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grep: Find Anything in Files

grep searches for text in files. That's it. But it's powerful.

Here's the thing: You'll use grep constantly. Searching logs. Finding patterns. Extracting information. Learn it well.

Basic grep​

grep "pattern" file.txt
grep "error" /var/log/syslog

That's it. Simple.

Common Options​

Case Insensitive​

grep -i "pattern" file.txt        # Ignore case
grep -i "error" /var/log/syslog   # Finds ERROR, error, Error

The -i flag: Makes search case-insensitive.

My take: Use -i when you don't care about case. Which is often.

Show Line Numbers​

grep -n "pattern" file.txt        # Show line numbers

The -n flag: Shows which line matches.

My take: Useful for finding where things are. Use it.

Invert Match​

grep -v "pattern" file.txt        # Show lines NOT matching
grep -v "^#" file.txt             # Exclude comment lines

The -v flag: Shows lines that don't match.

My take: Useful for filtering. Exclude what you don't want.

grep -r "pattern" directory/      # Search recursively
grep -R "pattern" .               # Search from current directory

The -r or -R flag: Searches in subdirectories.

Real example:

grep -r "password" /etc/          # Find "password" in all config files

My take: Use this for searching multiple files. Very common.

Show Filenames Only​

grep -l "pattern" *.txt           # Show only filenames with matches
grep -L "pattern" *.txt           # Show filenames without matches

The -l flag: Just filenames. Not the matches.

My take: Useful when you just need to know which files have matches.

Count Matches​

grep -c "pattern" file.txt        # Count matches per file

The -c flag: Shows count, not matches.

My take: Useful for statistics. How many times does something appear?

Context Lines​

grep -A 3 "pattern" file.txt      # 3 lines after match
grep -B 3 "pattern" file.txt      # 3 lines before match
grep -C 3 "pattern" file.txt      # 3 lines before and after

Real example:

grep -C 5 "error" /var/log/syslog  # Show 5 lines before and after errors

My take: Useful for logs. See what happened before and after errors.

Regular Expressions (The Powerful Part)​

Basic Patterns​

grep "^pattern" file.txt          # Lines starting with pattern
grep "pattern$" file.txt          # Lines ending with pattern
grep "^$" file.txt                # Empty lines

The ^ and $: Start and end of line.

My take: Learn these. They're useful.

Character Classes​

grep "[0-9]" file.txt             # Contains digit
grep "[a-z]" file.txt             # Contains lowercase
grep "[A-Z]" file.txt             # Contains uppercase
grep "[0-9][0-9][0-9]" file.txt   # Three digits

My take: Useful for finding patterns. Numbers. Letters. Combinations.

Wildcards​

grep "file.*\.txt" file.txt       # file followed by anything, then .txt
grep "error.*" file.txt           # error followed by anything

The .*: Matches anything.

My take: Useful for flexible patterns.

Common Patterns​

Find Errors​

grep -i error /var/log/syslog
grep -i "error\|fail\|critical" /var/log/syslog

My take: Searching logs for errors. You'll do this constantly.

Find IP Addresses​

grep -E "[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}" file.txt

The -E flag: Extended regex.

My take: Complex pattern. But useful when you need it.

Find Empty Lines​

grep "^$" file.txt

My take: Simple. But useful for cleaning files.

Exclude Comments​

grep -v "^#" file.txt             # Exclude lines starting with #
grep -v "^$" file.txt             # Exclude empty lines
grep -v "^#" file.txt | grep -v "^$"  # Exclude both

My take: Useful for processing config files.

Combining with Other Commands​

Pipe to grep​

ps aux | grep nginx
df -h | grep "/$"

My take: This is what you'll do most. Filter command output.

grep with find​

find /etc -name "*.conf" -exec grep -l "pattern" {} \;

My take: Find files, then grep in them. Powerful combination.

Common Mistakes (I've Made These)​

  1. Not quoting patterns: grep pattern file works, but grep "pattern" file is safer. Quote it.

  2. Forgetting -i: Case-sensitive by default. Use -i when you don't care about case.

  3. Not using -r for directories: grep pattern dir doesn't work. Use grep -r pattern dir.

  4. Wrong regex syntax: Basic grep uses basic regex. Use -E for extended regex.

  5. Not escaping special characters: Characters like . and * are special. Escape them with \.

Real-World Examples​

Search Logs for Errors​

grep -i error /var/log/syslog | tail -20

Find Configuration Values​

grep -r "Listen" /etc/nginx/

Count Occurrences​

grep -c "GET" /var/log/nginx/access.log

Find with Context​

grep -C 10 "error" /var/log/syslog

What's Next?​

Now that you can search files, you can find anything. Combine grep with other commands. Build powerful pipelines.

Or learn about sed and awk for more advanced text processing.

Personal note: When I started, I'd open files and search manually. Then I learned grep. Now I grep everything. It's faster. It's more powerful. grep changed how I work with files.