Is 'which' a built-in command? Can I override it?

Question

In openSUSE, the 'which' command actually updates and is very likely to be a separate app, while in some other distro, it is a built-in command.

So if there's a 'which' command in the PATH, will it override the default, built-in one?

Does this apply to the cd command and other built-in commands?

note: https://software.opensuse.org/package/which

Answer 1

which is a built-in in zsh, but not in most other shells. If you are using zsh and invoke which, it will use the shell built-in, which is the typical behavior when a command and a shell built-in share the same name.

If you want to invoke the command, then POSIX provides the command builtin to do that: command which. If you want to know where a command is located, the POSIX way to do that is command -v.

Answer 2

You have many questions. Lets try to answer them:

Is 'which' a built-in command?

Yes

• A builtin in some shells (csh, tcsh and zsh),
• A tracked alias (what the shell prints on type which for a hashed command (read about hash)) in (lksh, mksh, ksh93, and attsh) and
• An external app in (dash, bash, yash).

So, there is not a general simple answer.

Can I override it?

Always, the order of execution is alias, special built-in, (and, after searching in the PATH) function, regular builtin, and external utility.

To override:

• an special builtin use an alias,
• a regular builtin (what which should be) use either an alias or a function.
• And, if the PATH is used, you can always add an executable ahead of others.
• Also, some shells may provide additional controls of builtins, like (%builtin) in path search of Minix I (1989) ash shell or enable/disable in ksh, bash, zsh

In openSUSE, Is the 'which' command a separate app?

Yes, as is also in Fedora, Debian and many others. But still, shells may have their particular say in this.

Is it a built-in command in other distros ?

That depends more on what shell you use than on what a distro decides to have as available files. A distro selects files, a shell selects builtins.

Will a 'which' command in the PATH override the default, built-in one?

No, as an app is the last executed element in the execution search sequence.

Does this also apply to the cd command and other built-in commands?

A cd is an regular builtin, as such it could be override by an alias or a function (in Posix terms: only if an executable of the same name exists in the PATH).

Answer 3

It's easy to find out using which:

On Fedora 32:

$ which which
alias which='(alias; declare -f) | /usr/bin/which --tty-only --read-alias --read-functions --show-tilde --show-dot'
    /usr/bin/which
$ rpm -qf /usr/bin/which
which-2.21-19.fc32.x86_64

So, a separate command.

Answer 4

$ help
 alias [-p] [name[=value] ... ]          logout [n]
 bg [job_spec ...]                       mapfile [-d delim] [-n count] [-O or>
 bind [-lpsvPSVX] [-m keymap] [-f file>  popd [-n] [+N | -N]
 break [n]                               printf [-v var] format [arguments]
 builtin [shell-builtin [arg ...]]       pushd [-n] [+N | -N | dir]
 caller [expr]                           pwd [-LP]
 case WORD in [PATTERN [| PATTERN]...)>  read [-ers] [-a array] [-d delim] [->
 cd [-L|[-P [-e]] [-@]] [dir]            readarray [-d delim] [-n count] [-O >
 command [-pVv] command [arg ...]        readonly [-aAf] [name[=value] ...] o>
 compgen [-abcdefgjksuv] [-o option] [>  return [n]
 complete [-abcdefgjksuv] [-pr] [-DEI]>  select NAME [in WORDS ... ;] do COMM>
 compopt [-o|+o option] [-DEI] [name .>  set [-abefhkmnptuvxBCHP] [-o option->
 continue [n]                            shift [n]
 coproc [NAME] command [redirections]    shopt [-pqsu] [-o] [optname ...]
 declare [-aAfFgilnrtux] [-p] [name[=v>  source filename [arguments]
 dirs [-clpv] [+N] [-N]                  suspend [-f]
 disown [-h] [-ar] [jobspec ... | pid >  test [expr]
 echo [-neE] [arg ...]                   time [-p] pipeline
 enable [-a] [-dnps] [-f filename] [na>  times
 eval [arg ...]                          trap [-lp] [[arg] signal_spec ...]
 exec [-cl] [-a name] [command [argume>  true
 exit [n]                                type [-afptP] name [name ...]
 export [-fn] [name[=value] ...] or ex>  typeset [-aAfFgilnrtux] [-p] name[=v>
 false                                   ulimit [-SHabcdefiklmnpqrstuvxPT] [l>
 fc [-e ename] [-lnr] [first] [last] o>  umask [-p] [-S] [mode]
 fg [job_spec]                           unalias [-a] name [name ...]
 for NAME [in WORDS ... ] ; do COMMAND>  unset [-f] [-v] [-n] [name ...]
 for (( exp1; exp2; exp3 )); do COMMAN>  until COMMANDS; do COMMANDS; done
 function name { COMMANDS ; } or name >  variables - Names and meanings of so>
 getopts optstring name [arg]            wait [-fn] [id ...]
 hash [-lr] [-p pathname] [-dt] [name >  while COMMANDS; do COMMANDS; done
 help [-dms] [pattern ...]               { COMMANDS ; }
root@machinexa:~# help | grep which
root@machinexa:~# 

Which is not a bash builtin ? Comment if i am wrong Also to view the path do env $PATH, you could also check for which in every path like ls /usr/bin/which where /usr/bin is replaced by all paths