M9 — Redirection, pipes, and quoting

Prerequisites#

• M3–M5 (files and paths) and M8 (expansions help you read examples)
• Comfort running simple commands and reading their on-screen output

Learning objectives#

By the end of this module you will be able to:

1. Redirect standard output with > and >>
2. Redirect standard error with 2> and merge streams with 2>&1
3. Feed a command from a file with <
4. Chain commands with pipes | and split a stream with tee
5. Choose single quotes, double quotes, or backslash escapes so the shell does what you mean

Three streams every process has#

Redirection and pipes reconnect those streams to files or other programs.

mkdir -p /tmp/linux-lab-pushpjeet/redirect
cd /tmp/linux-lab-pushpjeet/redirect

Redirecting stdout: > and >>#

Overwrite a file with stdout:

ls /tmp/linux-lab-pushpjeet > listing.txt
cat listing.txt

Example output written to the file:

brace-demo
globs
redirect
vim-lab

(Your directory listing will reflect whatever you already created.)

Append instead of overwriting:

echo "append line" >> listing.txt
cat listing.txt

Danger: > truncates the target immediately. Mistyping > instead of >> can wipe a file you cared about. Prefer >> when you mean “add to the log”.

Redirecting stderr: 2>#

Errors use stream 2:

ls /no/such/path 2> errors.txt
cat errors.txt

Real output captured in errors.txt:

ls: cannot access '/no/such/path': No such file or directory

The terminal stays quiet because stderr went to the file.

Merging stderr into stdout: 2>&1#

Often you want both streams in one place (a log, or a pipe):

ls /tmp /no/such 2>&1 | head -5

Example:

ls: cannot access '/no/such': No such file or directory
/tmp:
...

Order matters. A common pattern to write everything to one file:

{ echo out; echo err >&2; } > both.txt 2>&1
cat both.txt

out
err

Read 2>&1 as “make stderr point at whatever stdout currently points at.”

stdin from a file: <#

printf 'alpha\nbeta\ngamma\n' > words.txt
wc -l < words.txt

Many commands also accept a filename argument (wc -l words.txt). The < form feeds stdin — useful for tools that only read stdin, or when you are building pipelines.

Pipes |#

A pipe connects stdout of the left command to stdin of the right:

printf 'lineA\nlineB\n' | tr 'a-z' 'A-Z'

LINEA
LINEB

Build filters step by step:

ls /tmp/linux-lab-pushpjeet | sort | head -n 5

Each stage does one job. That is the UNIX style you will use forever.

tee — see it and save it#

tee copies stdin to a file and passes it through to stdout:

printf 'lineA\nlineB\n' | tee captured.txt
cat captured.txt

lineA
lineB

(and the same lines are on the screen when you run the pipe). Append with tee -a.

Quoting: single, double, and backslash#

The shell treats spaces and special characters specially. Quotes protect them.

echo 'literal $HOME'
echo "expanded $HOME"
echo "escaped: \$HOME"

Real lab output:

literal $HOME
expanded /home/box
escaped: $HOME

Filenames with spaces need quotes:

echo 'hello' > "my notes.txt"
cat "my notes.txt"

Without quotes, the shell splits on spaces and you get the wrong arguments.

Combining ideas#

Save a directory listing and its errors together, then search:

ls /tmp/linux-lab-pushpjeet /missing 2>&1 | tee ls-mixed.log | grep -i 'cannot\|vim' || true

You now have a log file and a filtered view.

Common mistakes#

1. > when you meant >> — truncated log or config. Pause before overwrite redirects.
2. Redirecting only stdout and wondering where the error went — use 2> or 2>&1.
3. Wrong order: >file 2>&1 vs 2>&1 >file — learn one reliable pattern: cmd >file 2>&1.
4. Unquoted spaces — cat my notes.txt looks for two files named my and notes.txt.
5. Piping after a redirect that already emptied stdout — sketch the stream path on paper once; it clicks.

Hands-on lab#

mkdir -p /tmp/linux-lab-pushpjeet/m9-lab
cd /tmp/linux-lab-pushpjeet/m9-lab

1. Run date > now.txt and date >> now.txt. Confirm two lines with cat -n now.txt.
2. Run ls /nope 2> err.txt and read err.txt.
3. Run ls /tmp /nope 2>&1 | tee mixed.txt | wc -l.
4. Create "weekly report.txt" with a one-line message using quotes.
5. Pipe echo 'linux from scratch' | tr 'a-z' 'A-Z' | tee shout.txt.

Verify: now.txt has two timestamps; mixed.txt contains both listing and error text; shout.txt is uppercase.

Practice: check your understanding#

Multiple choice#

1. Which operator appends stdout to a file?

• A. >
• B. >>
• C. 2>
• D. |

2. What does 2>&1 do?

• A. Duplicate stdin
• B. Send stdout to stderr only
• C. Point stderr at the same place as stdout
• D. Open file descriptor 21

3. In ls | grep vim, what does grep read?

• A. The keyboard only
• B. A file named ls
• C. The stdout of ls as its stdin
• D. stderr of ls

4. Which echo keeps $HOME as literal text?

• A. echo "$HOME"
• B. echo '$HOME'
• C. echo $HOME
• D. echo $(HOME)

5. What is tee for?

• A. Terminating pipes
• B. Writing stdin to a file while still printing it
• C. Replacing > always
• D. Editing files in place like vim

Match the columns#

Fill in the blank#

1. Save errors only: make 2> build.err
2. Save everything: make > build.log 2>&1
3. Uppercase and save: echo hi | tr a-z A-Z | tee hi.txt
4. Quoted file: cat "my notes.txt"

Order the steps#

Build a pipeline that lists a directory, saves the full stream (including errors), and counts lines:

1. Append | wc -l
2. Run ls /tmp /missing
3. Add 2>&1
4. Add | tee full.log

Mini terminal challenge#

Goal: Produce a small log with both success and failure lines.

Setup

mkdir -p /tmp/linux-lab-pushpjeet/m9-challenge
cd /tmp/linux-lab-pushpjeet/m9-challenge

Tasks

1. Create three empty files with touch a.txt b.txt c.txt.
2. Run a command that lists them and also tries a missing path; merge streams; tee to run.log.
3. grep -i cannot run.log to show the error line.
4. Create a file named team notes.txt (with a space) and cat it using correct quoting.

Verify: run.log exists; grep finds the missing-path message; cat "team notes.txt" works.

Stretch: Append a second run to run.log with tee -a.

Next: M10 — Variables, history, and line editing — customise your shell session and reuse commands.

Continue

← Linux from Scratch hub · Cheat sheet · All tutorials