Grep

grep, awk, and sed are the foundational text processing tools in Unix. They’re old, they’re cryptic, and they’re incredibly powerful once you learn them. grep: Search and Filter grep searches for patterns in text. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 # Basic search grep "error" logfile.txt # Case insensitive grep -i "error" logfile.txt # Show line numbers grep -n "error" logfile.txt # Count matches grep -c "error" logfile.txt # Invert (lines NOT matching) grep -v "debug" logfile.txt # Recursive search grep -r "TODO" ./src/ # Only filenames grep -l "password" *.conf Regex with grep 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 # Extended regex (-E or egrep) grep -E "error|warning|critical" logfile.txt # Word boundary grep -w "fail" logfile.txt # Matches "fail" not "failure" # Line start/end grep "^Error" logfile.txt # Lines starting with Error grep "done$" logfile.txt # Lines ending with done # Any character grep "user.name" logfile.txt # user1name, username, user_name # Character classes grep "[0-9]" logfile.txt # Lines with digits grep "[A-Za-z]" logfile.txt # Lines with letters Context 1 2 3 4 5 6 7 8 # Lines before match grep -B 3 "error" logfile.txt # Lines after match grep -A 3 "error" logfile.txt # Lines before and after grep -C 3 "error" logfile.txt Real Examples 1 2 3 4 5 6 7 8 9 10 11 # Find IP addresses grep -E "\b[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\b" access.log # Find function definitions grep -n "^def \|^function " *.py *.js # Exclude directories grep -r "config" . --exclude-dir={node_modules,.git} # Find files NOT containing pattern grep -L "copyright" *.py sed: Stream Editor sed transforms text line by line. ...

You know grep "error" logfile.txt. But grep can do so much more — recursive searches, context lines, inverse matching, and regex patterns that turn hours of manual searching into seconds. The Basics 1 2 3 4 5 6 7 8 9 10 11 12 13 14 # Search for pattern in file grep "error" app.log # Case-insensitive grep -i "error" app.log # Show line numbers grep -n "error" app.log # Count matches grep -c "error" app.log # Only show filenames with matches grep -l "error" *.log Recursive Search 1 2 3 4 5 6 7 8 9 10 11 12 13 14 # Search all files in directory tree grep -r "TODO" ./src # With line numbers grep -rn "TODO" ./src # Include only certain files grep -r --include="*.py" "import os" . # Exclude directories grep -r --exclude-dir=node_modules "console.log" . # Multiple excludes grep -r --exclude-dir={node_modules,.git,dist} "function" . Context Lines When you find a match, you often need surrounding context: ...