📘 Standard Streams

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🎯 What You Will Learn

1. Understand the three standard streams in Bash: input, output, and error.
2. Learn how Bash commands communicate through file descriptors (0, 1, 2).
3. Differentiate between stdout, stderr, and stdin clearly.
4. Redirect streams using operators like >, <, 2>, and &>.
5. Apply stream management to WordPress automation, logging, and safe error capture.
6. Implement output filtering for CLI commands like wp, tar, and grep.

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1. 5W + 1H Framework

Element	Description
What	Standard streams are the default communication channels between a process and the terminal — input, output, and error.
Why	They allow command results, logs, and errors to be separated, redirected, or combined for automation safety.
Who	Bash users, WordPress admins, and DevOps engineers managing scripts and cron jobs.
Where	Everywhere in Linux and Bash scripting (CLI tools, system daemons, or cron jobs).
When	When automating WP-CLI commands, capturing logs, or redirecting data safely to files.
How	File descriptors 0, 1, and 2 correspond to stdin, stdout, and stderr, respectively, and can be controlled with redirection operators.

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2. Prerequisites

• Completed Module 3.5 (Parameter Expansion).
• Basic familiarity with executing scripts and viewing terminal output.
• Understanding of Linux file paths and permissions.

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3. Core Syntax & Concept (Revised and Verified)

No	Syntax Formula	Type	Syntax Example	Description	Behavior / Output
1	0	Standard Input (stdin)	cat 0< file.txt	Reads data into a command from a file via descriptor 0. Equivalent to cat < file.txt.	Displays contents of file.txt on screen.
2	1	Standard Output (stdout)	echo "Hello" 1> out.txt	Redirects output (descriptor 1) to a file instead of screen.	Creates out.txt containing “Hello”.
3	2	Standard Error (stderr)	ls /root 2> error.log	Redirects error messages (descriptor 2) to error.log.	Captures permission-denied messages.
4	>	Redirect stdout (overwrite)	wp plugin list > plugins.txt	Redirects normal output to file (replaces existing).	Saves plugin list.
5	>>	Append stdout	echo "Backup done" >> log.txt	Appends output to an existing file.	Adds “Backup done” at end.
6	<	Redirect stdin	sort < unsorted.txt	Reads file as input instead of keyboard.	Outputs sorted content.
7	2>	Redirect stderr	tar -czf backup.tar.gz /missing 2> err.log	Captures error messages only.	Errors go to file.
8	2>>	Append stderr	grep TODO *.sh 2>> error.log	Appends errors (like missing files) to log.	Keeps previous logs.
9	&>	Redirect stdout + stderr	wp plugin update --all &> update.log	Combines both output and errors into one file.	Unified log.
10	1>	Explicit stdout redirect	ls /etc 1> list.txt	Same as > — explicit descriptor version.	Creates list.txt.
11	1>>	Explicit append stdout	date 1>> system.log	Appends timestamp to log file.	Adds line at end.
12	2>&1	Merge stderr → stdout	`wp core update 2>&1	tee update.log`	Combines stderr into stdout before piping.
13	/dev/null	Null device (discard)	command > /dev/null 2>&1	Discards both output and errors silently.	Used in cron jobs.
14	exec	Redirect whole script	exec > script.log 2>&1	Redirects all future output to a log file.	Persistent log.
15	read var	Capture stdin into variable	read user; echo "Hi $user"	Reads user input interactively.	Displays greeting.
16	tee	Split output stream	`ls	tee files.txt`	Displays and writes output simultaneously.
17	>&2	Manual stderr redirection	echo "Error: Invalid path" >&2	Sends custom message to stderr.	Helpful for logging errors.
18	< input > output	Dual redirection	sort < names.txt > sorted.txt	Reads from one file, writes to another.	Sorted file created.

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4. Demonstration Example

Input Script:

#!/bin/bash
wp plugin list 1> plugins.log 2> errors.log
echo "Logs written successfully."

Expected Output:

Logs written successfully.

File Output:

• plugins.log → plugin list (stdout)
• errors.log → errors (stderr) Explanation: Here, 1> explicitly redirects normal output, while 2> redirects errors. Bash automatically uses these file descriptors.

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5. Use Cases (WordPress-VPS Focused)

5.1 (Ref: #2) – Save Plugin List to File

wp plugin list 1> /var/log/wp_plugins.log

Use Case: Audit plugin lists automatically.

5.2 (Ref: #3) – Capture Backup Errors

tar -czf /home/wpbackup/backup.tar.gz /wrong/path 2> /var/log/backup_error.log

Output: (silent) Explanation: Errors stored separately for debugging.

5.3 (Ref: #9) – Combine Output and Errors

wp plugin update --all &> /var/log/wp_update.log

Use Case: Single log file for automation summary.

5.4 (Ref: #13) – Quiet Cron Jobs

wp transient delete --expired > /dev/null 2>&1

Use Case: Suppress unwanted cron job output.

5.5 (Ref: #12) – Log While Viewing

wp core update 2>&1 | tee /var/log/wp_core_update.log

Output:

Success: WordPress updated successfully.

Explanation: Displays progress and records logs simultaneously.

5.6 (Ref: #14) – Redirect Full Script to Log

#!/bin/bash
exec > /var/log/wp_daily.log 2>&1
echo "Starting daily maintenance..."
wp plugin update --all
wp theme update --all
echo "Completed at $(date)"

Output (terminal): (none) File: /var/log/wp_daily.log

5.7 (Ref: #17) – Manual Error Reporting

if [ ! -d "/var/www/html" ]; then
  echo "❌ WordPress directory not found!" >&2
fi

Output:

❌ WordPress directory not found!

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5.8 (Ref: #16) – Mirror Output to Log

wp cache flush | tee -a /var/log/wp_cache.log

Output:

Success: Object cache flushed.

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6. Best Practices

Practice	Description
Use file descriptors explicitly (1>, 2>)	Improves script readability.
Separate logs for stdout and stderr	Simplifies debugging.
Always use >> for long-term logs	Avoids overwriting.
Use /dev/null sparingly	Don’t lose critical output.
Prefer tee for monitoring	Enables real-time feedback.
Write intentional errors to >&2	Distinguishes between normal and failure output.

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7. Common Mistakes & Fixes

Mistake	Wrong Example	Correct Example	Explanation
Missing error redirection	command > log.txt	command 1> log.txt 2> err.txt	Errors remain visible.
Overwriting logs	> used repeatedly	Use >>	Appends logs safely.
Reversed merge order	2>&1 > file	> file 2>&1	Must redirect stdout first.
Unquoted paths	echo >> /path with space/file.txt	echo >> "/path with space/file.txt"	Prevents word splitting.
Losing old logs with tee	`command	tee file`	`command

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8. Quick Lab – Unified WordPress Update Logger

Script: wp_update_log.sh

#!/bin/bash
# --------------------------------------------
# Script Name : wp_update_log.sh
# Purpose     : Log both output and errors during WordPress update
# --------------------------------------------

LOG="/var/log/wp_update_$(date +%F).log"

wp plugin update --all 1>> "$LOG" 2>&1
wp theme update --all 1>> "$LOG" 2>&1

echo "✅ Updates logged to $LOG"

Expected Output:

✅ Updates logged to /var/log/wp_update_2025-10-09.log

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9. Troubleshooting Matrix

Issue	Symptom	Cause	Solution
Log empty	Nothing recorded	Wrong redirection order	Use > file 2>&1.
Old logs missing	Overwritten	Use >> for append.
Errors shown	stderr not redirected	Add 2> file.
Cron job silent	Overused /dev/null	Test before silencing output.

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10. Static vs Dynamic Framing

Framing	Description	Behavior
Static	Fixed file paths for logging.	Predictable but rigid.
Dynamic	Uses variables or timestamps.	Flexible and self-documenting.

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11. Cheat Sheet

Task	Syntax / Example
Redirect stdin	0< file.txt
Redirect stdout	1> file.txt
Redirect stderr	2> error.txt
Redirect both	&> all.log
Merge stderr → stdout	2>&1
Discard all	> /dev/null 2>&1
Split output	`command
Global redirect	exec > logfile.txt 2>&1
Manual error	echo "Error" >&2

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12. Mini-Quiz

#	Question	Answer
1	Which file descriptor represents stdin?	0
2	Which descriptor handles normal output?	1
3	Which redirects errors only?	2>
4	How to merge stdout and stderr?	2>&1
5	Which operator discards all output?	/dev/null

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