Linux expr Command: Evaluate Math and Strings

The expr command is a fundamental Unix and Linux utility designed to evaluate expressions and return the corresponding result to standard output. This article provides a straightforward guide to understanding the role of expr, covering its syntax, how it performs integer arithmetic, its utility in string manipulation, and how it handles boolean logic within shell scripts.

What is the expr Command?

The expr command parses and evaluates positional arguments as expressions. It is a standard POSIX utility, making it crucial for writing portable shell scripts that run across various Unix-like environments without relying on shell-specific built-in features.

A critical syntax requirement of expr is that every operator and operand must be separated by a space. Missing spaces will cause the command to output the literal arguments rather than evaluating them.

Evaluating Arithmetic Expressions

The expr command performs basic integer arithmetic. It does not support floating-point operations natively.

Supported Arithmetic Operators

Addition (+): Adds two integers.
Subtraction (-): Subtracts the second integer from the first.
Multiplication (*): Multiplies two integers. Because the asterisk is a shell wildcard, it must be escaped with a backslash (\*).
Division (/): Performs integer division, discarding the remainder.
Modulus (%): Computes the remainder of an integer division.

Examples

# Addition
expr 10 + 5
# Output: 15

# Subtraction
expr 20 - 8
# Output: 12

# Multiplication (requires escaping)
expr 6 \* 7
# Output: 42

# Division
expr 20 / 3
# Output: 6

# Modulus
expr 20 % 3
# Output: 2

Evaluating String Expressions

Beyond numeric calculation, expr provides built-in operations for analyzing and manipulating strings.

Calculating String Length

To find the number of characters in a string:

expr length "Hello World"
# Output: 11

Extracting Substrings

The substr keyword extracts characters starting from a specified 1-based index for a set length:

# Syntax: expr substr STRING POSITION LENGTH
expr substr "LinuxOS" 1 5
# Output: Linux

Finding Character Index

The index keyword returns the position of the first matching character from a set of characters:

# Find the first occurrence of 'x' or 'u'
expr index "LinuxOS" "xu"
# Output: 4

Regular Expression Matching

The colon (:) operator performs regular expression matching against the start of a string. It returns either the number of matching characters or the matched capture group:

# Match from the start of the string
expr "sample.txt" : '.*\.txt'
# Output: 10

# Capture a pattern
expr "file_123.log" : '.*_\([0-9]*\)'
# Output: 123

Logical and Relational Operations

expr can evaluate relational conditions, returning 1 for true and 0 for false. Operators such as <, >, <=, and >= must be escaped to prevent shell redirection.

# Equality check
expr 10 = 10
# Output: 1

# Greater than check (escaped)
expr 15 \> 20
# Output: 0

# Logical OR (returns first non-empty/non-zero argument, else 0)
expr "" \| "fallback"
# Output: fallback

When to Use expr

While modern shells like Bash provide internal arithmetic expansion such as $(( ... )), the expr command remains widely used when writing scripts for POSIX-compliant environments like standard sh, or when working on legacy UNIX systems where modern bashisms are not supported. Its ability to mix string parsing, pattern matching, and arithmetic in a single standardized binary makes it a versatile utility for Linux systems administrators.