2.2 Input-Output Redirection and Pipes

Red Hat Certified System Administrator RHCSA

Track your progress as you read

Finished sections get a check mark.

1 Introduction & Exam Overview

1.1 Current Exam Facts 1.2 Lab Setup & Performance-Based Study Strategy

2 Essential Tools: Shell, Redirection & Text Analysis

2.1 Shell Prompt and Command Syntax 2.2 Input-Output Redirection and Pipes 2.3 grep and Regular Expressions for Text Analysis

3 Essential Tools: Remote Access, Users & Text Editing

3.1 Access Remote Systems Using SSH 3.2 Log In and Switch Users in Multiuser Targets 3.3 Create and Edit Text Files

4 Essential Tools: Files, Links & Archives

4.1 Create, Delete, Copy, and Move Files and Directories 4.2 Hard and Soft Links 4.3 Archive and Compress with tar, gzip, and bzip2

5 Essential Tools: Permissions & System Documentation

5.1 List, Set, and Change Standard ugo/rwx Permissions 5.2 System Documentation: man, info, and /usr/share/doc

6 Manage Software: RPM and Flatpak

6.1 Configure Access to RPM Repositories 6.2 Install and Remove RPM Software Packages 6.3 Configure Access to Flatpak Repositories 6.4 Install and Remove Flatpak Software Packages

7 Create Simple Shell Scripts

7.1 Conditional Execution: if, test, and [] 7.2 Looping Constructs for Files and Input 7.3 Process Script Inputs ($1, $2, and Beyond) 7.4 Process Shell Command Output in Scripts

8 Operate Running Systems: Boot and Recovery

8.1 Boot, Reboot, and Shut Down a System Normally 8.2 Boot Systems into Different Targets Manually 8.3 Interrupt the Boot Process to Gain Access

9 Operate Running Systems: Processes and Tuning

9.1 Identify and Kill CPU/Memory Intensive Processes 9.2 Adjust Process Scheduling 9.3 Manage Tuning Profiles

10 Operate Running Systems: Logs, Services & File Transfer

10.1 Locate and Interpret System Log Files and Journals 10.2 Preserve System Journals 10.3 Start, Stop, and Check Status of Network Services 10.4 Securely Transfer Files Between Systems

11 Configure Local Storage: Partitions and LVM

11.1 List, Create, and Delete Partitions on GPT Disks 11.2 Create and Remove Physical Volumes 11.3 Assign Physical Volumes to Volume Groups 11.4 Create and Delete Logical Volumes

12 Configure Local Storage: Mounting and Expansion

12.1 Mount File Systems at Boot by UUID or Label 12.2 Add Partitions, Logical Volumes, and Swap Non-Destructively

13 Create and Configure File Systems: Local FS and Permissions

13.1 Create, Mount, and Use VFAT, ext4, and XFS File Systems 13.2 Extend Existing Logical Volumes 13.3 Diagnose and Correct File Permission Problems

14 Create and Configure File Systems: NFS and autofs

14.1 Mount and Unmount Network File Systems Using NFS 14.2 Configure autofs

15 Deploy and Maintain: Scheduling, Services & Targets

15.1 Schedule Tasks with at, cron, and systemd Timers 15.2 Start, Stop, and Enable Services at Boot 15.3 Configure Systems to Boot into a Specific Target

16 Deploy and Maintain: Time, Software Updates & Bootloader

16.1 Configure Time Service Clients 16.2 Install and Update Software from CDN, Remote Repos, and Local Files 16.3 Modify the System Bootloader

17 Manage Basic Networking

17.1 Configure IPv4 and IPv6 Addresses 17.2 Configure Hostname Resolution 17.3 Configure Network Services to Start Automatically at Boot 17.4 Restrict Network Access Using firewalld and firewall-cmd

18 Manage Users and Groups

18.1 Create, Delete, and Modify Local User Accounts 18.2 Change Passwords and Adjust Password Aging 18.3 Create, Delete, and Modify Local Groups and Memberships 18.4 Configure Privileged Access

19 Manage Security: Firewall, umask & SSH Keys

19.1 Configure Firewall Settings with firewall-cmd and firewalld 19.2 Manage Default File Permissions 19.3 Configure Key-Based Authentication for SSH

20 Manage Security: SELinux Modes and Contexts

20.1 Set Enforcing and Permissive Modes for SELinux 20.2 List and Identify SELinux File and Process Contexts 20.3 Restore Default File Contexts

21 Manage Security: SELinux Ports and Booleans

21.1 Manage SELinux Port Labels 21.2 Use Boolean Settings to Modify SELinux Settings

2.2 Input-Output Redirection and Pipes

Key Takeaways Explain This Topic AI

Every process has three standard streams: stdin (0), stdout (1), and stderr (2); redirection operators reconnect those streams to files or other commands.

> truncates/overwrites; >> appends; 2> redirects errors; &> or >file 2>&1 merges stdout and stderr (order matters).

Pipes | send stdout of the left command to stdin of the right; use tee when you need both a file and a pipeline continuation.

Here-documents ( <<EOF ) and here-strings ( <<< ) feed multi-line or single-line stdin without temporary files—common in scripts and quick config writes.

Never confuse > with | : redirecting to a filename overwrites that file; piping connects processes.

Last updated: August 2026

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

• FD Name Default Typical content
• 0 stdin keyboard Input you type or feed
• 1 stdout terminal Normal command output
• 2 stderr terminal Errors and diagnostics

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

• Operator Meaning
• > Redirect stdout, overwrite
• >> Redirect stdout, append
• 2> Redirect stderr, overwrite
• 2>> Redirect stderr, append
• 2>&1 Send stderr to wherever stdout currently goes
• &> / &>> Redirect both streams (overwrite / append)
• < Take stdin from file
• <<DELIM Here-document until DELIM
• <<< string Here-string
• | Pipe stdout to next command
• |& Pipe stdout+stderr to next command
• tee file Duplicate stdin to file and stdout
• /dev/null Discard data written to it
• Exam Traps Specific to Redirection

Overwriting the source with command file > file .

Wrong merge order 2>&1 >file vs >file 2>&1 .

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'

Piping when you meant to save — cmd | file tries to run file as a command.

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.