2

I want to get the text which an alias expands into.

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.

  • So what exactly are you missing? – RudiC Aug 18 '18 at 10:16
  • 1
    Note that the zsh output is the standard one. bash will give you that one as well when in POSIX mode. – Stéphane Chazelas Aug 18 '18 at 18:06
  • why are the single quotes a problem? – Henno Brandsma Aug 19 '18 at 6:04
  • @HennoBrandsma I want to define an alias based on the contents of an existing one. – Tom Hale Aug 19 '18 at 7:03
  • why define aliases that way? aliases are meant for interactive use, and usually only set once in a config file. I don't see a use case for generating aliases from other aliases. I'm just trying to understand the problem.. – Henno Brandsma Aug 19 '18 at 7:11
4

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 -- "$1")"
  eval 'printf "%s\n" "${'"$#"'#*=}"'
}
  • I wondered why parentheses are used instead of the usual braces... a function body can be a single statement, and in this case that's a subshell. – Tom Hale Aug 19 '18 at 5:50
  • 1
    @TomHale, that's to limit the scope of that alias function we declare. – Stéphane Chazelas Aug 19 '18 at 6:35
3

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

# Expand an alias as text - https://unix.stackexchange.com/q/463327/143394
function expand_alias {
  if [[ -n $ZSH_VERSION ]]; then
    # shellcheck disable=2154  # aliases referenced but not assigned
    printf '%s\n' "${aliases[$1]}"
  else  # bash
    printf '%s\n' "${BASH_ALIASES[$1]}"
  fi
}

(with bash 4.0 or newer).

  • 1
    function force-expand { local e="$(expand_alias "$1")" ; test -z "$e" && e="$1" ; echo "$e" } – HappyFace Jun 19 at 13:28
0

How about defining a shell function:

EXA() { alias $@ | sed 's/^/expand_/; s/=/ == /; s/\o047//g; '; }
  • Syntax error when I copy and paste – Tom Hale Aug 19 '18 at 5:12
  • show the syntax error. – RudiC Aug 19 '18 at 6:48
  • unmatched '. How will this remove single quotes? – Tom Hale Aug 19 '18 at 8:34
  • Rats! Corrected typo... – RudiC Aug 19 '18 at 10:57
  • Breaks with: alias ls='colourify ls -CF --block-size=\'\''1 --color=always --quoting-style=shell-escape' – Tom Hale Aug 22 '18 at 8:23
-1

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' }

  • This leaves the single quotes in. Please re-read the question. – Tom Hale Aug 19 '18 at 5:10
  • @TomHale Inthe OP's example, it's not quite clear to me why the single quotes are a problem. They don't hurt for single commands, or one word substitutions; they are needed for commands with spaces. So that's why I left in the spaces, as I don't want to complicate things by adding logic to remove unnecessary single quotes. If they always have to go, just add ` | tr -d \' ` to the pipe. – Henno Brandsma Aug 19 '18 at 6:04
  • Fair enough assumption. I can't upvote you again now. The | tr wouldn't allow for a ' in the middle of an alias though. – Tom Hale Aug 19 '18 at 7:06

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