Code-Memo

Grep

grep is a powerful command-line utility in Linux used for searching plain-text data sets for lines that match a regular expression, and its name stands for “global regular expression print.”

Basic Syntax

grep [OPTIONS] PATTERN [FILE...]

• PATTERN: The regular expression or string to search for.
• FILE: The file or files to search within. If no file is specified, grep searches the standard input.

Commonly Used Options

• -i: Ignore case distinctions in both the PATTERN and the input files.
• -v: Invert the sense of matching, to select non-matching lines.
• -r or -R: Read all files under each directory, recursively.
• -n: Prefix each line of output with the line number within its input file.
• -l: Print only the names of files with at least one matching line.
• -c: Print only a count of matching lines per input file.
• -H: Print the file name for each match.

Examples

1. Basic Search

   grep "hello" file.txt
   

   Searches for the string “hello” in file.txt.

2. Case-Insensitive Search

   grep -i "hello" file.txt
   

   Searches for “hello” in a case-insensitive manner in file.txt.

3. Search Recursively

   grep -r "hello" /path/to/directory
   

   Searches for “hello” in all files under /path/to/directory.

4. Count Matching Lines

   grep -c "hello" file.txt
   

   Counts the number of lines that contain “hello” in file.txt.

5. Display Line Numbers

   grep -n "hello" file.txt
   

   Displays matching lines along with their line numbers in file.txt.

6. Search for Whole Words

   grep -w "hello" file.txt
   

   Searches for lines containing the whole word “hello” in file.txt.

7. Invert Match

   grep -v "hello" file.txt
   

   Displays lines that do not contain “hello” in file.txt.

8. Multiple Files

   grep "hello" file1.txt file2.txt
   

   Searches for “hello” in both file1.txt and file2.txt.

Using Regular Expressions

grep supports extended regular expressions (ERE) with the -E option, which allows for more complex pattern matching.

grep -E "hello|world" file.txt

This searches for lines containing either “hello” or “world” in file.txt.

Piping and Redirection

grep is often used in combination with other commands using pipes.

cat file.txt | grep "hello"

This searches for “hello” in the output of cat file.txt.

Grep in Scripts

grep is frequently used in shell scripts for processing and filtering text data. Here’s a simple example:

#!/bin/bash
# Script to find and count occurrences of "error" in log files
for file in /var/log/*.log; do
    echo "Processing $file"
    grep -c "error" "$file"
done