Order of executables started in bash

bash shell command shell-builtin

5,545

Solution 1

Highest priority is bash alias, then special builtins (only in POSIX mode), then functions, then builtins, then a search in $PATH.

To execute a builtin, use builtin test.
To execute an external application, use an explicit path: /bin/test.
To ignore functions and aliases, use command test.
To bypass just alias, use \test or any other kind of expansion.

It's possible to disable/enable a builtin with enable test.

(Updated according to comments below)
(Fixed incorrect admin edit that bash has disable builtin - in fact, there is only enable)

Solution 2

Built-in commands are always preferred to external commands. The rationale is that the built-in command is faster (and in a few cases, such as cd or test -o BASH_OPTION, only the built-in command can have the desired effect).

Sometimes the external command may have capabilities that the shell builtin doesn't. In that case, you can call the external command by giving an explicit path (i.e. containing a slash) (this bypasses any concern about the order in $PATH). If you don't want to hard-code the external path but you do want to prevent the use of the builtin, you can use "$(type -P test)" (note capital P) in bash, "$(whence -p test)" in ksh, and =test in zsh. Another way to force the use of an external command is to go through the env utility (env test …).

In zsh, you can disable a builtin with disable test. This is permanent (for the current shell or subshell) until the builtin is reenabled with enable test. In bash, you can do the same with enable -n test to disable and enable test to reenable.

You can use an alias or function to force the execution of a different command, for example alias test=/usr/bin/test or test () { /usr/bin/test "$@"; }. If you have such an alias, you can prevent its use by quoting any part of it, e.g. \test will perform the normal function/builtin/external lookup. Note that depending on the shell and its settings, alias definitions in a function may be expanded when a function is read or when it is executed. If you've defined a function, you can use command test to prevent function lookup as well as alias lookup (so here the test builtin will be invoked unless disabled).

5,545

comeback4you

Updated on September 18, 2022

Comments

comeback4you almost 2 years
If I execute the test command in bash, test(evaluates conditional expression) built-in utility is started:
```
$ type test
test is a shell builtin
$ type -a test
test is a shell builtin
test is /usr/local/bin/test
test is /usr/bin/test
$ 
```
However, as seen in output of type -a test above, there is another test in /usr/local/bin directory and yet another one in /usr/bin directory. How are executables ordered, i.e. are the built-in commands always preferred and then the rest of the commands depend on the directory order in $PATH variable? In addition, is it possible to change the order of the executables started, e.g. if I type in test, then /usr/bin/test is started instead of bash-builtin test?
- jasonwryan about 10 years
  
  You can specify the full path when calling the command, eg., /usr/bin/test -f "$file"...
- comeback4you about 10 years
  
  @jasonwryan I'm aware of this, but I'm just interested if there is a way to change the order of executables started.
Mathias Begert about 10 years

The order you have specified is different than the order mentioned in an answer from one of the gurus on this site. Which one is right?
Gilles 'SO- stop being evil' about 10 years

@1_CR gena2x is right. My answer omitted special builtins, which take precedence over functions as per POSIX (though some shells are not compliant; bash complies only in POSIX mode).
Olivier Dulac about 10 years

And to bypass just the alias : \test (this will either start the builtin, or, if no builtin of that name or it is disabled, will start any function of that name, or look for it in the PATH)
John Kugelman about 10 years

Suggested edit: Aliases are disabled when you quote the command (or any part of it), as in \test or 'test' or tes't'.
gena2x about 10 years

That's not full picture. Seems any kind of expansion (in bash manual, all the substitution, tilde expansion and so on called expansion) disables aliases. I tried.
John Kugelman about 10 years

Quote from the bash man page: "The first word of each simple command, if unquoted, is checked to see if it has an alias. If so, that word is replaced by the text of the alias. The characters /, $, backtick, and = and any of the shell metacharacters or quoting characters listed above may not appear in an alias name."
gena2x about 10 years

My statement says that any kind of expansion prevents alias substitution. This comply to the man page which says that any word which contains no symbols out of restricted set will never been substituted. Your statement that unquoted words prevent alias substitution. Both statements comply to bash manual. My statement is just broader than your and still holds truth. If you think that my statement is not true, please provide an example of the expansion which doesn't prevent alias substitution.
twan163 over 7 years

+1 for hints in helping me find the source of this information: it is in the bash man page, under the section COMMAND EXECUTION, second and third paragraphs.
papo over 5 years

so, if the shell runs from BusyBox, are other, usually external commands from the same BusyBox considered as internals? e.g. I added full df to a PATH on first position, removed alias 'df', which df shows /opt/bin/df, but df runs /bin/df -> busybox
Gilles 'SO- stop being evil' over 5 years

@papo which df does not necessarily show you what df runs. unix.stackexchange.com/questions/85249/…
Tinmarino almost 5 years

If the search is unsuccessful, the shell searches for a defined shell function named command_not_found_handle.
papo over 3 years

I bounced back to this issue. This is not a problem of which or command or type. I concluded this to be a bug of that busybox (v1.1.1). It is prioritizing its compiled-in binaries over PATH. And even when a link: xxx -> busybox is removed, the program xxx still runs. This must mean that this old busybox was considering all compiled-in binaries to be built-ins. New busybox does not behave this way. Just another thing to consider. btw command -v and type shows df is df or touch is touch and not /bin/touch. I think that is giving away it's considered to be a sort of internal.
Gilles 'SO- stop being evil' over 3 years

@papo I see the same behavior with the fairly recent Busybox 1.30.1 on Ubuntu 20.04. It makes sense to me that the “compiled-in binaries” — which you could also call “built-in utilities” — are considered builtins in the shell sense of the term. P.S. There was a mistake in my answer: command -p does not bypass builtins, only functions and aliases.
papo over 3 years

on v1.22.1, type touch touch is /bin/touch, on v1.1.1 touch is touch, removing touch symlink file, on 1.22.1, touch does not work, on 1.1.1 touch still works in shell. I don't like the fact that I can't upgrade something like df, date, touch, put new binary in PATH and use this upgraded version in a script without static path. There is no mention nowhere that applets compiled-in busybox are special. I don't argue it could not be, but if that is not a bug, the behavior should be stable and it should be noted somewhere.
Gilles 'SO- stop being evil' over 3 years

@papo Maybe this depends on compilation options (e.g. hush vs ash) rather than on the busybox version