Searching text with grep
grep filters lines of text, keeping only those that match a pattern.
Syntax
grep [options] pattern [file...]grep searches text for lines matching a pattern and prints them. It reads from files, or from a pipe.
The most useful options
-i— ignore case.-r— search recursively through a directory.-n— show line numbers.-v— invert: show lines that do not match.-c— count matching lines instead of printing them.-E— use extended regular expressions.
Patterns can be plain text or full regular expressions, making grep extremely flexible.
Example
# Find a word in a file
grep "timeout" config.yaml
# Case-insensitive, with line numbers
grep -in "error" app.log
# Search a whole project recursively
grep -rn "TODO" ./src
# Lines that do NOT contain 'debug'
grep -v "debug" app.logWhen to use it
- A developer uses 'grep -rn "TODO" ./src/' to find all TODO comments across a codebase before a release.
- A sysadmin runs 'grep -i "error" /var/log/syslog' to spot error messages regardless of capitalisation.
- A security analyst uses 'grep -E "^(FAILED|Invalid)" /var/log/auth.log' to extract failed authentication events.
More examples
Basic search and case-insensitive
Shows basic grep usage, the -i flag for case-insensitive matching, and -n to include line numbers.
# Search for exact word in a file
grep 'error' /var/log/syslog
# Case-insensitive search
grep -i 'ERROR' /var/log/syslog
# Show line numbers
grep -n 'error' /var/log/syslogRecursive search in directory
Uses -r to search all files recursively, -n for line numbers, and -C for surrounding context lines.
# Search all .py files for a function name
grep -rn 'def authenticate' ./src/
# Search and show 2 lines of context
grep -rn -C 2 'database_url' ./config/Extended regex with grep -E
Uses -E for extended regular expressions to match alternatives and extract IP addresses with -o (only matching).
# Match lines starting with FAILED or Invalid
grep -E '^(FAILED|Invalid)' /var/log/auth.log
# Extract IPv4 addresses
grep -oE '[0-9]{1,3}(\.[0-9]{1,3}){3}' access.log | sort -u
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