Pipes, redirection and quoting in Linux

Everything happens in a new folder, ~/lab13. You’ll make one real file and ask for one that doesn’t exist, so every command has something to say on both stdout and stderr. The outputs below are exact, except times, dates and counts that depend on your machine.

1See the two output streams

Paste this whole block:

mkdir -p ~/lab13 && cd ~/lab13
printf 'web01\nweb02\ndb01\n' > servers.txt
ls servers.txt nofile.txt

You should see this:

ls: cannot access 'nofile.txt': No such file or directory
servers.txt

Both lines look the same on screen, but the first came through stderr (fd 2) and the second through stdout (fd 1). The next steps pull them apart.

✅ Checkpoint: You see one error line and one file name.

2Send stdout to a file

Run this:

ls servers.txt nofile.txt > out.txt
cat out.txt

You should see this:

ls: cannot access 'nofile.txt': No such file or directory
servers.txt

The error still hit your screen because > only moved fd 1. The file name went into out.txt, which cat then printed.

✅ Checkpoint: out.txt contains only servers.txt.

3Overwrite or append

Run this:

echo one > n.txt
echo two > n.txt
cat n.txt
echo three >> n.txt
cat n.txt

You should see this:

two
two
three

> empties the file before writing, so “one” is gone. >> adds to the end. Logs and reports you build over time always use >>.

Protect yourself from accidental overwrites with set -o noclobber. Then echo x > n.txt fails with “cannot overwrite existing file”, and >| forces it when you mean it.

✅ Checkpoint: n.txt ends with the lines two and three.

Example output from rocky01: 2> keeps only the error, and > all.txt 2>&1 puts both streams in one file

4Split errors from output, or join them

Run this:

ls servers.txt nofile.txt 2> err.txt
cat err.txt
ls servers.txt nofile.txt > all.txt 2>&1
cat all.txt

You should see this:

servers.txt
ls: cannot access 'nofile.txt': No such file or directory
ls: cannot access 'nofile.txt': No such file or directory
servers.txt

The first command printed servers.txt on screen and saved only the error. The second put both streams in all.txt, which is the form you want for a cron job log. &> all.txt does the same in bash.

✅ Checkpoint: all.txt contains both the error line and servers.txt.

5Throw output away with /dev/null

Run this:

ls nofile.txt 2>/dev/null
echo "exit status: $?"

You should see this:

exit status: 2

The error is gone from the screen, but the command still failed, and $? (the exit status of the last command) says so. Hiding errors is fine in a quick check; in a script, check the status, or you’ll hide real problems.

✅ Checkpoint: Nothing printed except the exit status 2.

6Pipe one command into another

Run this:

ls /etc | wc -l
wc -l < servers.txt
ls servers.txt nofile.txt 2>&1 | wc -l

You should see something like this:

178
3
2

The first line counts the entries in /etc (your number will differ). wc -l < servers.txt prints only the number, because wc never saw a file name. The last one counts 2 lines because 2>&1 sent the error into the pipe as well; without it the error skips the pipe and wc counts 1.

✅ Checkpoint: wc -l < servers.txt prints 3.

7Write to a root-owned file with sudo tee

Run this:

sudo echo "10.10.10.31 kafka01.saatlab.test kafka01" >> /etc/hosts

You’ll get:

-bash: /etc/hosts: Permission denied

sudo ran echo as root, but the >> was opened by your own shell as student, before sudo started. Use tee, which reads stdin and writes it both to the screen and to a file, and run tee itself with sudo:

Run this:

grep -q kafka01 /etc/hosts || echo "10.10.10.31 kafka01.saatlab.test kafka01" | sudo tee -a /etc/hosts
tail -n 1 /etc/hosts

You should see this:

10.10.10.31 kafka01.saatlab.test kafka01
10.10.10.31 kafka01.saatlab.test kafka01

The first line is tee echoing what it wrote, the second is tail proving it’s in the file. The -a matters: without it tee overwrites the file, and an /etc/hosts with one line breaks name lookups on the server. The grep -q ... || part only adds the line if it isn’t already there, so running the block twice is safe. kafka01 is the VM you’ll build in Lesson 58, and this entry matches the lab plan.

✅ Checkpoint: tail -n 1 /etc/hosts shows the kafka01 line.

8Save and watch at the same time

Run this:

df -h / | tee disk-report.txt
wc -l disk-report.txt

You should see something like this:

Filesystem           Size  Used Avail Use% Mounted on
/dev/mapper/rl-root   17G  1.9G   16G  11% /
2 disk-report.txt

This is the everyday use of tee: you see the output and you keep a copy for the ticket.

✅ Checkpoint: disk-report.txt has the same two lines you saw on screen.

9Turn a list into arguments with xargs

Paste this whole block:

mkdir -p logs && touch logs/a.log logs/b.log "logs/my app.log"
find logs -name "*.log" | xargs ls -l

You’ll get:

ls: cannot access 'logs/my': No such file or directory
ls: cannot access 'app.log': No such file or directory
-rw-rw-r--. 1 student student 0 Oct 27 12:14 logs/a.log
-rw-rw-r--. 1 student student 0 Oct 27 12:14 logs/b.log

xargs split “my app.log” into two words. Now do it the safe way, with null separators:

Run this:

find logs -name "*.log" -print0 | xargs -0 ls -l
echo a b c d | xargs -n 2 echo

You should see something like this:

-rw-rw-r--. 1 student student 0 Oct 27 12:14 logs/a.log
-rw-rw-r--. 1 student student 0 Oct 27 12:14 logs/b.log
-rw-rw-r--. 1 student student 0 Oct 27 12:14 logs/my app.log
a b
c d

-n 2 passes two arguments per run. For deleting files you’ll usually use find’s own -delete after a dry run with -print (Lesson 25), but xargs is what you reach for when the next command isn’t find.

✅ Checkpoint: The -print0 / -0 version lists all three files, including my app.log.

10Command substitution and quoting

Paste this whole block:

name="Priya"
echo "Hello $name"
echo 'Hello $name'
echo "Cost: \$5"
count=$(wc -l < servers.txt)
echo "$count servers, kernel $(uname -r)"
v="a   b"
echo $v
echo "$v"

You should see something like this:

Hello Priya
Hello $name
Cost: $5
3 servers, kernel 6.12.0-211.16.1.el10_2.0.1.x86_64
a b
a   b

Double quotes expand, single quotes don’t, and a backslash protects one character. $( ) dropped the output of two commands into the sentence. The last pair is the one that matters most: without quotes, bash split $v into two words and echo joined them with one space. That same splitting is what turns a file called “my app.log” into two files in a script.

✅ Checkpoint: echo “$v” keeps the three spaces and echo $v does not.