Cheatsheet & Examples: grep

The grep command is a command-line utility for searching text using patterns. It scans files or input streams for lines that match a specified regular expression, outputting those lines. It is widely used for filtering and analyzing text data.

Search for a pattern in a file

Example Usage:
grep "search_term" filename.txt

What it does:
Searches for the exact string "search_term" in the specified file and prints matching lines.

Command-line Arguments Explained:

• "search_term": The string or regular expression to search for.
• filename.txt: The file to search within.

Search for a pattern case-insensitively

Example Usage:
grep -i "error" log.txt

What it does:
Searches for "error" in log.txt while ignoring case (e.g., matches "Error", "ERROR", etc.).

Command-line Arguments Explained:

• -i: Ignores case when matching patterns.
• "error": The pattern to search for.
• log.txt: The file to search within.

Recursively search in directories

Example Usage:
grep -r "warning" /var/log/

What it does:
Searches all files in the /var/log/ directory and its subdirectories for lines containing "warning".

Command-line Arguments Explained:

• -r: Recursively searches through directories.
• "warning": The pattern to find.
• /var/log/: The directory to search in.

Count matching lines

Example Usage:
grep -c "success" results.txt

What it does:
Counts the number of lines in results.txt that match "success" and outputs the total.

Command-line Arguments Explained:

• -c: Counts the number of matching lines.
• "success": The pattern to count.
• results.txt: The file to analyze.

Show line numbers of matches

Example Usage:
grep -n "debug" file.txt

What it does:
Prints lines containing "debug" from file.txt, prefixed with their line numbers.

Command-line Arguments Explained:

• -n: Displays line numbers for matching lines.
• "debug": The pattern to locate.
• file.txt: The file to search.

Invert match to exclude lines

Example Usage:
grep -v "fail" output.txt

What it does:
Prints lines from output.txt that do not contain "fail".

Command-line Arguments Explained:

• -v: Inverts the match (prints non-matching lines).
• "fail": The pattern to exclude.
• output.txt: The file to process.

Match whole words only

Example Usage:
grep -w "apple" fruits.txt

What it does:
Finds lines in fruits.txt containing the exact word "apple" (not part of another word).

Command-line Arguments Explained:

• -w: Matches only whole words, not substrings.
• "apple": The word to search for.
• fruits.txt: The file to check.

Use extended regular expressions

Example Usage:
grep -E "error|warning" file.txt

What it does:
Searches for lines containing either "error" or "warning" using extended regex syntax.

Command-line Arguments Explained:

• -E: Enables extended regular expressions (like egrep).
• "error|warning": The regex pattern (| = "or").
• file.txt: The target file.

Search in multiple files

Example Usage:
grep "error" file1.txt file2.txt

What it does:
Searches for "error" in both file1.txt and file2.txt, printing matching lines.

Command-line Arguments Explained:

• "error": The pattern to find.
• file1.txt, file2.txt: Files to search in.

Search via pipeline input

Example Usage:
ps aux | grep "python"

What it does:
Filters the output of the ps aux command to show only lines containing "python".

Command-line Arguments Explained:

• "python": The pattern to match.
• ps aux: Command whose output is piped into grep.

Highlight matches in color

Example Usage:
grep --color "error" log.txt

What it does:
Prints lines containing "error" with the matched text highlighted in color.

Command-line Arguments Explained:

• --color: Enables colored output for matches.
• "error": The pattern to highlight.
• log.txt: The file to search.

Search specific file types recursively

Example Usage:
grep -r --include="*.log" "error" /data/

What it does:
Recursively searches files ending with .log in the /data/ directory for "error".

Command-line Arguments Explained:

• -r: Recursively searches directories.
• --include="*.log": Limits search to files with the .log extension.
• "error": The pattern to find.
• /data/: Directory to search in.

Show context around matches

Example Usage:
grep -A 2 "warning" file.txt

What it does:
Prints lines containing "warning" along with the two lines after each match.

Command-line Arguments Explained:

• -A 2: Displays 2 lines of "after" context.
• "warning": The pattern to find.
• file.txt: The file to check.

Search for lines starting with a pattern

Example Usage:
grep "^start" file.txt

What it does:
Finds lines in file.txt that begin with "start".

Command-line Arguments Explained:

• "^start": Regular expression matching lines starting with "start".
• file.txt: The file to search.

Print filenames with matches

Example Usage:
grep -l "success" *.txt

What it does:
Lists filenames ending with .txt that contain at least one line matching "success".

Command-line Arguments Explained:

• -l: Outputs only filenames with matches.
• "success": The pattern to find.
• *.txt: Files to search (wildcard for all .txt files).

Search for patterns in binary files

Example Usage:
grep -a "magic" binary_file.bin

What it does:
Treats binary files as text and searches for "magic" within them.

Command-line Arguments Explained:

• -a: Processes binary files as text (avoids "Binary file ... matches" messages).
• "magic": The pattern to find.
• binary_file.bin: The binary file to search.

Search multiple patterns

Example Usage:
grep -e "error" -e "timeout" file.txt

What it does:
Prints lines from file.txt that contain either "error" or "timeout".

Command-line Arguments Explained:

• -e "error": First pattern to match.
• -e "timeout": Second pattern to match.
• file.txt: The file to search.

Use grep with a file of patterns

Example Usage:
grep -f patterns.txt file.txt

What it does:
Searches file.txt for lines matching any of the patterns listed in patterns.txt.

Command-line Arguments Explained:

• -f patterns.txt: Reads patterns from the specified file.
• file.txt: The file to search.