三剑客之 grep

Grep Command Summary

Introduction: Grep is a command-line utility on UNIX systems for searching a given pattern within files and outputting the matching lines. Created in 1974 by Ken Thompson, it is an essential tool for regular expression handling.

Basic Usage:

The basic grep format is:

grep [options] pattern [file or directory]

Main Options:

• -E: Supports extended regular expressions
• -i: Ignores case
• -n: Prints line numbers
• -o: Outputs only the matched parts of the line
• -c: Outputs the count of matching lines
• -B: Outputs n lines before the match
• -A: Outputs n lines after the match
• -C: Outputs n lines around the match
• -v: Outputs lines that do not match
• -e: Allows multiple patterns

Regular Expressions:

Using regular expressions can enhance the functionality of grep:

• *: Matches the previous character 0 or more times
• ^: Matches the start of a line
• $: Matches the end of a line
• .*: Matches any number of any characters
• [ ]: Matches any character inside the brackets
• [^ ]: Excludes any character inside the brackets
• \<: Anchors the match to the start of a word
• \>: Anchors the match to the end of a word
• {m,n}: Matches the preceding character at least m and at most n times

Examples:

Examples are based on the 'passwd.txt' file and demonstrate various grep functionalities, such as:

• Filtering out blank lines or comments
• Searching with or without case sensitivity
• Outputting only the matching content or the line number
• Showing lines before or after the matching line
• Using multiple search patterns with the -e option