test

Evaluate conditional expressions.

Synopsis

test [expr]
# Or use square brackets (must have spaces inside):
[ EXPRESSION ]

Main Purpose

• Evaluate conditional expressions and return boolean values (true/false)
• Commonly used with if statements for logical control in shell scripts
• Can perform three types of tests: numeric, string, and file tests

File Test Operators

The test command is most commonly used to check file attributes:

Example Script:

#!/bin/bash
file="/etc/passwd"
if [ -e "$file" ]; then
    echo "$file exists"
    if [ -r "$file" ]; then
        echo "and is readable"
    fi
else
    echo "$file does not exist"
fi

Output:

/etc/passwd exists
and is readable

String Operators

Important: Always quote string variables to prevent syntax errors from empty variables.

Example:

#!/bin/bash
read -p "Enter username: " username
if [ -z "$username" ]; then
    echo "Error: username cannot be empty"
    exit 1
elif [ "$username" = "root" ]; then
    echo "Warning: not recommended to use root account"
else
    echo "Welcome, $username"
fi

Numeric Comparison Operators

Example:

#!/bin/bash
read -p "Enter age: " age
if [ "$age" -lt 0 ]; then
    echo "Age cannot be negative"
elif [ "$age" -lt 18 ]; then
    echo "Minor"
elif [ "$age" -ge 18 ] && [ "$age" -lt 60 ]; then
    echo "Adult"
else
    echo "Senior"
fi

Logical Operators

Modern recommended syntax: Use && and || instead of -a and -o for POSIX compliance:

[ "$a" -eq 1 ] && [ "$b" -eq 2 ]  # AND
[ "$a" -eq 1 ] || [ "$b" -eq 2 ]  # OR

Other Operators

Advanced Syntax: [[ ]] and (( ))

Bash provides more powerful test syntax:

Double brackets [[ ]]

• Pattern matching: [[ "$var" == *.txt ]]
• Regular expressions: [[ "$var" =~ ^[0-9]+$ ]]
• Safer string handling

Double parentheses (( ))

• Designed for numeric comparisons: (( a > b ))
• Supports complex arithmetic expressions

if [[ "$file" == *.log ]]; then
    echo "This is a log file"
fi

if (( $count > 10 )); then
    echo "Count exceeds 10"
fi

Return Value

Returns 0 if the expression evaluates to success, and returns 1 if the expression evaluates to failure or if invalid parameters are given.

Practical Examples

1. Check if a service is running

#!/bin/bash
service="nginx"
if systemctl is-active --quiet "$service"; then
    echo "$service is running"
else
    echo "$service is not running"
    # Can add command to start the service here
fi

2. Backup file check

#!/bin/bash
backup_file="/backups/data_$(date +%Y%m%d).tar.gz"
if [ ! -f "$backup_file" ]; then
    echo "Error: backup file $backup_file does not exist"
    exit 1
elif [ ! -s "$backup_file" ]; then
    echo "Warning: backup file is empty"
else
    echo "Backup verification successful"
fi

3. Basic usage examples

# Execute a conditional expression and show the return value
[root@pc root]$ test ! "abc" == 123; echo $?
0

# Equivalent form using square brackets (note the spaces)
[root@pc root]$ [ ! "abc" == 123 ]; echo $?
0

# Using double brackets
[root@pc root]$ [[ ! "abc" == 123 ]]; echo $?
0

Common Errors and Debugging Tips

Common Mistakes:

1. Missing spaces: [ "$a"="$b" ] is incorrect, correct is [ "$a" = "$b" ]
2. Unquoted variables: [ -f $file ] should be [ -f "$file" ]
3. Confusing string and numeric comparison: Use = for strings, -eq for numbers

Debugging Tips:

Add set -x at the beginning of the script to enable debug mode, or use echo to print test expressions:

echo "Test expression: [ $a -eq $b ]"
[ "$a" -eq "$b" ] && echo "True" || echo "False"

Notes

1. This command is equivalent to [.
2. To write bash conditional expressions, you can use the built-in command test, [, or the compound command [[.
   • For conditional expressions, see here.
   • For an index of built-in commands, see here.
   • For an index of compound commands, see here.
3. This command is a bash built-in; see the help command for related help information.