Mastering grep: How to Search Multiple Files

Introduction

grep is a representative command for performing string searches on Linux and macOS.

It is especially useful because it can search across multiple files at once, making it valuable for log analysis, source code investigation, and many other situations.

This article explains, in an easy-to-understand way, everything from the basic syntax for searching multiple files with grep, how to specify search targets, wildcard searches, display settings for search results, condition specification, recursive search, and how to handle errors.

Reference: GNU grep

Basic syntax for searching multiple files with grep

Create file

cat << 'EOF' > file1.txt
apple
banana
orange
EOF

Create file

cat << 'EOF' > file2.txt
grape
banana
melon
EOF

Create file

cat << 'EOF' > file3.txt
apple
peach
melon
EOF

Command to run

grep "banana" file1.txt file2.txt file3.txt

Result

file1.txt:banana
file2.txt:banana

Command to run

grep "apple" file*.txt

Result

file1.txt:apple
file3.txt:apple

Command to run

grep -n "melon" file1.txt file2.txt file3.txt

Result

file2.txt:3:melon
file3.txt:3:melon

How it works

Element	Description
grep	Text search command
"banana"	String to search for
file1.txt file2.txt file3.txt	Multiple target files to search
file*.txt	Specifying multiple files using a wildcard
-n	Displays the matching line number
filename:content	When searching multiple files, the matching filename is also displayed

Explanation

grep can search multiple files at the same time by specifying the target files separated by spaces.
Using a wildcard (*) lets you search a large number of files all at once.

How to specify search targets for multiple files with grep

Create file

cat << 'EOF' > sample1.txt
Linux
grep command
error log
EOF

Create file

cat << 'EOF' > sample2.txt
Windows
grep multiple files
warning log
EOF

Create file

cat << 'EOF' > sample3.txt
MacOS
grep test
info log
EOF

Command to run

grep "grep" sample1.txt sample2.txt sample3.txt

Result

sample1.txt:grep command
sample2.txt:grep multiple files
sample3.txt:grep test

Command to run

grep "log" sample*.txt

Result

sample1.txt:error log
sample2.txt:warning log
sample3.txt:info log

Command to run

grep -n "grep" sample1.txt sample2.txt sample3.txt

Result

sample1.txt:2:grep command
sample2.txt:2:grep multiple files
sample3.txt:2:grep test

How it works

Item	Description
Specifying search targets	Specify multiple filenames separated by spaces
Specifying with a wildcard	Multiple files can be specified at once, such as sample*.txt
Output format	The filename is displayed at the beginning of each matching line
-n option	Also displays the line number of the match
Processing flow	grep reads and searches each specified file in order

Explanation

grep can search multiple files at the same time and shows which file the match was found in.

When dealing with a large number of files, using a wildcard makes the search more efficient.

Searching multiple files with grep using wildcard specification

Create file

cat << 'EOF' > app.log
INFO: Application started
ERROR: Database connection failed
INFO: Retry processing
EOF

Create file

cat << 'EOF' > system.log
INFO: System boot
ERROR: Disk space low
INFO: Cleanup completed
EOF

Create file

cat << 'EOF' > web.log
INFO: Request received
ERROR: Internal Server Error
INFO: Response sent
EOF

Command to run

grep "ERROR" *.log

Result

app.log:ERROR: Database connection failed
system.log:ERROR: Disk space low
web.log:ERROR: Internal Server Error

Command to run

grep -n "INFO" *.log

Result

app.log:1:INFO: Application started
app.log:3:INFO: Retry processing
system.log:1:INFO: System boot
system.log:3:INFO: Cleanup completed
web.log:1:INFO: Request received
web.log:3:INFO: Response sent

Command to run

grep -l "ERROR" *.log

Result

app.log
system.log
web.log

How it works

Element	Description
grep	Command that searches for strings inside files
"ERROR"	String to search for
*.log	Specifies multiple files ending in .log via wildcard
-n	Displays results with line numbers
-l	Displays only the matching filenames
Search process	Scans through all specified files in order and outputs matching lines

Explanation

Using a wildcard (*.log) with grep allows you to search multiple files at once. This is a convenient method commonly used for log analysis and configuration file investigation.

Displaying filenames in grep search results for multiple files

Create file

cat << 'EOF' > file1.txt
apple
banana
orange
EOF

Create file

cat << 'EOF' > file2.txt
grape
banana
melon
EOF

Create file

cat << 'EOF' > file3.txt
apple
peach
melon
EOF

Command to run

grep "banana" file1.txt file2.txt file3.txt

Result

file1.txt:banana
file2.txt:banana

Command to run

grep -H "apple" file1.txt file2.txt file3.txt

Result

file1.txt:apple
file3.txt:apple

Command to run

grep -nH "melon" file1.txt file2.txt file3.txt

Result

file2.txt:3:melon
file3.txt:3:melon

How it works

Item	Description
grep	Command that searches for a specified string
Specifying multiple files	Listing multiple filenames lets you search them all together
Filename display	When multiple files are targeted, the filename is automatically shown at the start of the search result
-H	Always displays the filename
-n	Displays the matching line number
Output format	filename:content or filename:line number:content

Explanation

When you search multiple files with grep, the filename is automatically displayed so you can tell which file the match came from.
If you want the filename displayed even for a single file, use the -H option.

Displaying line numbers in grep search results for multiple files

Create file

cat << 'EOF' > file1.txt
apple
banana
orange
grape
EOF

Create file

cat << 'EOF' > file2.txt
melon
banana
peach
apple
EOF

Create file

cat << 'EOF' > file3.txt
kiwi
lemon
banana
EOF

Command to run

grep -n "banana" file1.txt file2.txt file3.txt

Result

file1.txt:2:banana
file2.txt:2:banana
file3.txt:3:banana

Command to run

grep -Hn "apple" file1.txt file2.txt file3.txt

Result

file1.txt:1:apple
file2.txt:4:apple

How it works

Item	Description
grep	Command that searches text
Specifying multiple files	List them like file1.txt file2.txt file3.txt
-n	Displays the line number of the matching line
-H	Always displays the filename
Output format	filename:line number:matching line

Explanation

Using grep -n lets you check the line numbers of matches when searching multiple files.

Combining it with -H also shows the target filename, making it easier to identify the results.

Searching multiple files with grep without case sensitivity

Create file

cat << 'EOF' > file1.txt
Linux
Ubuntu
GREP
EOF

Create file

cat << 'EOF' > file2.txt
linux server
CentOS
grep command
EOF

Command to run

grep -i "grep" file1.txt file2.txt

Result

file1.txt:GREP
file2.txt:grep command

Command to run

grep -i "linux" file1.txt file2.txt

Result

file1.txt:Linux
file2.txt:linux server

Command to run

grep -i "grep\|linux" file1.txt file2.txt

Result

file1.txt:Linux
file1.txt:GREP
file2.txt:linux server
file2.txt:grep command

How it works

Element	Description
grep	Command that searches text
-i	Searches without distinguishing between uppercase and lowercase
file1.txt file2.txt	Searches multiple files at the same time
grep\|linux	Searches for lines matching grep or linux
filename:content	When searching multiple files, the matching filename is also shown

Explanation

The grep command can take multiple target files, and adding the -i option lets you search without distinguishing uppercase from lowercase.
Since multiple files can be searched at once, this is efficient for log investigation and checking configuration files.

Searching for multiple keywords across multiple files with grep

Create file

cat << 'EOF' > file1.txt
grep is a text search command
You can search across multiple files
EOF

Create file

cat << 'EOF' > file2.txt
grep can search multiple keywords
Extract strings from files
EOF

Create file

cat << 'EOF' > file3.txt
grep is useful for log file analysis
EOF

Command to run

grep -E 'grep|multiple|file' file1.txt file2.txt file3.txt

Result

file1.txt:grep is a text search command
file1.txt:You can search across multiple files
file2.txt:grep can search multiple keywords
file2.txt:Extract strings from files
file3.txt:grep is useful for log file analysis

Command to run

grep -E 'grep|multiple' file*.txt

Result

file1.txt:grep is a text search command
file1.txt:You can search across multiple files
file2.txt:grep can search multiple keywords
file3.txt:grep is useful for log file analysis

How it works

Element	Description
grep	Command that performs text searches
-E	Enables extended regular expressions
grep|multiple|file	| performs an OR search across multiple keywords
file1.txt file2.txt file3.txt	Multiple target files to search
file*.txt	Specifying multiple files using a wildcard

Explanation

Using grep's -E option lets you search for multiple keywords together with an OR condition.

When multiple files are specified at once, the matching lines and their filenames are displayed.

Using regular expressions to search multiple files with grep

Create file

cat << 'EOF' > file1.txt
Linux
grep
sed
EOF

Create file

cat << 'EOF' > file2.txt
awk
grep
find
EOF

Create file

cat << 'EOF' > file3.txt
docker
kubernetes
grep
EOF

Command to run

grep -E 'grep|find' file1.txt file2.txt file3.txt

Result

file1.txt:grep
file2.txt:grep
file2.txt:find
file3.txt:grep

Command to run

grep -E '^grep$' file1.txt file2.txt file3.txt

Result

file1.txt:grep
file2.txt:grep
file3.txt:grep

Command to run

grep -En 'grep|find' file1.txt file2.txt file3.txt

Result

file1.txt:2:grep
file2.txt:2:grep
file2.txt:3:find
file3.txt:3:grep

How it works

Item	Description
grep	Command that searches for strings inside files
-E	Uses extended regular expressions
-n	Displays the matching line number
grep|find	Matches either grep or find
^grep$	Matches only when the entire line is exactly "grep"
Specifying multiple files	Can specify multiple files like file1.txt file2.txt file3.txt
Filename display	When searching multiple files, the matching filename is also shown

Explanation

grep can search multiple files at once, and using -E lets you write OR conditions and other regular expressions concisely.
Since results are shown with the filename, you can immediately tell which file the match was found in.

Searching for an exact match string across multiple files with grep

Create file

cat << 'EOF' > file1.txt
apple
orange
banana
EOF

Create file

cat << 'EOF' > file2.txt
grape
apple
melon
EOF

Create file

cat << 'EOF' > file3.txt
pineapple
apple
Apple
EOF

Command to run

grep -x "apple" file1.txt file2.txt file3.txt

Result

file1.txt:apple
file2.txt:apple
file3.txt:apple

Command to run

grep -x -n "apple" file*.txt

Result

file1.txt:1:apple
file2.txt:2:apple
file3.txt:2:apple

How it works

Item	Description
grep	Command that searches text
Specifying multiple files	List them like file1.txt file2.txt file3.txt
-x	Only displays a line when the entire line exactly matches the search string
-n	Displays the matching line number
Output format	filename:content or filename:line number:content

Explanation

Searching multiple files at the same time with grep lets you check all at once which files contain the matching string.

Using the -x option restricts the search to exact matches rather than partial matches.

Searching multiple files for an AND condition with grep

Create file

cat << 'EOF' > file1.txt
server=web01
status=active
env=prod
EOF

Create file

cat << 'EOF' > file2.txt
server=web02
status=inactive
env=prod
EOF

Create file

cat << 'EOF' > file3.txt
server=web03
status=active
env=dev
EOF

Command to run

grep -l "status=active" file*.txt | xargs grep -l "env=prod"

Result

file1.txt

Command to run

grep -l "status=active" file*.txt | xargs grep "env=prod"

Result

file1.txt:env=prod

How it works

Item	Description
First grep	Extracts filenames containing status=active from multiple files
-l option	Displays only the matching filenames
xargs	Passes the result of the first grep as arguments to the next grep
Second grep	Searches the narrowed-down files for env=prod
Achieving the AND condition	Narrows down step by step using the first and second grep

Explanation

Since grep alone has no AND condition option, this is achieved by combining grep -l with xargs.
This is convenient when you want to identify target files across multiple files using multiple conditions.

Searching multiple files for a NOT condition with grep

Create file

cat << 'EOF' > file1.txt
apple
orange
banana
EOF

Create file

cat << 'EOF' > file2.txt
grape
orange
melon
EOF

Create file

cat << 'EOF' > file3.txt
apple
kiwi
peach
EOF

Command to run

grep -L "orange" file1.txt file2.txt file3.txt

Result

file3.txt

Command to run

grep -v "orange" file1.txt file2.txt

Result

file1.txt:apple
file1.txt:banana
file2.txt:grape
file2.txt:melon

Command to run

grep -L "apple" file*.txt

Result

file2.txt

How it works

Option	Mechanism	Use case
-L	Displays only the names of files that do not contain the specified string	Finding files with a NOT condition across multiple files
-v	Excludes matching lines from the output	Extracting lines using a NOT condition
file1.txt file2.txt file3.txt	Searches multiple files at the same time	grep search across multiple files

Explanation

With grep, using -L lets you easily identify "files that do not contain a string."
Also, using -v lets you extract only the lines that exclude a specific string from multiple files.

Recursively searching multiple files with grep

Create file

mkdir -p sample/app sample/log

Create file

cat << 'EOF' > sample/app/main.txt
database_host=localhost
api_endpoint=https://example.com
EOF

Create file

cat << 'EOF' > sample/app/config.txt
database_user=admin
database_password=secret
EOF

Create file

cat << 'EOF' > sample/log/app.log
INFO: application started
ERROR: database connection failed
EOF

Command to run

grep -r "database" sample

Result

sample/app/config.txt:database_user=admin
sample/app/config.txt:database_password=secret
sample/app/main.txt:database_host=localhost
sample/log/app.log:ERROR: database connection failed

Command to run

grep -rn "database" sample

Result

sample/app/config.txt:1:database_user=admin
sample/app/config.txt:2:database_password=secret
sample/app/main.txt:1:database_host=localhost
sample/log/app.log:2:ERROR: database connection failed

Command to run

grep -rl "database" sample

Result

sample/app/config.txt
sample/app/main.txt
sample/log/app.log

How it works

Option	Description
grep	Command that searches for strings
-r	Searches recursively through a directory's contents
-n	Displays the matching line number
-l	Displays only the matching filenames
"database"	Search keyword
sample	Target directory to search

Explanation

Using grep -r lets you recursively search through a directory containing multiple files.
Combining it with -n or -l lets you efficiently check just the line numbers or just the target files.

Excluding files when searching multiple files with grep

Create file

cat << 'EOT' > file1.txt
error: connection failed
info: process started
EOT

Create file

cat << 'EOT' > file2.txt
warning: disk usage high
error: timeout occurred
EOT

Create file

cat << 'EOT' > ignore.log
error: test data
EOT

Command to run

grep "error" file*.txt --exclude=file2.txt

Result

file1.txt:error: connection failed

Command to run

grep -r "error" . --include="*.txt" --exclude="file2.txt"

Result

./file1.txt:error: connection failed

How it works

Item	Description
grep	Searches for a specified string
file*.txt	Targets multiple files together for the search
--exclude=file2.txt	Excludes the specified file from the search target
-r	Searches recursively through a directory's contents
--include="*.txt"	Restricts the file type that is targeted by the search

Explanation

When searching multiple files, grep lets you exclude unwanted files from the search target by using --exclude.

This is often used when you want to exclude log files or temporary files.

Combining grep and find to search multiple files

Create file

cat << 'EOF' > file1.txt
Linux
grep command
find command
EOF

Create file

cat << 'EOF' > file2.txt
grep multiple files
search target
EOF

Create file

cat << 'EOF' > file3.txt
sample text
grep test
EOF

Command to run

find . -type f -name "*.txt" -exec grep -H "grep" {} \;

Result

./file2.txt:grep multiple files
./file3.txt:grep test
./file1.txt:grep command

Command to run

find . -type f -name "*.txt" | xargs grep -H "grep"

Result

./file2.txt:grep multiple files
./file3.txt:grep test
./file1.txt:grep command

How it works

Command element	Role
find .	Searches under the current directory
-type f	Targets only regular files
-name "*.txt"	Targets .txt files for the search
-exec grep -H "grep" {} \;	Runs grep against each file that was found
xargs grep -H "grep"	Passes find's results to grep all at once
grep -H	Displays the matching line and its filename

Explanation

By narrowing down target files with find and searching their content with grep, you can efficiently search multiple files.

When handling a large number of files, using xargs improves execution efficiency.

Saving grep search results from multiple files to another file

Create file

cat << 'DATA' > file1.txt
error: database connection failed
info: application started
DATA

Create file

cat << 'DATA' > file2.txt
warning: disk usage high
error: memory allocation failed
DATA

Create file

cat << 'DATA' > file3.txt
info: backup completed
error: network timeout
DATA

Command to run

grep "error" file1.txt file2.txt file3.txt > result.txt

Command to run

cat result.txt

Result

file1.txt:error: database connection failed
file2.txt:error: memory allocation failed
file3.txt:error: network timeout

How it works

Element	Description
grep	Searches for a specified string
"error"	Target search keyword
file1.txt file2.txt file3.txt	Multiple target files to search
> result.txt	Redirect that saves the search results to a file
cat result.txt	Displays the saved content

Explanation

grep can search multiple files at the same time, and you can save the search results to a separate file using >.
This is commonly used when extracting and managing a specific string from a large number of log files.

Common errors and solutions when searching multiple files with grep

Create file

cat << 'EOT' > file1.txt
error: database connection failed
info: application started
EOT

Create file

cat << 'EOT' > file2.txt
warning: disk usage 80%
error: timeout occurred
EOT

Create file

cat << 'EOT' > file3.txt
info: backup completed
EOT

Command to run

grep "error" file1.txt file2.txt file3.txt

Result

file1.txt:error: database connection failed
file2.txt:error: timeout occurred

Command to run

grep "error" file1.txt file2.txt file4.txt

Result

file1.txt:error: database connection failed
file2.txt:error: timeout occurred
grep: file4.txt: No such file or directory

Command to run

grep "error" *.log

Result

no output

Command to run

grep "error" file1.txt file2.txt file4.txt 2>/dev/null

Result

file1.txt:error: database connection failed
file2.txt:error: timeout occurred

How it works

Situation	Cause	Solution
No such file or directory	The specified file does not exist	Check the filename
grep: *.log: No such file or directory	No files match the wildcard	Use ls to check the target files
No results are displayed	The search string does not exist	Review the search condition
Many errors are displayed	A nonexistent file is included	Suppress standard error output with 2>/dev/null

Explanation

When searching multiple files with grep, errors can occur due to nonexistent files or failed wildcard expansion.

Checking for the file's existence beforehand, and using 2>/dev/null when necessary, allows for more efficient searching.

Key points for searching multiple files with grep

When searching multiple files with grep, it is important to first understand the basic syntax.

Building on that, making use of wildcard target specification and the options for displaying filenames and line numbers improves work efficiency and allows you to use grep in a more practical way.