I want to look into the source code behind the command cd
, but cd
doesn't seem to have its own package so which package is it in?
5 Answers
Find out what type a command is using the type
command.
$ type cd
cd is a shell builtin
$ type ls
ls is aliased to `ls --color=auto'
$ type cat
cat is /bin/cat
You can see, cd
is a shell builtin.
This means, it's a part of your shell which is by default Bash. That's of course also the software package it's contained in.
For installed commands that are not shell builtins but executable files, use dpkg -S
to find out the package:
$ dpkg -S $(which cat)
coreutils: /bin/cat
To get help for built in commands, use the help
command (which is also built in):
$ help cd
cd: cd [-L|[-P [-e]] [-@]] [dir]
Change the shell working directory.
[... output shortened ...]
-
5
dpkg -S
only applies to package installed software - if they installed from a tarball or a bin file it won't help them there - buttype
will still identify it as a different binary and not a built in or an alias– Thomas Ward ♦Mar 11, 2016 at 15:26 -
4Ok true but I hope we assume a vanilla Ubuntu when answering about default applications. And Ubuntu without bash installed seems unlikely ;-)– RinzwindMar 11, 2016 at 15:41
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1You can use apt-file search <file> instead of dpkg -S for searching for files in packages, that are not installed.– ortangMar 11, 2016 at 16:04
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4I recommend
type -a cd
to showa
ll locations of this command. There still should only be the shell builtin. Compare, for example, withtype -a printf
which comes in both builtin and /usr/bin varieties. Mar 11, 2016 at 19:25 -
2@DigitalTrauma: Very good comment! Indeed, there are two sorts of builtins: ones which must be builtins because they cannot possibly work as external commands (e.g.
cd
,set
, which alter the calling process's environment) and "convenience" builtins which are builtins for performance reasons (avoiding the overhead of a subshell) or because you would like to have them available even on a system with a completely hosed$PATH
or something like that (e.g.[
,echo
,printf
). The latter can sometimes lead to confusion because e.g.man echo
will show the command's manpage, but … Mar 12, 2016 at 1:25
cd
is necessarily a shell built-in. If the shell spawned a child process that changed the working directory and then exited, the parent process (the shell itself) would not be affected.
As to the source code of cd
, all it needs to do is call chdir(2)
, which changes the working directory of the process. See chdir at opengroup.org which states:
The chdir() function shall cause the directory named by the pathname pointed to by the path argument to become the current working directory; that is, the starting point for path searches for pathnames not beginning with '/'.
-
3
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3Right. I only added an answer because the other answers didn't address why
cd
must be a built-in. What is more misleading is that sometimes there actually is a/usr/bin/cd
.– Tim BMar 11, 2016 at 16:03
Here is a man page for cd but it not an official one since cd
is part of the "Shell Builtin Commands". Just like some other commands ...
alias, bg, bind, break, builtin, command, compgen, complete,
continue, declare, dirs, disown, echo, enable, eval, exec, exit,
export, fc, fg, getopts, hash, help, history, jobs, kill, let, local,
logout, popd, printf, pushd, pwd, read, readonly, return, set, shift,
shopt, source, suspend, test, times, trap, type, typeset, ulimit,
umask, unalias, unset, wait
See the man page for bash. From the link cd
states:
cd [-L|-P] [dir]
Change the current directory to dir. The variable HOME is the default dir. The variable CDPATH defines the search path for the directory containing dir. Alternative directory names in CDPATH are separated by a colon (:). A null directory name in CDPATH is the same as the current directory, i.e., ''.''. If dir begins with a slash (/), then CDPATH is not used. The -P option says to use the physical directory structure instead of following symbolic links (see also the -P option to the set builtin command); the -L option forces symbolic links to be followed. An argument of - is equivalent to $OLDPWD. If a non-empty directory name from CDPATH is used, or if - is the first argument, and the directory change is successful, the absolute pathname of the new working directory is written to the standard output. The return value is true if the directory was successfully changed; false otherwise.
Which package is the command 'cd' in?
That would be bash
-
5Adding to this, while many of the builtin tools could be external commands, there is no way that the
cd
command could be implemented as an external command, since it's impossible for a subprocess to change the working directory of its parent process. Mar 11, 2016 at 18:35 -
+1 for the link to the package, from which link the source code can be readily obtained. Mar 12, 2016 at 9:56
The actual source code of cd
is for
- Bash in
builtins/cd.def
at the end of the file - TCSH in
sh.dir.c
as the functiondochngd
- PDKSH (Public Domain Korn shell found in some BSDs) in
c_ksh.c
right at the top of the file - Korn 93 (Original Korn shell, old version) in
src/cmd/ksh93/bltins/cd_pwd.c
The sys-call chdir
for
- Linux kernel is in
fs/open.c
- FreeBSD is where the link at the beginning of this item points to
- other BSD-type OSs and OpenSolaris work quite similar
It might be of note that the sys-calls are still not the end of the chain, that would be down at the individual file system level.
Right, so as everyone has written, cd
is built into whatever shell you use, because a process cannot affect the working directory of its parent. cd
has a few bells and whistles tacked on (I mean pushd
and friends, cd
without an argument, etc.), but if you take the trouble to dig through the source you'll be underwhelmed: the main thing is that cd
calls the chdir(2)
system call.
There's not much to see because unlike, say, an environment variable like HOME
, the "working directory" is a system primitive: It's one of the attributes that every process has, like (real) user id or scheduling priority. It is the starting point when opening files with relative paths.
sudo cd /var/named
work?which
command work forcd
? I can't find the executable forcd
either!fg
,bg
andjobs
installed?)