grep searches for text patterns in files. It's how you find things inside files, not just file names.

Basic Usage

grep "pattern" filename

Shows lines containing the pattern.

Search a File

grep "error" log.txt

Shows every line containing "error".

Search Multiple Files

grep "pattern" file1.txt file2.txt
grep "pattern" *.txt

Shows matches with filenames.

Search Recursively (-r)

Search all files in a directory:

grep -r "pattern" .

The . means current directory. Goes through all subdirectories.

Case Insensitive (-i)

grep -i "error" log.txt

Matches "error", "Error", "ERROR", etc.

Show Line Numbers (-n)

grep -n "pattern" file.txt

Output:

12:This line contains pattern
45:Another pattern here

Show Only Filenames (-l)

grep -l "pattern" *.txt

Lists files that contain the pattern, not the matching lines.

Count Matches (-c)

grep -c "error" log.txt

Returns just the number of matches.

Invert Match (-v)

Show lines that DON'T contain the pattern:

grep -v "debug" log.txt

Filters out debug messages.

Whole Word Match (-w)

grep -w "error" log.txt

Matches "error" but not "errors" or "erroring".

Show Context

grep -A 2 "error" log.txt  # 2 lines After
grep -B 2 "error" log.txt  # 2 lines Before
grep -C 2 "error" log.txt  # 2 lines Context (both)

Helpful for understanding errors in context.

Regular Expressions

grep supports patterns:

Examples:

grep "^Error" log.txt     # Lines starting with "Error"
grep "error\$" log.txt     # Lines ending with "error"
grep "log[0-9]" file.txt  # log0, log1, log2, etc.

Extended Regex (-E)

For more patterns:

grep -E "error|warning" log.txt

Matches "error" OR "warning".

grep -E "[0-9]{3}-[0-9]{4}" file.txt

Matches phone number patterns like "555-1234".

Common Combinations

# Case insensitive, recursive, line numbers
grep -rni "pattern" .

# Search code, exclude directories
grep -r --exclude-dir=node_modules "pattern" .

# Search specific file types
grep -r --include="*.js" "function" .

Exclude Files and Directories

grep -r --exclude="*.log" "pattern" .
grep -r --exclude-dir=".git" "pattern" .
grep -r --exclude-dir={node_modules,dist,.git} "pattern" .

Practical Examples

Find a function in code:

grep -rn "function processData" --include="*.js" .

Find TODO comments:

grep -rni "TODO" --include="*.{js,py,rb}" .

Find errors in logs:

grep -i "error\|failed\|exception" /var/log/*.log

Search command history:

history | grep "docker"

Find files containing secrets (audit):

grep -rn "password\|api_key\|secret" --include="*.{js,json,py}" .

Colorized Output

grep --color=auto "pattern" file.txt

Make it default:

echo 'alias grep="grep --color=auto"' >> ~/.zshrc

Exit Status

grep returns:

• 0 if matches found
• 1 if no matches
• 2 if error

Useful in scripts:

if grep -q "error" log.txt; then
    echo "Errors found!"
fi

The -q flag is quiet - no output, just the exit status.

Quick Reference

Keep Learning

grep is essential for finding things in code. The free course covers search and more.

Check it out at Mac Terminal for Humans.