Mastering Grep and Replace: A Comprehensive Guide

Introduction

grep is a command commonly used for string searching, but it is also frequently used in combination with replacement.

On the other hand, since grep alone cannot perform replacement, it is important to use it together with commands such as sed, find, and xargs.

This article explains everything in detail, from the basics to more advanced applications.

Reference: GNU grep

Basic syntax for grep replace

Creating a file

cat << 'EOF' > input.txt
apple
banana
apple pie
grape
pineapple
EOF

Command to run

grep "apple" input.txt

Result

apple
apple pie
pineapple

Command to run

grep "apple" input.txt | sed 's/apple/orange/g'

Result

orange
orange pie
pineorange

How it works

Item	Description
Command	grep
Role	Searches for and displays lines that match a pattern
Can it replace?	No (grep alone cannot replace)
How to replace	Use it together with a replacement command such as sed

Explanation

grep is a search-only command and has no string replacement feature.
So when people refer to "grep replace," they generally mean passing the results found by grep to sed for replacement.

How to bulk replace a specific string with grep replace

Creating a file

cat << 'EOF' > input.txt
apple
banana
apple pie
orange
apple juice
EOF

Command to run

grep -rl "apple" . | xargs sed -i 's/apple/orange/g'

Result

no output

Command to run

cat input.txt

Result

orange
banana
orange pie
orange
orange juice

How it works

Element	Description
grep -rl "apple" .	Recursively searches for file names containing apple
xargs	Passes the resulting file names to sed
sed -i	Edits the file directly (in-place editing)
s/apple/orange/g	Replaces every instance of apple with orange

Explanation

grep itself has no replacement feature, but combining it with sed lets you efficiently bulk-replace only the files that were found. This combination is commonly used for grep replace.

How to use regular expressions with grep replace

Creating a file

cat << 'EOF' > input.txt
apple
banana
grape
pineapple
Apple
EOF

Command to run

grep -E 'apple|grape' input.txt

Result

apple
grape
pineapple

Command to run

grep -E '^apple$|^grape$' input.txt

Result

apple
grape

How it works

Item	Description
grep	A command that searches a file for lines matching a condition
-E	Option that enables extended regular expressions (ERE)
apple|grape	Matches apple or grape (OR condition)
^	Anchor representing the start of a line
$	Anchor representing the end of a line
^apple$	Matches only when the entire line is exactly apple

Explanation

grep has no replacement feature, but it allows flexible string searches using regular expressions.
When you need to perform replacement, it's common to combine it with a command such as sed or perl.

Replacing with capture groups in grep replace

Creating a file

cat << 'EOF' > input.txt
name: taro
name: hanako
name: jiro
EOF

Command to run

grep '^name:' input.txt | sed -E 's/^name: (.*)$/user=\1/'

Result

user=taro
user=hanako
user=jiro

How it works

Item	Description
grep '^name:'	Extracts only lines starting with name:
sed -E	Enables extended regular expressions
(.*)	Captures the text after name: as a capture group
\1	Expands the first capture group into the replacement text
s/search/replace/	Replaces the text matched by the regular expression

Explanation

Here, grep extracts the target lines, and sed's capture group ( ) and \1 are used to reuse the captured value while performing the replacement.
Since grep alone cannot replace, it is generally combined with a replacement command such as sed.

How to replace only matching lines with grep replace

Creating a file

cat << 'EOF' > input.txt
apple
banana
apple pie
orange
apple juice
EOF

Command to run

grep 'apple' input.txt | sed 's/apple/APPLE/'

Result

APPLE
APPLE pie
APPLE juice

Command to run

sed '/apple/s/apple/APPLE/' input.txt

Result

APPLE
banana
APPLE pie
orange
APPLE juice

How it works

Command	How it works	Characteristics
grep 'apple' input.txt | sed 's/apple/APPLE/'	grep extracts only the matching lines, then sed replaces the piped output	Only matching lines are output
sed '/apple/s/apple/APPLE/' input.txt	sed performs the replacement only on lines containing apple	Replaces only the matching lines while keeping all the original lines

Explanation

If you only want to replace the lines extracted by grep, pipe them to sed.
If you want to keep the original file structure while replacing only the matching lines, using sed's address specification (/pattern/) is convenient.

How to bulk replace across multiple files with grep replace

Creating a file

mkdir -p sample

Creating a file

cat << 'EOF' > sample/file1.txt
grep is useful
Search with grep
EOF

Creating a file

cat << 'EOF' > sample/file2.txt
Learn grep
grep is a search command
EOF

Command to run

grep -rl "grep" sample | xargs sed -i 's/grep/rg/g'

Result

no output

Command to run

grep -R "rg" sample

Result

sample/file2.txt:Learn rg
sample/file2.txt:rg is a search command
sample/file1.txt:rg is useful
sample/file1.txt:Search with rg

How it works

Item	Description
grep -rl "grep" sample	Recursively retrieves only file names containing "grep"
xargs	Passes the retrieved file names to the following command
sed -i	Edits files directly and replaces the string
s/grep/rg/g	Replaces every "grep" on each line with "rg"

Explanation

grep extracts the target files, and sed performs the replacement, allowing you to bulk-update multiple files.
Because the target files can be narrowed down, this lets you replace strings safely and efficiently.

How to target only a specific file extension with grep replace

Creating a file

mkdir -p src docs

Creating a file

cat << 'EOF' > src/main.py
grep foo
EOF

Creating a file

cat << 'EOF' > src/app.py
grep bar
EOF

Creating a file

cat << 'EOF' > src/index.js
grep baz
EOF

Creating a file

cat << 'EOF' > src/util.py
grep hoge
EOF

Creating a file

cat << 'EOF' > docs/readme.md
grep fuga
EOF

Command to run

grep -r 'grep' src/*.py docs/*.py | sed 's/grep/replace/g'

Result

grep: docs/*.py: No such file or directory
src/app.py:replace bar
src/main.py:replace foo
src/util.py:replace hoge

How it works

Item	Description
mkdir -p src docs	Creates the src and docs directories (no error if they already exist)
src/*.py docs/*.py	Uses shell globbing to target only .py files
grep -r 'grep'	Searches the specified .py files for lines containing grep
sed 's/grep/replace/g'	Replaces grep with replace on the extracted lines
Scope restriction	src/index.js and docs/readme.md don't match *.py, so they are excluded from the search and replacement

Explanation

Using the *.py glob pattern lets you limit the search target to only Python files.
This means you can safely perform the replacement without worrying about files with other extensions.

How to specify exclusion conditions with grep replace

Creating a file

cat << 'EOF' > input.txt
apple
banana
orange
grape
apple pie
banana split
orange juice
grape soda
EOF

Command to run

grep -v "apple" input.txt | sed 's/banana/BANANA/g'

Result

BANANA
orange
grape
BANANA split
orange juice
grape soda

How it works

Item	Description
grep -v "apple"	Excludes lines containing apple (specifies the exclusion condition)
sed 's/banana/BANANA/g'	Replaces banana with BANANA in only the remaining lines
Pipe |	Passes grep's output to sed for continuous processing

Explanation

Since grep itself has no string replacement feature, exclusion conditions are specified with grep -v while the replacement is done with sed.

This allows you to safely replace strings after excluding specific lines.

How to specify multiple conditions (AND / OR) with grep replace

Creating a file

cat << 'EOF' > input.txt
apple red
apple green
banana yellow
banana green
orange orange
grape green
apple yellow
EOF

Command to run

grep -E 'apple|banana' input.txt | sed 's/apple/APPLE/g; s/banana/BANANA/g'

Result

APPLE red
APPLE green
BANANA yellow
BANANA green
APPLE yellow

Command to run

grep 'apple' input.txt | grep 'green' | sed 's/apple/APPLE/g; s/green/GREEN/g'

Result

APPLE GREEN

Command to run

grep 'apple' input.txt | grep 'yellow' | sed 's/apple/APPLE/g; s/yellow/YELLOW/g'

Result

APPLE YELLOW

Command to run

grep -E 'apple|banana' input.txt | grep 'green' | sed 's/apple/APPLE/g; s/banana/BANANA/g; s/green/GREEN/g'

Result

APPLE GREEN
BANANA GREEN

Command to run

sed -i -e 's/apple/APPLE/g' -e 's/banana/BANANA/g' input.txt

Result

no output

Command to run

cat input.txt

Result

APPLE red
APPLE green
BANANA yellow
BANANA green
orange orange
grape green
APPLE yellow

How it works

Condition	Example command	How it works
OR search + replace	grep -E 'apple|banana' input.txt | sed 's/apple/APPLE/g; s/banana/BANANA/g'	Uses grep's -E option to perform an OR search with a regular expression, then replaces the matching lines with sed.
AND search + replace	grep 'apple' input.txt | grep 'green' | sed 's/apple/APPLE/g'	Chains multiple grep commands with a pipe, and only the lines matching all conditions are changed by sed.
OR + AND search + replace	grep -E 'apple|banana' input.txt | grep 'green' | sed 's/apple/APPLE/g; s/banana/BANANA/g; s/green/GREEN/g'	Narrows down the target with an OR condition, then further narrows it with an AND condition before replacing with sed.
Direct file replacement	sed -i -e 's/old/new/g' file	With the -i option, the changes are written to the original file instead of standard output.

Explanation

Since grep is a search-only command, it has no replacement feature.

You can achieve "grep replace" by building the search condition with grep and passing the replacement processing to sed.

How to combine grep replace with the find command

Creating a file

mkdir -p src docs

Creating a file

cat << 'EOF' > src/main.txt
This is a grep replacement sample.
EOF

Creating a file

cat << 'EOF' > src/test.txt
Run grep replacement.
EOF

Creating a file

cat << 'EOF' > docs/readme.md
Explanation of grep replacement.
EOF

Command to run

find . \( -path "./src/*" -o -path "./docs/*" \) -type f -exec grep -H "replacement" {} \;

Result

./docs/readme.md:Explanation of grep replacement.
./src/main.txt:This is a grep replacement sample.
./src/test.txt:Run grep replacement.

Command to run

find . \( -path "./src/*" -o -path "./docs/*" \) -type f -exec sed -i 's/replacement/search and replace/g' {} \;

Result

no output

Command to run

find . \( -path "./src/*" -o -path "./docs/*" \) -type f -exec grep -H "search and replace" {} \;

Result

./docs/readme.md:Explanation of grep search and replace.
./src/main.txt:This is a grep search and replace sample.
./src/test.txt:Run grep search and replace.

How it works

Element	Description
find .	Searches recursively under the current directory
-path "./src/*" -o -path "./docs/*"	Targets only files under src or docs
-type f	Targets only files
-exec grep "replacement" {} \;	Searches each found file for the string
-exec sed -i 's/replacement/search and replace/g' {} \;	Bulk-replaces the string in each found file
{}	Holds the file name found by find
\;	Marks the end of -exec

Explanation

By narrowing down target files with find, searching with grep, and replacing with sed, you can efficiently process multiple files.

Using -exec lets you run any command against each file that was found.

How to bulk replace using xargs with grep replace

Creating a file

cat << 'EOF' > input.txt
apple
orange
apple
grape
apple
EOF

Command to run

grep -rl "apple" . --include="*.txt" | xargs sed -i 's/apple/banana/g'

Result

no output

Command to run

cat input.txt

Result

banana
orange
banana
grape
banana

How it works

Element	Role
grep -rl "apple" .	Recursively searches for only file names containing apple
--include="*.txt"	Limits the target to only .txt files
xargs	Passes the list of files retrieved by grep to the following command
sed -i 's/apple/banana/g'	Bulk-replaces apple with banana inside each file
cat input.txt	Checks the content after replacement

Explanation

By using grep to extract only the target files and passing that list to sed via xargs, you can perform bulk replacement efficiently.

Even in an environment with a large number of files, you avoid processing unnecessary files, so it runs fast.

How to preview replacement results with grep replace (dry run)

Creating a file

cat << 'EOF' > input.txt
apple
banana
apple pie
grape
pineapple
EOF

Command to run

grep 'apple' input.txt

Result

apple
apple pie
pineapple

Command to run

grep 'apple' input.txt | sed 's/apple/orange/g'

Result

orange
orange pie
pineorange

Command to run

sed 's/apple/orange/g' input.txt

Result

orange
banana
orange pie
grape
pineorange

Command to run

cat input.txt

Result

apple
banana
apple pie
grape
pineapple

How it works

Command	Role	File change
grep 'apple' input.txt	Extracts only the lines to be replaced	None
grep 'apple' input.txt | sed 's/apple/orange/g'	Previews the replacement result for the extracted lines only	None
sed 's/apple/orange/g' input.txt	Displays the replacement result for the entire file to standard output	None
cat input.txt	Confirms that the original file has not been changed	None

Explanation

Since grep lets you check the target lines beforehand, you can use it as a dry run before actually performing the replacement.
Combined with sed, you can safely check the results of "grep replace" without modifying the file.

Causes and fixes when grep replace doesn’t work

Creating a file

cat << 'EOF' > input.txt
apple
banana
grape
EOF

Command to run

grep "apple" input.txt | sed 's/apple/orange/'

Result

orange

Command to run

grep "orange" input.txt | sed 's/orange/apple/'

Result

no output

Command to run

sed -i 's/apple/orange/' input.txt

Result

no output

Command to run

cat input.txt

Result

orange
banana
grape

How it works

Item	Description
Cause	grep is a search-only command and has no feature to rewrite files (no replacement feature).
Why it can't replace	grep only prints matching lines to standard output; it does not change the input file.
Fix	Replace the standard output with sed 's/search string/replace string/'.
How to replace the file directly	Use sed -i 's/search string/replace string/' filename.
Role of grep	Searches for and extracts the target string.
Role of sed	Performs string replacement/editing.

Explanation

Since grep is a search-only command, it cannot replace strings on its own.
If you want to change a string, you need to combine it with a text-editing command such as sed or perl.

How to automate grep replace with a shell script

Creating a file

cat << 'EOF' > input.txt
apple
banana
apple
orange
EOF

Creating a file

cat << 'EOF' > replace.sh
#!/bin/bash

# Search with grep and replace with sed
TARGET="apple"
REPLACE="grape"

grep -n "$TARGET" input.txt

sed -i "s/$TARGET/$REPLACE/g" input.txt

echo "After replacement:"
cat input.txt
EOF

Command to run

chmod +x replace.sh

Result

no output

Command to run

./replace.sh

Result

1:apple
3:apple
After replacement:
grape
banana
grape
orange

How it works

Process	Command used	Description
Search for the target string	grep	Checks the file for the string to be replaced
Replacement	sed	Changes the specified string to another string
Automation	Shell script	Runs the grep search and sed replacement as a single sequence
Granting execute permission	chmod	Makes the script directly executable

Explanation

By checking the target string with grep and turning the sed replacement into a shell script, you can automate string replacement work across a large number of files.
Using this approach lets you repeat the same replacement process, reducing work time and preventing input mistakes.

Key points on grep replace

grep replace is an efficient way of working that combines searching and replacing.

Once you understand the basic syntax, learning regular expressions, multiple conditions, and how to combine grep with find and xargs will let you edit even large numbers of files quickly.

By learning step by step from the basics, even beginners can acquire practical command-line skills.