Basic regex in grep in Bash Scripting

Introduction

You use grep with basic regex to find text patterns in files or outputs quickly and easily.

You want to find all lines containing a specific word in a file.

You need to search for lines starting with a certain letter or word.

You want to find lines that end with a specific character or word.

You want to find lines containing either one word or another.

You want to search for simple patterns like digits or letters in text.

Syntax

Bash Scripting

grep 'pattern' filename

The pattern is a basic regular expression (regex) that describes what you want to find.

Single quotes around the pattern prevent the shell from changing special characters.

Examples

Finds lines containing the exact word 'cat' anywhere in the file.

Bash Scripting

grep 'cat' file.txt

Finds lines that start with 'Hello'. The ^ means start of line.

Bash Scripting

grep '^Hello' file.txt

Finds lines that end with 'end'. The $ means end of line.

Bash Scripting

grep 'end$' file.txt

Finds lines containing 'cat' or 'dog'. The \| means OR in basic regex.

Bash Scripting

grep 'cat\|dog' file.txt

Sample Program

This script creates a file with some words and sentences. Then it uses grep with basic regex to find lines starting with 'cat', lines ending with 'og', and lines containing either 'cat' or 'dog'.

Bash Scripting

# Create a sample file
cat << EOF > sample.txt
cat
caterpillar
dog
catalog
end
Hello world
Hello there
EOF

# Find lines starting with 'cat'
grep '^cat' sample.txt

# Find lines ending with 'og'
grep 'og$' sample.txt

# Find lines containing 'cat' or 'dog'
grep 'cat\|dog' sample.txt

Important Notes

In basic regex for grep, the OR operator is written as \| and needs to be escaped.

Use ^ to match the start of a line and $ to match the end of a line.

Always quote your pattern to avoid shell interpreting special characters.

Summary

grep with basic regex helps you find text patterns in files easily.

Use ^ and $ to match start and end of lines.

Use \| for OR conditions in basic regex.