Hi Christian, Christian Weisgerber wrote on Thu, Dec 14, 2017 at 03:20:22PM -0000: > On 2017-12-13, Ingo Schwarze <schwa...@usta.de> wrote:
>> That is not a stand-alone command, but a shell built-in. >> Actually, it is not even possible to implement it as a stand-alone >> command because the effect of the intended change would end when >> the command exits. > I think there is/was a POSIX requirement, or at least some people > interpreted it that way, that all shell builtins must also exist > as external commands, You are almost right. Not all shell built-ins, but all *regular* shell built-ins. That requirement exists at the very end of http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap01.html below "1.6 Built-In Utilities": However, all of the standard utilities, including the regular built-ins in the table, but not the special built-ins described in Special Built-In Utilities, shall be implemented in a manner so that they can be accessed via the exec family of functions as defined in the System Interfaces volume of POSIX.1-2008 and can be invoked directly by those standard utilities that require it (env, find, nice, nohup, time, xargs). And the table right above includes cd(1), alias(1), ulimit(1), and umask(1). > and I seem to remember one system, AIX I think, > that actually had an external version of cd(1). > It didn't do anything sensible, of course, but it was there. It would still provide an EXIT STATUS and a message on stderr in case of error, i presume. Not enough to call it "sensible", though, IMHO. We are actually violating POSIX here: schwarze@isnote $ env -i cd . env: cd: No such file or directory schwarze@isnote $ echo . | xargs cd xargs: cd: No such file or directory I'm not convinced we should implement that. Providing a stand-alone cd(1) could not do much harm, but might be a waste of space and time. It looks like a striking example of standards' insanity. > Indeed, proof that I'm not hallucinating this: > https://www.ibm.com/support/knowledgecenter/en/ssw_aix_72/com.ibm.aix.cmds1/cd.htm > | Note > | Running /usr/bin/cd from a shell does not change the shell's working > | directory. The shell's built-in cd command must be used. http://pubs.opengroup.org/onlinepubs/9699919799/utilities/cd.html says something similar below APPLICATION USAGE... :-o It looks like certified UNIX systems have no choice. Look here: schwarze@unstable11s [unstable11s]:~ > uname -a SunOS unstable11s 5.11 11.2 sun4u sparc SUNW,SPARC-Enterprise schwarze@unstable11s [unstable11s]:~ > file /usr/bin/cd /usr/bin/cd: ELF 64-bit MSB executable SPARCV9 Version 1, UltraSPARC1 Extensions Required, dynamically linked, not stripped schwarze@unstable11s [unstable11s]:~ > /usr/bin/cd /tmp schwarze@unstable11s [unstable11s]:~ > echo $? 0 schwarze@unstable11s [unstable11s]:~ > /usr/bin/cd /murks /usr/bin/cd[9]: cd: /murks: [No such file or directory] schwarze@unstable11s [unstable11s]:~ > echo $? 1 schwarze@unstable9s :~ > uname -a SunOS unstable9s 5.9 Generic_Virtual sun4u sparc SUNW,SPARC-Enterprise-T5220 schwarze@unstable9s :~ > file /usr/bin/cd /usr/bin/cd: executable /bin/ksh script schwarze@unstable9s :~ > cat /usr/bin/cd #!/bin/ksh -p # #ident "@(#)alias.sh 1.2 00/02/15 SMI" # # Copyright (c) 1995 by Sun Microsystems, Inc. # cmd=`basename $0` $cmd "$@" schwarze@unstable9s :~ > /usr/bin/cd /tmp schwarze@unstable9s :~ > echo $? 0 schwarze@unstable9s :~ > /usr/bin/cd /murks /usr/bin/cd[8]: /murks: not found schwarze@unstable9s :~ > echo $? 1 Hilarious. Yours, Ingo
