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grep.md

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description
Search for text inside of files

Grep

Description

Grep is a really useful tool for quickly finding what you're looking for. If you know a file somewhere has some content, or just want to find all files with a certain pattern in them, Grep is the perfect tool for the job. It's written in C and highly optimized, meaning you can quickly search through lots of files. 

$ grep [OPTIONS...] PATTERNS [FILES...]
  • OPTIONS can be any flags to change the way the search works, or matches are displayed
  • PATTERNS are a string containing one or more patterns to search for, separated by newline characters (\n). To put a newline character in an argument you can use the $'first\nsecond' syntax
  • FILES are the files to search through for the PATTERNS. If not specified, it will read from standard input (piping into grep). If in recursive mode with -r, it will default to the current directory but can be any directory
$ grep something file.txt
And here is something.

{% hint style="info" %} See all documentation about the options with man grep {% endhint %}

Options

The are a few common and really useful options to know in Grep:

  • -r: Recursively search a directory (default: current)
  • -v: Invert search, matching lines where no match
  • -i: Search case-insensitive (uppercase/lowercase doesn't matter)
  • -n: Print the line number of the match in the file
  • -o: Only output match (no text around)
  • -a: Show all matches (also binary files)
  • -b: Show byte-offset of matches
  • -l: List files that match instead of showing the match
  • Simple regular-expressions-regex.md are enabled by default in PATTERNS
    • -F: Treat PATTERNS as fixed strings, not regular expressions
    • -P: Use perl-compatible regular expressions (PCRE) including all advanced RegEx features

Some options are also available by using egrep (-E), fgrep (-F) and rgrep (-r) to quickly set the options without having to add the flag.

{% code title="Examples" %}

# # Select files and output
$ grep -r "something"  # Search recursively in current directory for "something"
$ grep -v "something" file.txt  # Find all lines in file that don't match "something"
$ grep "something" *.txt  # Search "something" in all .txt files (current directory only)
$ grep -r "something" --include "*.txt"  # Recursivly search "something" in .txt files
$ grep -ab "something" file.bin  # Show all (binary) matches and byte-offset
$ grep -r -l "something"  # List filenames that match "something" recursively
$ grep -B2 -A5 "something" file.txt  # Show 2 lines before, and 5 lines after match

# # Patterns
$ grep -r -i "something"  # Search case-insensitively for "something"
$ grep "CTF{.*}" file.txt  # Search for flag format in file
$ grep -P "\x73\x6f\x6d\x65\x74\x68\x69\x6e\x67" file.txt  # Search for hex bytes in file
$ xxd -p file.txt | grep "aabbccdd"  # Search for hex bytes using xxd
$ grep $'first\nsecond' file.txt  # Search for multiple patterns in one file

{% endcode %}

{% hint style="info" %} Tip: Also check out ripgrep for a Rust implementation of most grep features, with better defaults for recursive searching while skipping unnecessary files {% endhint %}