Substitute

The s (substitute) command in sed is a powerful tool for search-and-replace operations on text streams. Whether you’re updating configuration files, automating code refactoring, or cleaning log files, mastering sed substitution will streamline your workflow.

By default, sed reads from stdin or an input file and writes to stdout. Use the -i option for in-place editing.

Basic Syntax
Quick Reference Commands
Practical Example: Updating a Salary
Substitution Scope
Targeting Specific Occurrences
Line Addressing
In-Place Editing (-i)
Inserting Text with i
Conclusion
References

Basic Syntax

Use the following pattern to substitute old_string with new_string:

sed 's/old_string/new_string/' input-file

s — substitute command
old_string — search pattern (regular expression supported)
new_string — replacement text
/ — delimiters (can be any character)
By default, only the first match per line is replaced.

Quick Reference Commands

Command	Description	Example
`s/original/replacement/`	Substitute first match on each line	`sed 's/foo/bar/' file.txt`
`-n '/pattern/p'`	Print only lines matching a pattern	`sed -n '/Error/p' /var/log/syslog`
`/pattern/d`	Delete lines matching a pattern	`sed '/^#/d' config.conf`

Practical Example: Updating a Salary

Given an employees.txt file:

The image shows a text file named "employees.txt" containing a list of employees with details such as name, department, job title, email, and salary.

To update Enrique Rivera’s salary from 65000 to 85000, run:

sed 's/65000/85000/' employees.txt

This command replaces the first occurrence of 65000 on each line and prints the result to stdout.

Substitution Scope

Global Replacement

Append the g flag to replace all matches in each line:

sed 's/\bIT\b/Information Technology/g' employees.txt

First-Match Only

Without g, only the first match is replaced:

sed 's/\bIT\b/Information Technology/' employees.txt

Targeting Specific Occurrences

You can replace only the nth occurrence on each line by specifying a number:

sed 's/\bIT\b/Information Technology/2' employees.txt

The above replaces only the second IT per line.

Line Addressing

Limit substitutions to certain lines or ranges:

Single line (line 7):

sed '7 s/\bHR\b/Human Resources/' employees.txt

Line range (lines 1–3):

sed '1,3 s/Finance/Taxes/' employees.txt

Another single line (line 5):

sed '5 s/\bSales\b/Transactions/' employees.txt

In-Place Editing (`-i`)

Modify the file directly using -i:

sed -i 's/\bcompany\b/KodeKloud/g' employees.txt

This updates all instances of company in employees.txt.

When using -i, changes are irreversible unless you create a backup:

sed -i.bak 's/old/new/g' file.txt

This creates file.txt.bak before editing.

Inserting Text with `i`

The i command inserts lines before the current pattern space or at a specified line:

sed '1iID|Name|FirstName|LastName|Job|Department|Email|Salary' employees.txt

Output:

ID|Name|FirstName|LastName|Job|Department|Email|Salary
1|Kriti|Shreshtha|Finance|Financial Analyst|kriti.shreshtha@KodeKloud.com|60000
2|Rajasekar|Vasudevan|Finance|Senior Accountant|rajasekar.vasudevan@KodeKloud.com|75000
...

If you omit the text after i, sed throws an error:

$ sed 'i' employees.txt
sed: -e expression #1, char 1: expected \ after 'a', 'c' or 'i'

Conclusion

In this guide, you learned how to:

Use s/old/new/ for basic substitution
Leverage g and numeric flags for global or specific replacements
Address lines and ranges for targeted edits
Apply in-place editing with -i (and backups)
Insert new lines using the i command

Together, these techniques form the foundation of efficient text processing with sed.

Introduction

Conventions

Refresher

Streams

Expansions - Part One

Globs

Expansions - Part Two

Special Shell Variables

Arrays

Good Practices applied

awk

sed

Table of Contents

Basic Syntax

Quick Reference Commands

Practical Example: Updating a Salary

Substitution Scope

Global Replacement

First-Match Only

Targeting Specific Occurrences

Line Addressing

In-Place Editing (`-i`)

Inserting Text with `i`

Conclusion

References

Watch Video

Practice Lab

​Table of Contents

​Basic Syntax

​Quick Reference Commands

​Practical Example: Updating a Salary

​Substitution Scope

​Global Replacement

​First-Match Only

​Targeting Specific Occurrences

​Line Addressing

​In-Place Editing (-i)

​Inserting Text with i

​Conclusion

​References

Watch Video

Practice Lab

Table of Contents

Basic Syntax

Quick Reference Commands

Practical Example: Updating a Salary

Substitution Scope

Global Replacement

First-Match Only

Targeting Specific Occurrences

Line Addressing

In-Place Editing (`-i`)

Inserting Text with `i`

Conclusion

References