mkdocs-pages/docs/examples/redirection.md

File Redirection Examples

This page has a few examples of some things that can be done using redirection and pipes

Pipes

Pipes are usefull when you need to do something with an programs output

In the following command output we may be looking for a file that is starting with 'pas'

[root@gunicorn paste]# ls -alh
total 16K
drwxr-xr-x. 6 root nginx  158 Feb 20 06:38 .
drwxr-xr-x. 4 root nginx   30 Feb 19 09:32 ..
drwxr-xr-x. 8 root nginx  163 Feb 19 09:29 .git
-rw-r--r--. 1 root nginx   20 Feb 19 09:29 .gitignore
-rwxr-xr-x. 1 root nginx 2.4K Feb 19 09:36 paste.py
drwxr-xr-x. 2 root nginx    6 Feb 19 09:33 pastes
drwxr-xr-x. 2 root nginx   61 Feb 19 09:36 __pycache__
-rw-r--r--. 1 root nginx    0 Feb 19 09:29 README.md
-rw-r--r--. 1 root nginx    6 Feb 19 09:29 requirements.txt
drwxr-xr-x. 2 root nginx   40 Feb 20 05:23 templates
-rw-r--r--. 1 root nginx   77 Feb 19 09:36 wsgi.py

We could just look through the output but that can take some time with larger file lists, in this case we can use grep

[root@gunicorn paste]# ls -alh | grep pas
-rwxr-xr-x. 1 root nginx 2.4K Feb 19 09:36 paste.py
drwxr-xr-x. 2 root nginx    6 Feb 19 09:33 pastes

As you can see this has filtered the output of the command and is now just showing the output that matches the 'pas' string provided to grep, something to keep in mind is by default grep matches the entire line

Another example of piping is the following command, this will find the message ID of all frozen emails on a mail server running exim and then pass the ID one at a time to exim -Mrm which removes the message from the queue.

exim -iz | xargs exim -Mrm

Redirection

There will be quite a few times when you will want to redirect the output of a command into a file, this could be a temporary file with some data in it that you need to work with

[root@gunicorn scratch]# ps aux > ps.out will save the output of ps aux to ps.out, this is useful as we now have a snapshot of what was running and we can run some commands on it without the data changing

You also may need to redirect errors from a command so they do not make a mess of your output that you have, for example you are running a find command on a directory and your user account does not have full permissions for all the directories/files your output will have

find: ‘/boot/efi/EFI/centos’: Permission denied
find: ‘/boot/grub2’: Permission denied
find: ‘/proc/tty/driver’: Permission denied
find: ‘/proc/1/task/1/fd’: Permission denied
find: ‘/proc/1/task/1/fdinfo’: Permission denied
find: ‘/proc/1/task/1/ns’: Permission denied

In this case we can redirect the STDERR out to a special file that nulls all data that is sent to it, the file in this case is /dev/null

[testingacc@gunicorn testing]$ find
.
./dir1
./dir1/file1
./dir1/file2
./dir1/file3
./dir2
find: ‘./dir2’: Permission denied

If we run the same command but append 2> /dev/null it will remove the errors from the output

[testingacc@gunicorn testing]$ find 2> /dev/null
.
./dir1
./dir1/file1
./dir1/file2
./dir1/file3
./dir2

You can also redirect the errors to a file for later use if required, this is done in the same way but you need to swap /dev/null with the filename/path you want to save the errors to

[testingacc@gunicorn testing]$ find 2> error.log
.
./dir1
./dir1/file1
./dir1/file2
./dir1/file3
./dir2
./error.log

[testingacc@gunicorn testing]$ cat error.log
find: ‘./dir2’: Permission denied

You can see we now have the error.log file and it contains the 'Permission denied' error and it was not printed to the screen