watch command alias expansion

Question

If a run the watch command containing an alias, it will not expand the alias. I have tried both with single quote and double quotes, in fact given the following alias:

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

The following command will fail

# watch ll
sh: ll: command not found

Shouldn't command line expansion work in this case?

Answer 1

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.

Answer 2

Maybe we could manually expand the alias before watch sees it?

watch $(alias ll | cut -d\' -f2)

Explanation

The output of alias ll looks like:

$ alias ll
alias ll='ls -lAGh'

So we set cut's delimeter to be single quote, and cut the 2nd field, leaving:

ls -lAGh

ie the expanded alias. That then forms the args given to watch.

Make a function to do it

function watcha {
    watch $(alias "$@" | cut -d\' -f2)
}

Then,

watcha ll

works as desired. This is awful and will fail in all sorts of situations. Sorry.

Answer 3

Another trick that I use (specifically for aliasing kubectl to k) is to create a symlink instead of an alias. That way the symlink still is found by watch and no trickery is required.

The downside of this is that you have to do this for every alias you want to work, and I'm not sure how it'd work for shell builtins.

Answer 4

There's a custom program to handle this over at github.com/antonmedv/watch. It supports multiple shells. Of course because it's custom software to install it's OK for your normal computers but not helpful on random servers you might be connected to. Here's how I run it so I can watch my zsh aliases and functions:

function watch-zsh() {
  WATCH_COMMAND='zsh -ci' /usr/local/bin/watch "$@"
}

Answer 5

Here's one that uses the --color option and allows you to use the -n argument to specify a refresh interval.

swatch_usage() {
    cat <<EOF >&2
NAME
       swatch - execute a program periodically with "watch". Supports aliases.

SYNOPSIS
       swatch [options] command

OPTIONS
       -n, --interval seconds (default: 1)
              Specify update interval.  The command will not allow quicker than
              0.1 second interval.
EOF
}

swatch() {
    if [ $# -eq 0 ]; then
        swatch_usage
        return 1
    fi
    seconds=1

    case "$1" in
    -n)
        seconds="$2"
        args=${*:3}
        ;;
    -h)
        swatch_usage
        return 1
        ;;
    *)
        seconds=1
        args=${*:1}
        ;;

    esac

    watch --color -n "$seconds" --exec bash -ic "$args || true"
}

I only needed color and timing support, but I'm sure you could add more if you wanted.

The meat of the function is that it executes your command with bash directly in interactive mode and can thus use any aliases or commands that are normally available to you in bash.

I'm not that experienced with scripting so fair warning, your mileage may vary. Sometimes I have to press Ctrl+C a few times to get it to stop, but for what it's worth, I've been using it frequently for 6 months without issue.

Gist form: https://gist.github.com/ablacklama/550420c597f9599cf804d57dd6aad131

Answer 6

I thought here basic problem is while executing watch, the argument is given to "sh -c" which means if alias ll is not defined in sh(dash shell's) environment then it will not expand it. But I was wrong and this is not the case, Chris' answer above is right.