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.

Operator	Description	Example
-e	True if the file exists.	[ -e /var/log/app.log ]
-d	True if the path is a directory.	[ -d /tmp/data ]
-f	True if the path is a regular file.	[ -f /bin/bash ]
-L	True if the path is a symbolic link.	[ -L /usr/bin/python ]
-r	True if the file is readable.	[ -r config.cfg ]
-w	True if the file is writable.	[ -w output.txt ]
-x	True if the file is executable.	[ -x ./install.sh ]
-nt	True if file1 is newer than file2.	[ backup.tar -nt source.tar ]
-ot	True if file1 is older than file2.	[ old.log -ot new.log ]

String Comparison Operators

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

Operator	Description	Example
-z	True if the string length is zero.	[ -z "$input" ]
-n	True if the string length is non-zero.	[ -n "$username" ]
= or ==	True if strings are equal.	[ "$host" = "localhost" ]
!=	True if strings are not equal.	[ "$mode" != "test" ]
<	True if string1 sorts before string2 (ASCII). Must be escaped in [ ]: \<.	[[ "apple" < "banana" ]]
>	True if string1 sorts after string2 (ASCII). Must be escaped in [ ]: \>.	[[ "zebra" > "apple" ]]

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.

Operator	Description	Example
-eq	Equal to.	[ "$count" -eq 10 ]
-ne	Not equal to.	[ "$retries" -ne 0 ]
-lt	Less than.	[ "$value" -lt 100 ]
-le	Less than or equal to.	[ "$index" -le "$max" ]
-gt	Greater than.	[ "$pid" -gt 0 ]
-ge	Greater than or equal to.	[ "$version" -ge 2 ]

With in 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 comamnds.

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

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.

Expression	Description
${var:-default}	Use default if var is unset or empty.
${var:=default}	Use and assign default to var if it is unset or empty.
${var:?error_msg}	Exit with error_msg if var is unset or empty.
${var:+replacement}	Use replacement only if var is set and non-empty.
${#var}	Length of the string in var.
${var#pattern}	Remove shortest leading match of pattern.
${var##pattern}	Remove longest leading match of pattern.
${var%pattern}	Remove shortest trailing match of pattern.
${var%%pattern}	Remove longest trailing match of pattern.
${var/old/new}	Replace first match of old with new.
${var//old/new}	Replace all matches of old with new.

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...