Exercise: Capture and combine output

Capture and combine output

• Write and append output to files.
• Separate errors from results, and discard them.
• Build pipelines, and keep a copy with tee.

Start the exercise

On workstation, start the exercise. It removes the result files of an earlier run from student's home on servera.

[student@workstation ~]$ lab start sa-redirect

Later tasks assume that you are logged in to the server they name: ssh student@servera from workstation, then sudo -i for a root shell.

Save a list of configuration files

On servera, as student, list the .conf files directly in /etc into conf-files.txt, then count and peek at it.

[student@workstation ~]$ ssh student@servera
[student@servera ~]$ ls /etc/*.conf > conf-files.txt
[student@servera ~]$ wc -l conf-files.txt
22 conf-files.txt
[student@servera ~]$ head -n 3 conf-files.txt
/etc/chrony.conf
/etc/dracut.conf
/etc/host.conf

The pattern expanded to 22 names, and ls printed one per line because its output went to a file rather than a terminal.

Add a footer line

Add a line with today's date to the end of the same file, without losing the list.

[student@servera ~]$ echo "--- listed on $(date +%F)" >> conf-files.txt
[student@servera ~]$ tail -n 2 conf-files.txt
/etc/yum.conf
--- listed on 2026-10-03

Keep results and errors apart

Search all of /etc for .conf files, sending the results to found.txt and the permission errors to errors.txt. Count both.

[student@servera ~]$ find /etc -name "*.conf" > found.txt 2> errors.txt
[student@servera ~]$ wc -l found.txt errors.txt
  78 found.txt
  13 errors.txt
  91 total

Nothing appeared on the screen: both streams went to files.

Count results, discarding the errors

[student@servera ~]$ find /etc -name "*.conf" 2> /dev/null | wc -l
78

Only stdout flows through the pipe; stderr went to /dev/null.

Find the three biggest files in /etc

ls -lS sorts by size, biggest first. Keep the first four lines (the first is the total line).

[student@servera ~]$ ls -lS /etc | head -4
total 1320
-rw-r--r--.  1 root root   692252 Jun 23  2020 services
-rw-r--r--.  1 root root    13507 Oct  3 09:28 ld.so.cache
-rw-r--r--.  1 root root    10373 May  2  2025 nanorc

See a report and save it at the same time

[student@servera ~]$ df -h | tee disk-report.txt
Filesystem      Size  Used Avail Use% Mounted on
devtmpfs        455M     0  455M   0% /dev
...output omitted...
/dev/sda2        20G  1.7G   18G   9% /
...output omitted...
[student@servera ~]$ wc -l disk-report.txt
9 disk-report.txt

Grade and finish

Check your work with lab grade sa-redirect, then put the exercise away with lab finish sa-redirect.