Can anyone explain the source of the cd shell command?

Question

$ cat $(which cd)
#!/bin/sh
# $FreeBSD: src/usr.bin/alias/generic.sh,v 1.2 2005/10/24 22:32:19   cperciva Exp $
# This file is in the public domain.
builtin `echo ${0##*/} | tr \[:upper:] \[:lower:]` ${1+"$@"}

My interest is in understanding who writes the state change after calling cd. Is this a file?

Answer 1

It is not built-in cd, and in fact it does not work, except to indicate whether the directory can be changed by rejecting a non-zero exit status if it cannot.

Try:

"$(which cd)" /

... will do nothing.

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The actual inline inline cdis part of your C shell and calls syscall chdir(). This syscall updates your process status - tracked by the kernel - to work with another working directory.

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... , script? :

builtin `echo ${0##*/} | tr \[:upper:] \[:lower:]` ${1+"$@"}

... -, echo ( , - ), tr , , cd, cd.

... second, ${1+"$@"} ("$@") , , (${foo+bar} bar, foo ).

, shell-builtin cd (, , syscall chdir()), . , , , , , , , shebang #!/bin/sh, , .

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, cd? , Busybox ash.

• - ...
• - .
• - syscall, .