Shell scripts rely on comparison operators to evaluate conditions. These operators are primarily used with the [ ] test construct or the test command. They are categorized based on the data they evaluate: files, strings, and integers.

File Test Operators

These operators check the properties of a file or directory.

String Comparison Operators

Always quote variables in string comparisons to prevent word splitting or globbing issues.

For pattern matching within strings, use the =~ operator inside the [[ ]] construct.

url="https://example.com"
if [[ "$url" =~ "example" ]]; then
    echo "Substring found."
fi

Integer Comparison Operators

These operators compare whole numbers. Do not use them for floating-point arithmetic.

Within double parentheses (( )), you can use standard arithmetic symbols (<, <=, >, >=, ==, !=).

if (( fileSize < 1024 )); then
    echo "File is small."
fi

Combining Conditions

Logical operators -a (AND), -o (OR), and ! (NOT) can combine multiple tests within a single [ ] or test command. The && and || operators are also commonly used between separate test commands.

primary="yes"
secondary="no"

if [ "$primary" = "yes" -a "$secondary" = "no" ]; then
    echo "Condition met."
fi

if [ ! -f "/tmp/lockfile" ]; then
    echo "Lock file not present."
fi

The test Command

The test command is functionally equivalent to the [ ] construct. The operators listed above work identically with it.

if test -d "$HOME/downloads"; then
    echo "Downloads directory exists."
fi

Parameter and Variable Manipulation

Bash provides powerful variable substitution and manipulation features.

Practical Example: Argument Handling

This script demonstrates checking the number of command-line arguments and providing a default value.

#!/bin/bash
# Set default port
connection_port=8080

# Check if at least one argument was provided
if [ $# -ge 1 ]; then
    connection_port=$1
fi

echo "Connecting to service on port: $connection_port"
# Command to connect would follow...