Bash Script – Why Can’t I Use Alias in Script Even If Defined Above?

aliasbashshell-script

I have a very wierd case… If I run a script with /bin/bash, it can't recognize aliases that I set even inside the script. And the most strange thing is

$ cat -n test.sh
    1 #!/bin/bash
    2 alias somecommand='ls -alF'
    3 alias
    4 somecommand
$ ./test.sh
alias somecommand='ls -alF'
./test.sh: line 4: somecommand: command not found

… as shown above, if I run the command "alias" in the script it turns out that bash has taken somecommand into the aliases, but if I run the somecommand itself it will still not be recognized!

Everything is right if I use the command "sh" to run the script.. so is it a bug of bash? Or is there something I'm missing?

Any help is appreciated!

Best Answer

Simply don't use aliases in scripts. It makes little sense to use a feature designed for interactive use in scripts. Instead, use functions:

somecommand () {
    ls -alF
}

Functions are much more flexible than aliases. The following would overload the usual ls with a version that always does ls -F (arguments are passed in $@, including any flags that you use), pretty much as the alias alias ls="ls -F" would do:

ls () {
    command ls -F "$@"
}

The command here prevents the shell from going into an infinite recursion, which it would otherwise do since the function is also called ls.

An alias would never be able to do something like this:

select_edit () (
    dir=${1:-.}
    if [ ! -d "$dir" ]; then
        echo 'Not a directory' >&2
        return 1
    fi
    shopt -s dotglob nullglob
    set --
    for name in "$dir"/*; do
        [ -f "$name" ] && set -- "$@" "$name"
    done
    select file in "$@"; do
        "${EDITOR:-vi}" "$file"
        break
    done
)

This creates the function select_edit which takes directory as an argument and asks the user to pick a file in that directory. The picked file will be opened in an editor for editing.

The bash manual contains the statement

For almost every purpose, aliases are superseded by shell functions.

Related Solutions

When to Use Alias, Script, or Function in Bash

An alias should effectively not (in general) do more than change the default options of a command. It is nothing more than simple text replacement on the command name. It can't do anything with arguments but pass them to the command it actually runs. So if you simply need to add an argument at the front of a single command, an alias will work. Common examples are

# Make ls output in color by default.
alias ls="ls --color=auto"
# make mv ask before overwriting a file by default
alias mv="mv -i"

A function should be used when you need to do something more complex than an alias but that wouldn't be of use on its own. For example, take this answer on a question I asked about changing grep's default behavior depending on whether it's in a pipeline:

grep() { 
    if [[ -t 1 ]]; then 
        command grep -n "$@"
    else 
        command grep "$@"
    fi
}

It's a perfect example of a function because it is too complex for an alias (requiring different defaults based on a condition), but it's not something you'll need in a non-interactive script.

If you get too many functions or functions too big, put them into separate files in a hidden directory, and source them in your ~/.bashrc:

if [ -d ~/.bash_functions ]; then
    for file in ~/.bash_functions/*; do
        . "$file"
    done
fi

A script should stand on its own. It should have value as something that can be re-used, or used for more than one purpose.

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

When to Use Alias, Script, or Function in Bash

Bash: Use an alias in a variable

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