Get the expansion of an alias (in both bash and zsh)

Question

I want to get the expansion of an alias.

For example, if I have:

alias g=hub
alias cdh='cd $HOME'

I want to have:

expand_alias g == hub

expand_alias cdh == cd $HOME

The tricky thing is that the two shells have different output: bash:

$ alias g cdh
alias g='git'
alias cdh='cd $HOME'

zsh:

% alias g cdh
g=hub
cdh='cd $HOME'

Note no alias prefix and no quotes around hub.

Answer 1

In zsh, you can just use

get_alias() {
  printf '%s\n' $aliases[$1]
}

With bash (assuming it's not in POSIX mode in which case its alias would give an output similar to zsh's), you could do:

get_alias() (
  eval '
    alias() { printf "%s\n" "${1#*=}"; }'"
    $(alias -- "$1")"
)

Basically, we evaluate the output of alias after having redefined alias as a function that prints what's on the right of the first = in its first argument.

You could use a similar approach for something compatible with most POSIX shells, zsh and bash:

get_alias() {
  eval "set -- $(alias -- "$1")"
  eval 'printf "%s\n" "${'"$#"'#*=}"'
}

Answer 2

Based on Stéphane Chazelas' answer, I came up with:

function alias_expand {
  if [[ $ZSH_VERSION ]]; then
    # shellcheck disable=2154  # aliases referenced but not assigned
    [ ${aliases[$1]+x} ] && printf '%s\n' "${aliases[$1]}" && return
  else  # bash
    [ "${BASH_ALIASES[$1]+x}" ] && printf '%s\n' "${BASH_ALIASES[$1]}" && return
  fi
  false  # Error: alias not defined
}

(with bash 4.0 or newer).

Answer 3

How about defining a shell function:

EXA() { alias $@ | sed 's/^/expand_/; s/=/ == /; s/\o047//g; '; }

Answer 4

My zsh had this bound to a key combo:

_expand_alias () {
    local word expl tmp pre sel what
    local -a tmpa suf
    eval "$_comp_setup"
    if [[ -n $funcstack[2] ]]
    then
        if [[ "$funcstack[2]" = _prefix ]]
        then
            word="$IPREFIX$PREFIX$SUFFIX"
        else
            word="$IPREFIX$PREFIX$SUFFIX$ISUFFIX"
        fi
        pre=()
    else
        local curcontext="$curcontext"
        if [[ -z "$curcontext" ]]
        then
            curcontext="expand-alias-word:::"
        else
            curcontext="expand-alias-word:${curcontext#*:}"
        fi
        word="$IPREFIX$PREFIX$SUFFIX$ISUFFIX"
        pre=(_main_complete - aliases)
    fi
    zstyle -s ":completion:${curcontext}:" regular tmp || tmp=yes
    case $tmp in
        (always) sel=r  ;;
        (yes | 1 | true | on) [[ CURRENT -eq 1 ]] && sel=r  ;;
    esac
    zstyle -T ":completion:${curcontext}:" global && sel="g$sel"
    zstyle -t ":completion:${curcontext}:" disabled && sel="${sel}${(U)sel}"
    tmp=
    [[ $sel = *r* ]] && tmp=$aliases[$word]
    [[ -z $tmp && $sel = *g* ]] && tmp=$galiases[$word]
    [[ -z $tmp && $sel = *R* ]] && tmp=$dis_aliases[$word]
    [[ -z $tmp && $sel = *G* ]] && tmp=$dis_galiases[$word]
    if [[ -n $tmp ]]
    then
        tmp=${tmp%%[[:blank:]]##}
        if [[ $tmp[1] = [[:alnum:]_] ]]
        then
            tmpa=(${(z)tmp})
            if [[ $tmpa[1] = $word && $tmp = $aliases[$word] ]]
            then
                tmp="\\$tmp"
            fi
        fi
        zstyle -T ":completion:${curcontext}:" add-space || suf=(-S '')
        $pre _wanted aliases expl alias compadd -UQ "$suf[@]" -- ${tmp%%[[:blank:]]##}
    elif (( $#pre )) && zstyle -t ":completion:${curcontext}:" complete
    then
        $pre _aliases -s "$sel" -S ''
    else
        return 1
    fi
}

Answer 5

In my zsh environment, when I type alias with no command I get on stdout a list of all aliases. The same happens in bash (on MacOS both). These are in a form alias=expansion

So you could filter further with sed: alias | sed -ne 's/cdh=\(.*\)$/\1/p' if "cdh" is the alias you want to know. You could make a shell function in your profile for this, if this is a common task for you: like

expand_alias() { alias $@ | sed -ne 's/^'$@'=\(.*\)$/\1/p' }