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Searching for text patterns across files and directories is a common task on Linux systems. The grep command lets you quickly locate lines that match a specified string or regular expression.

Table of Common Options

Option	Description	Example
-i	Ignore case (case-insensitive search)	`grep -i 'error' logfile.txt`
-r	Recursive search in all subdirectories	`grep -r 'TODO' /home/user/projects`
-v	Invert match (show non-matching lines)	`grep -v 'DEBUG' app.log`
-w	Match whole words only	`grep -w 'user' /etc/passwd`
-o	Show only the part of a line that matches	`grep -o '[0-9]\+' data.txt`

General Syntax

grep [OPTIONS] PATTERN [FILE...]

PATTERN
The text or regular expression to search for.
FILE
One or more files or directories to scan. If no file is given, grep reads from standard input.

Note

Always quote patterns that contain spaces or shell metacharacters:

grep -i 'error message' /var/log/syslog

1. Basic, Case-Sensitive Search

By default, grep performs a case-sensitive search. To find lines containing CentOS in /etc/os-release:

$ grep 'CentOS' /etc/os-release
NAME="CentOS Stream 8"
PRETTY_NAME="CentOS Stream 8"
REDHAT_SUPPORT_PRODUCT_VERSION="CentOS Stream"

2. Case-Insensitive Search

Use -i to ignore case differences. This matches “centos”, “CentOS”, or “CENTOS” alike:

$ grep -i 'centos' /etc/os-release
NAME="CentOS Stream 8"
ID="centos"
PRETTY_NAME="CentOS Stream 8"
CPE_NAME="cpe:/o:centos:centos:8"
HOME_URL="https://centos.org/"
REDHAT_SUPPORT_PRODUCT_VERSION="CentOS Stream"

3. Recursive Search in Directories

To search through all files in a directory and its subdirectories, combine -r with your pattern:

$ grep -r 'CentOS' /etc/
/etc/centos-release:CentOS Stream release 8
/etc/yum.repos.d/CentOS-Stream-AppStream.repo:# CentOS-Stream-AppStream.repo

Combine -r and -i for a recursive, case-insensitive search:

$ grep -ir 'centos' /etc/
/etc/centos-release:CentOS Stream release 8
/etc/krb5.conf.d/kcm_default_ccache:# On Fedora/RHEL/CentOS, this is /etc/krb5.conf.d/
…

Warning

You may encounter “Permission denied” messages when scanning protected directories. Use sudo if you need elevated privileges:

sudo grep -ir 'centos' /etc/

4. Invert Match

Show only the lines that do not contain the pattern. Use the -v option:

$ grep -vi 'centos' /etc/os-release
VERSION="8"
ID_LIKE="rhel fedora"
VERSION_ID="8"
PLATFORM_ID="platform:el8"
ANSI_COLOR="0;31"
BUG_REPORT_URL="https://bugzilla.redhat.com/"
REDHAT_SUPPORT_PRODUCT="Red Hat Enterprise Linux 8"

5. Match Whole Words Only

To avoid matching substrings (e.g., “redhat” when you want “red”), add the -w flag:

$ echo -e "red Red redhat RED" | grep -iw 'red'
red
Red
RED

6. Show Only the Matched Text

By default, grep prints the entire line containing the match. Use -o to display only the portion that matches:

$ grep -oi 'centos' /etc/os-release
CentOS
centos
CENTOS
centos

You’ve now covered the essentials of grep for searching text in files. In upcoming lessons, we will explore more advanced text-processing tools like awk and sed.

Links and References

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