15

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?

2
  • You can specify the full path when calling the command, eg., /usr/bin/test -f "$file"... – jasonwryan May 29 '14 at 22:43
  • @jasonwryan I'm aware of this, but I'm just interested if there is a way to change the order of executables started. – Martin May 29 '14 at 22:48
29

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)

9
  • 1
    @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). – Gilles 'SO- stop being evil' May 29 '14 at 23:55
  • 1
    Suggested edit: Aliases are disabled when you quote the command (or any part of it), as in \test or 'test' or tes't'. – John Kugelman May 30 '14 at 12:58
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    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. – gena2x May 30 '14 at 13:05
  • 1
    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." – John Kugelman May 30 '14 at 15:02
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    +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. – twan163 Dec 14 '16 at 18:21
6

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).

7
  • wouldnt env be appropriate here too? – Steven Penny Jan 23 '17 at 0:58
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    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 – papo Dec 24 '18 at 11:02
  • @papo which df does not necessarily show you what df runs. unix.stackexchange.com/questions/85249/… – Gilles 'SO- stop being evil' Dec 24 '18 at 14:15
  • 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. – papo Jan 19 at 21:34
  • @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. – Gilles 'SO- stop being evil' Jan 19 at 21:43

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