Linux Finding Text in Files

Finding Text in Files

grep (General Regular Expression Print)

grep [options] pattern files

grep searches the specified files (or standard input if none are given) for lines matching a pattern.
By default, it prints matching lines.

Example:

grep hello *.txt

Searches all text files in the current directory for lines containing “hello”.

Useful Options

• -c → print the count of matching lines
• -i → ignore case
• -v → invert match (show lines that don’t match)
• -n → show line numbers

Example (ignore case + invert match):

grep -vi hello *.txt

Finds lines that do not contain any case variation of “hello”.

Searching Recursively

Combine with find to search across a directory tree:

grep hello `find . -name "*.txt" -print`

Or use the recursive option:

grep -r hello .

Regular Expressions

grep uses regular expressions for powerful pattern matching.

• ^ → start of line
• $ → end of line
• . → any character
• [0-9] → any digit
• [^a-z] → any non-lowercase letter

Example:

grep ^..[l-z]$ hello.txt

Matches any 3-character line ending with letters l to z.

You can find a more detailed explanation on regex here: Regular Expressions

Variants

• egrep → supports extended regex (|, (), +, {} ranges, etc.)
• fgrep → searches for fixed strings (like grep -F), faster for literal matches

Example with egrep:

egrep '(^[0-9]{1,5}[a-zA-Z]+$)|none' file.txt

Matches lines that either:

• Start with up to 5 digits, followed by letters/spaces
• Or contain the word “none”

📌 More info: man grep

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Comparison

Command	Purpose	Example
grep	Search inside files for text patterns	grep -i "error" *.log
egrep	Extended regex search	egrep “foo &#124 bar” file.txt
fgrep	Fixed string search	fgrep "hello world" file.txt

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Other Useful Tips

• Search text in compressed files:
  zgrep pattern file.gz

• Count matches across many files:
  grep -r -c "error" /var/log

• Get top 10 most frequent words in a file:

  tr -cs '[:alnum:]' '[\n*]' < file.txt | sort | uniq -c | sort -nr | head -10