The Linux Command Tutorial series provides rigorous, upstream-verified references for essential system commands across Linux distributions and UNIX-like environments. Each article focuses on a single executable, combining exhaustive option documentation, verified real-world examples, security boundaries, and best practices directly derived from official source documentation and POSIX standards.

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1. Introduction

Upstream: GNU Sed 4.9 | POSIX: POSIX.1-2024 (with GNU extensions) | Safety Tier: unprivileged-filesystem-write | Scope: Stream editing, in-place regex replacement (-i) & text transformations

sed (stream editor) is a non-interactive text editor that processes input streams line by line according to an execution cycle. It applies user-defined commands (such as substitutions, deletions, insertions, and branch logic) to an internal pattern buffer, writing transformed results to standard output or updating files in place.

• Upstream Project & Provenance: Developed and maintained under GNU Sed (sed).
• Portability & Standards Baseline: Standardized in IEEE Std 1003.1-2024 (POSIX.1-2024). GNU sed introduces in-place editing (-i), extended regular expressions (-E), zero-terminated streams (-z), and case-insensitive matching flags (/I).
• Target Research Implementation: Audited against GNU Sed 4.9 (sed(1)).
• Applicability & Lifecycle: The standard command for programmatic text transformations, automated configuration editing, and stream sanitization in Linux.

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2. Syntax and Command Model

2.1 Canonical Synopsis

sed [OPTION]... {script-only-if-no-other-script} [input-file]...
sed [OPTION]... -e script... [input-file]...
sed [OPTION]... -f script-file... [input-file]...

2.2 Execution Cycle: Pattern Space vs Hold Space

sed operates by executing an infinite loop over each input line:

1. Read: Reads a line from the input stream, strips the trailing newline, and places it into the Pattern Space (temporary working buffer).
2. Execute: Evaluates commands in order. If an address matches (e.g. /regex/ or line_num), the command executes on the pattern space.
3. Output: By default, prints the pattern space contents followed by a newline, unless -n (quiet mode) is specified.
4. Hold Space: A secondary persistent buffer used for multi-line accumulators, swapping data via h (hold), H (append to hold), g (get from hold), and x (exchange).

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3. Options

3.1 Primary Operational Flags

Short Flag	Long Flag	Description	POSIX Defined	Default
-n	--quiet, --silent	Suppress automatic printing of pattern space.	Yes	Auto-print
-e script	--expression=script	Add script commands to be executed.	Yes	N/A
-f file	--file=script-file	Add script commands contained in file.	Yes	N/A
-i[SUFFIX]	--in-place[=SUFFIX]	Edit files in place (creates backup if SUFFIX supplied).	No	Standard output
-E, -r	--regexp-extended	Use extended regular expressions (ERE).	Yes (in 2024)	Basic (BRE)
-s	--separate	Treat files as separate rather than a single continuous stream.	No	Off
-z	--zero-terminated	Separate lines by NUL characters (\0).	No	Off

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4. Basic Usage

4.1 Quick Reference & Common Invocations

Task / Scenario	Command	Key Flags / Behavior
First match substitution	sed 's/foo/bar/' file.txt	Replaces first occurrence per line
Global substitution	sed 's/foo/bar/g' file.txt	Replaces all occurrences per line
In-place edit with backup	sed -i.bak 's/old/new/g' config.conf	Modifies file and saves config.conf.bak
Filter and print matches only	sed -n '/ERROR/p' app.log	-n suppresses auto-print; p prints matches
Delete matching lines	sed '/^#/d' config.conf	d drops lines matching pattern
Delete blank lines	sed '/^$/d' input.txt	Deletes empty lines
Print line number range	sed -n '10,20p' file.txt	Prints lines 10 through 20
Replace using custom delimiter	sed 's|/var/www|/srv/www|g' file	Avoids escaping / forward slashes
Extended regex substitution	sed -E 's/([0-9]+)/[\1]/g' file	-E supports modern regex groupings without \(

4.2 Simple String Substitution

echo "Server status: offline" | sed 's/offline/online/'

Sample terminal output:

Server status: online

4.3 Suppressing Default Output with -n and /p

Printing only lines that match a regular expression:

sed -n '/ERROR/p' /var/log/syslog

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5. Practical Operations

5.1 Safe In-Place Configuration Modification with Backup

Updating a configuration directive in /etc/default/ufw with a timestamped backup:

sed -i.bak 's/^IPV6=no/IPV6=yes/' /etc/default/ufw

• Technical Analysis: -i.bak writes changes to /etc/default/ufw while saving the pre-modified original file as /etc/default/ufw.bak.

5.2 Global Substitution with Custom Delimiters

When replacing paths with slashes, escaping every / with \/ is prone to errors. sed allows using alternative delimiters (such as | or #):

sed 's|/var/www/html|/srv/www/public|g' nginx.conf

5.3 Deleting Commented and Blank Lines

Filtering out comments and whitespace lines:

sed -E '/^[[:space:]]*(#|$)/d' /etc/redis/redis.conf | head -n 5

• Technical Analysis: Matches lines where the first non-whitespace character is # or the end of the line ($), and executes the d (delete) command.

5.4 Slicing Specific Line Ranges

Printing lines 15 through 25 of a file:

sed -n '15,25p' /etc/passwd

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6. Advanced Usage

6.1 Multi-Line Join via Pattern Space Branching

Joining two lines when a line ends with a backslash:

cat <<'EOF' | sed -E ':a; /\\$/ { N; s/\\\n//; ta }'
HOSTS="node1 \
node2 \
node3"
EOF

Sample terminal output:

HOSTS="node1 node2 node3"

• :a: Defines branch label a.
• N: Appends the next line to pattern space.
• s/\\\n//: Strips the backslash and newline.
• ta: Loops back to label a if the substitution succeeded.

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7. Exit Status, Environment, and Configuration

7.1 Exit Status Codes

Exit Code	Meaning
0	Success: all scripts executed cleanly.
>0	An error occurred (syntax error in script, file unreadable, write failure in -i).

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8. Safety, Security, and Portability

8.1 Critical Portability Trap: GNU sed -i vs macOS/BSD sed -i

WARNING

In-Place Flag Incompatibility Hazard:

• GNU sed: The backup extension argument is optional (sed -i 's/foo/bar/' file works without backup; sed -i.bak creates a backup).
• BSD/macOS sed: The backup argument is mandatory. Passing sed -i 's/foo/bar/' file on macOS treats 's/foo/bar/' as the backup extension and file as the script, causing errors or corrupted files. On BSD/macOS, an explicit empty string sed -i '' 's/foo/bar/' file is required.

8.2 In-Place Editing Inode Replacement

IMPORTANT

When sed -i modifies a file, it creates a temporary file in the same directory and renames it over the original file. This changes the file's inode number, breaking existing hard links and replacing symlinks with regular files unless --follow-symlinks is passed.

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9. Best Practices

1. Always Supply an Extension to -i in Production:

   IMPORTANT

   Guidance: Write sed -i.bak '...' file when modifying production configurations. Authoritative Justification: Provides an instantaneous rollback file in the event of an erroneous regex match.

2. Use Alternative Delimiters (| or #) for Paths:

   TIP

   Guidance: Avoid escaping slashes; write s|old|new| instead of s/\/old/\/new\/. Authoritative Justification: GNU documentation notes that any single character can serve as the delimiter in s commands.

3. Use -E for Modern Readable Regular Expressions:

   TIP

   Guidance: Pass -E to avoid backslash escaping on (, ), +, and {}. Authoritative Justification: Standardized in POSIX.1-2024 and natively supported across all modern versions of GNU and BSD sed.

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References

1. GNU Sed Manual: https://www.gnu.org/software/sed/manual/sed.html
2. POSIX.1-2024 sed Specification: The Open Group Base Specifications Issue 8. https://pubs.opengroup.org/onlinepubs/9799919799/utilities/sed.html