2.2 Input-Output Redirection and Pipes

2.2 Input-Output Redirection and Pipes

Quick Answer: Use > / >> for stdout to files, 2> / 2>> for stderr, < for stdin from a file, and | to connect commands. Merge streams with command >out 2>&1 or command &>out. Append with >> when you must not destroy existing content.

The EX200 objective Use input-output redirection (>, >>, |, 2>, etc.) appears constantly: save command output as evidence, filter logs, create files non-interactively, and hide errors while capturing data. Redirection is pure bash syntax—the operators are processed by the shell, not by ls or grep themselves.

Standard Streams

Programs write “normal” results to stdout and diagnostics to stderr so you can separate them. Example:

ls /etc/passwd /no/such/file

You see a successful listing and an error. They look mixed on the terminal, but they are different file descriptors—and can be redirected independently.

Redirecting Standard Output

command > file        # overwrite/create file with stdout
command >> file       # append stdout to file
command 1> file       # explicit FD 1 (same as >)

Examples:

date > /tmp/timestamp.txt
uname -r >> /tmp/timestamp.txt
ls /etc > /tmp/etc-list.txt

Destructive trap: > truncates the target before the command runs. This classic mistake destroys data:

sort file.txt > file.txt     # BAD: truncates file.txt first; result often empty

Safe patterns:

sort file.txt > file.sorted && mv file.sorted file.txt
sort -o file.txt file.txt    # sort’s own in-place-safe output option

Redirecting Standard Error

command 2> errors.txt        # overwrite error log
command 2>> errors.txt       # append errors
command 2> /dev/null         # discard errors

Practical exam use—find files while ignoring “Permission denied” noise:

find /etc -name '*.conf' 2>/dev/null

Capture only errors:

find /etc -name '*.conf' > /tmp/found.txt 2> /tmp/find-errors.txt

Merging stdout and stderr

You often want everything in one file for a lab report or grading artifact:

command > all.txt 2>&1
command &> all.txt          # bash shorthand: both streams to all.txt
command &>> all.txt         # append both streams

Order is critical:

command > all.txt 2>&1      # correct: point stdout to file, then send stderr where stdout goes
command 2>&1 > all.txt      # WRONG intent: stderr follows old stdout (terminal), then stdout goes to file

Memorize the working form: >file 2>&1 or &>file.

Discard everything:

command > /dev/null 2>&1
command &>/dev/null

Redirecting Standard Input

command < input.txt

Examples:

wc -l < /etc/passwd
tr 'a-z' 'A-Z' < message.txt
mail -s "Report" admin@example.com < body.txt

Many commands also accept filenames as arguments (wc -l /etc/passwd). Using < forces stdin mode—useful for programs that only read stdin or when building pipelines from files.

Here-Documents and Here-Strings

Here-document feeds multi-line stdin until a delimiter:

cat > /tmp/motd <<'EOF'
Welcome to the exam lab system.
Unauthorized access is prohibited.
EOF

• <<EOF allows parameter expansion inside the body.
• <<'EOF' (quoted delimiter) treats the body literally—prefer this when pasting configs with $ characters.

Here-string feeds a single string:

grep -i error <<<"ERROR: disk full"
bc <<<"2+2"

These avoid temporary files and are faster for small content during the exam.

Pipes

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

command1 | command2 | command3

Examples:

ps aux | head -n 5
journalctl -u sshd | grep -i fail | tail -n 20
cut -d: -f1 /etc/passwd | sort | uniq
rpm -qa | grep httpd

Notes:

• By default, stderr does not go through the pipe—only stdout. To pipe errors too:

command 2>&1 | less
command |& less            # bash: pipe stdout and stderr

• Each stage runs concurrently; the kernel buffers the stream.
• Exit status of a pipeline is the status of the last command unless set -o pipefail is enabled (important in scripts; optional at interactive prompt).

tee: Save and Continue

tee writes stdin to a file and passes it to stdout:

dnf list installed | tee /tmp/packages.txt | grep kernel

Append with tee -a:

date | tee -a /var/tmp/run.log

Use tee when graders (or you) need a saved artifact and you still want to filter on screen.

Combining Operators in Real Tasks

Task pattern: inventory with clean logs

rpm -qa --qf '%{NAME}\n' 2>/dev/null | sort > /root/pkg-names.txt

Task pattern: run a command, keep full transcript

bash -x /usr/local/bin/setup.sh &> /root/setup-trace.log

Task pattern: create a config non-interactively

cat > /etc/issue <<'EOF'
RHEL 10 Lab — authorized use only
EOF

Task pattern: filter a large log for later review

grep -i error /var/log/messages 2>/dev/null | tee /root/errors.txt | wc -l

noclobber and Forced Overwrite

If set -o noclobber is active, > will not overwrite existing files:

set -o noclobber
echo hi > existing.txt      # fails if exists
echo hi >| existing.txt     # force overwrite despite noclobber

Default interactive shells on RHEL usually do not enable noclobber, but knowing >| avoids panic if a task environment sets it.

Process Substitution (Awareness)

Bash process substitution is useful though less central than basic pipes:

diff <(rpm -qa | sort) <(cat /root/baseline-pkg.txt | sort)

This feeds command output where a filename is expected. Prefer ordinary pipes for EX200 unless a comparison task needs two live streams.

Operator Quick Reference

Exam Traps Specific to Redirection

1. Overwriting the source with command file > file.
2. Wrong merge order 2>&1 >file vs >file 2>&1.
3. Forgetting root permissions when redirecting as a user: sudo ls > /root/out.txt still opens /root/out.txt as the user shell. Fix:

sudo ls | sudo tee /root/out.txt >/dev/null
sudo bash -c 'ls > /root/out.txt'

1. Piping when you meant to save—cmd | file tries to run file as a command.
2. Assuming stderr is in the pipe—add 2>&1 when searching error text that programs write to stderr.

RHEL 10 Practice Drill

Run this sequence in a lab and verify each file’s contents with cat and ls -l:

mkdir -p /tmp/io-lab && cd /tmp/io-lab
echo first > a.txt
echo second >> a.txt
ls /etc/passwd /nope > out.txt 2> err.txt
ls /etc/passwd /nope > both.txt 2>&1
cat a.txt out.txt err.txt both.txt
printf 'alpha\nbeta\ngamma\n' | tee lines.txt | grep e
wc -l < lines.txt

You should see appended lines in a.txt, only success in out.txt, only failure in err.txt, and both outcomes in both.txt.

Why This Matters for Later Objectives

Storage reports, journalctl filters, package queries, network dumps, and script debugging all depend on redirection fluency. If you still hunt for GUI “Save output” buttons, you will lose minutes on every multi-step graded item. Make operators muscle memory so cognitive load stays on the Linux problem, not on shell syntax.