[SOLVED] xargs cd is not working

doru · 03-18-2010, 02:29 AM

Please explain why

Code:

echo some_directory | xargs cd

is not working.

Code:

echo some_directory | xargs ls

is working as expected.

HasC · 03-18-2010, 06:15 AM

What means 'not working'
What error message gives to you?

catkin · 03-18-2010, 06:41 AM

Quote:

Originally Posted by doru

Please explain why

Code:

echo some_directory | xargs cd

is not working.

Code:

echo some_directory | xargs ls

is working as expected.

If I presume correctly ...

It is working but it is not meeting your expectations. You expect that, after the cd pipe, your current directory will be the new directory. It doesn't work because only the first command in a pipleine is run in the current shell; the xargs runs in a subshell; in the subshell, cd does work (but you can't see it). When the pipeline has finished the current directory is unchanged because it never was changed in the current shell.

doru · 03-20-2010, 03:10 AM

Thanks for your answers!

HasC: This is what I get:

Quote:

$ echo test | xargs cd
xargs: cd: No such file or directory

catkin: I would be happy to know that your explanation is right, but then, why do I get the above mentioned error message?

catkin · 03-21-2010, 02:55 AM

Quote:

Originally Posted by doru

... why do I get the above mentioned error message?

Because xargs is searching the directories listed in $PATH for a file called cd and there isn't one because cd is (must be) a shell builtin

Code:

[c@CW8:~$ type cd
cd is a shell builtin

Do you know why cd must be a builtin? If you don't, can you figure it out?

doru · 03-21-2010, 06:30 AM

Thanks, indeed, cd is builtin and ls is not.

Quote:

Do you know why cd must be a builtin? If you don't, can you figure it out?

Maybe because cd has to change an environment variable in the current shell, and non-builtins run in subshells?

Could you please explain to me why does echo builtin have a file in /bin, while cd does not? So the relevant difference between cd and ls is not that one is builtin and the other one is not, but that cd has to change an environment variable in the current shell, and the other doesn't? And xargs is simply protecting me from a meaningless command, by refusing to run it if it does not have a file in /bin? All this would be amazing! Thanks for your answer, please verify my guess.

MTK358 · 03-21-2010, 07:01 AM

It does seem to have both here! I wonder why.

Code:

$ type echo
echo is a shell builtin
$ which echo
/bin/echo
$ echo "Test"
Test
$ /bin/echo "Test"
Test

I don't know if this could be true, but maybe it's for other shells that don't have an echo command built-in?

doru · 03-21-2010, 07:19 AM

Quote:

Originally Posted by MTK358

I don't know if this could be true, but maybe it's for other shells that don't have an echo command built-in?

This is possible,

Quote:

$ type /bin/echo
/bin/echo is /bin/echo

but xargs is using echo by default, if no command is specified, so it has to be a /bin/echo, if there is xargs.

catkin · 03-21-2010, 07:43 AM

Quote:

Originally Posted by doru

Thanks, indeed, cd is builtin and ls is not.

Maybe because cd has to change an environment variable in the current shell, and non-builtins run in subshells?

cd does change a variable in the current shell but that is not the most significant factor. Non-builtins ("external commands" in shell-speak) do run in child processes and that is key to why cd must be a builtin. Try these commands (the first runs cd in a subshell)

Code:

(cd /tmp; pwd)
pwd

Quote:

Originally Posted by doru

Could you please explain to me why does echo builtin have a file in /bin, while cd does not?

Historically echo was an external command but it was very commonly used in shellscripts which meant creating a sub-process each time which used a lot of system resources so it was made a builtin for efficiency and performance. The external /usr/bin/echo was still required for the likes of xargs, as you mentioned, because xargs is a binary running in a sub-process so does not have access to the shell builtin echo.

Quote:

Originally Posted by doru

And xargs is simply protecting me from a meaningless command, by refusing to run it if it does not have a file in /bin?

More simply, xargs can't find the cd command; the most helpful thing it can do is to report that.

doru · 03-22-2010, 04:29 AM

Quote:

Originally Posted by catkin

The external /usr/bin/echo was still required for the likes of xargs, as you mentioned, because xargs is a binary running in a sub-process so does not have access to the shell builtin echo.

More simply, xargs can't find the cd command; the most helpful thing it can do is to report that.

Thank you for your explanation. Could you please clarify why is cd not available in the subprocess. The holy bash manual says:

Quote:

Each command in a pipeline is executed as a separate process (i.e., in a subshell).

Quote:

(list) list is executed in a subshell environment. [...] Variable assignments and builtin commands that affect the shell’s environment do not remain in effect after the command completes.

Quote:

Builtin commands that are invoked as part of a pipeline are also executed in a subshell environment.

The subshell inherits the environment, so it is more than a simple sub-process, it is a shell. Therefore, shell builtins should have been available there, too. (Is it possible to have a process which runs outside any shell?)

catkin · 03-22-2010, 05:21 AM

Quote:

Originally Posted by doru

Thank you for your explanation. Could you please clarify why is cd not available in the subprocess. The holy bash manual says: The subshell inherits the environment, so it is more than a simple sub-process, it is a shell. Therefore, shell builtins should have been available there, too. (Is it possible to have a process which runs outside any shell?)

To answer you last question first: yes, it is not only possible but common to have a process which is executing a program which is not a shell. That is the only way a program that is not a script can run. Shells run non-scripts in child processes, not sub-shells.

A shell is a process in which one of the shell binary executables (usually listed in /etc/shells) is running. When bash runs a pipleline, say echo foo | xargs, the first command in the pipleine (echo foo) runs in the current shell and bash starts a child process to run xargs. Bash does not execute a shell in the child process but the /usr/bin/xargs binary executable. Thus there is no shell running in the xargs child process so xargs does not have shell features available including shell builtins.

To understand this you need to know about processes, programs (especially the difference between a process and a program) and shells. Wikipedia may be helpful.

doru · 03-22-2010, 06:58 AM

Quote:

Originally Posted by catkin

To answer you last question first: yes, it is not only possible but common to have a process which is executing a program which is not a shell. That is the only way a program that is not a script can run. Shells run non-scripts in child processes, not sub-shells.

A shell is a process in which one of the shell binary executables (usually listed in /etc/shells) is running. When bash runs a pipleline, say echo foo | xargs, the first command in the pipleine (echo foo) runs in the current shell and bash starts a child process to run xargs. Bash does not execute a shell in the child process but the /usr/bin/xargs binary executable. Thus there is no shell running in the xargs child process so xargs does not have shell features available including shell builtins.

To understand this you need to know about processes, programs (especially the difference between a process and a program) and shells. Wikipedia may be helpful.

I read an introductory book on Unix, so I know some about processes and programs. Only the practical application of these notions seems a bit cumbersome.

It seems that xargs simply refuses to start yet another subshell to run the command it receives as an argument in it, and expects external commands only. This explanation fits both the Unix and the bash manuals.

Code:

echo test | xargs cd

bash creates subshells for echo test and for xargs cd, but xargs refuses to create another subshell for cd. That is all. PS: bash is external.

Thanks for your help.

doru · 03-23-2010, 05:05 AM

Quote:

Originally Posted by catkin

A shell is a process in which one of the shell binary executables (usually listed in /etc/shells) is running.

Shells run non-scripts in child processes, not sub-shells.

When bash runs a pipeline, say echo foo | xargs, the first command in the pipeline (echo foo) runs in the current shell and bash starts a child process to run xargs.

The first two statements are correct, the third one is false, but all three are very important.

You can see that the third statement is incorrect by running:

Code:

dave@router2:~$ ls -dl tmp tst
drwxr-xr-x 2 dave dave 4096 2010-02-25 12:46 tmp
drwxr-xr-x 2 dave dave 4096 2010-03-22 18:33 tst
dave@router2:~$ cd tmp | cd tst
dave@router2:~$ pwd
/home/dave

See?

catkin · 03-23-2010, 05:32 AM

Quote:

Originally Posted by doru

The first two statements are correct, the third one is false

Thanks for the correction and the demo doru

The GNU Bash Reference says: "Each command in a pipeline is executed in its own subshell ..."

doru · 03-23-2010, 12:42 PM

Quote:

Originally Posted by catkin

Thanks for the correction and the demo doru

I am glad that I have contributed a little bit.