it'll "see" the aliases defined in the shell you invoked it from:

bash -i aliastest.sh ll

It will answer ll is an alias.

Answer 4

You can write like this

[[ $(type your_alias) == *"alias" ]] && echo "alias defined" || echo "alias undefined"

Example:

[[ $(type fd) == *"alias" ]] && echo "fd is an alias" || echo "fd is not an alias"

Another option can be

[[ -z $(alias | grep 'll=') ]] && echo "alias not found" || echo "alias found"