Mastering the Text Trio: grep, sed, and awk for Powerful Text Processing

The Text Trio: grep, sed, and awk

grep

Concept

grep is a powerful text-search tool that uses pattern matching (including regular expressions) to search files and outputs matching lines by default.

Syntax

grep [options] pattern file

Options

-i   ignore case
-n   show line numbers
-c   count matching lines
-v   invert match (show non‑matching lines)
-o   show only matching part
-q   quiet mode (no output)
-A N show N lines after a match
-B N show N lines before a match
-C N show N lines around a match
-E   use extended regex (egrep)
-F   treat pattern as a fixed string (fgrep)

Regular Expressions

Regular expressions are text patterns composed of literal and special characters, used to match strings that satisfy a given syntax. They support character classes, quantifiers, anchors, groups, and are supported by many programming languages.

Metacharacters

.   matches any single character
[]  matches any character in the set
[^] matches any character not in the set
[:alnum:] alphanumeric characters
[:alpha:] alphabetic characters
[:lower:] lowercase letters
[:upper:] uppercase letters
[:blank:] space and tab
[:space:] any whitespace
\b word boundary
\w word characters
\W non‑word characters
\s whitespace
\S non‑whitespace
*   matches preceding element zero or more times (greedy)
+   matches preceding element one or more times
?   matches preceding element zero or one time
{n} exactly n times
{n,} at least n times
{n,m} between n and m times
^   start of line
$   end of line

sed

sed is a stream editor that reads input line by line, applies editing commands, and outputs the result.

Syntax

sed [options] 'script' file

Common Options

-n   suppress automatic printing
-e   add script commands
-f   read script from file
-r, -E use extended regular expressions
-i.bak edit files in place with backup

Addressing

Commands can be applied to specific lines, line ranges, or patterns.

Examples

sed -n '2,5p' file      # print lines 2 to 5
sed -n '/^start/,/^end/p' file   # print from line matching start to line matching end
sed -n '1~2p' file      # print odd-numbered lines
sed -n '2~2p' file      # print even-numbered lines

Commands

p   print pattern space
d   delete pattern space
a\text   append text after a line
i\text   insert text before a line
c\text   replace line with text
w file   write matching lines to file
r file   read file and insert after matching line
=   print line number
q   quit

awk

awk is a programming language for pattern scanning and processing. It splits input into fields and provides built‑in variables.

Usage

awk [options] 'program' file

Important Variables

FS field separator (default space or tab)

OFS output field separator

NF number of fields in the current record

NR number of records processed so far

$0 the entire current line

$n the nth field

FILENAME name of the current input file

RS record separator (default newline)

These tools together form a powerful toolkit for searching, editing, and reporting on text data directly from the Unix command line.