test

Understanding the Basics

The test command’s primary function is to perform comparisons and checks on various aspects of your system. It operates by examining an expression and returning an exit code. This exit code is then used by control structures like if statements to determine the flow of your script.

The [ ] (square brackets) are an alias for the test command; they’re functionally identical. Using square brackets is generally preferred for readability within scripts. The closing bracket must be separated from the following argument by a space.

test expression  # Equivalent to
[ expression ]

Common Test Operators

test and [ support a wide range of operators. Here are some of the most frequently used:

String Comparisons:

• =: Checks for string equality.
• !=: Checks for string inequality.
• -z string: Checks if a string is empty.
• -n string: Checks if a string is not empty.

Numerical Comparisons:

• -eq: Equal to.
• -ne: Not equal to.
• -gt: Greater than.
• -ge: Greater than or equal to.
• -lt: Less than.
• -le: Less than or equal to.

File Tests:

• -e file: Checks if a file exists.
• -f file: Checks if a file exists and is a regular file.
• -d file: Checks if a file exists and is a directory.
• -r file: Checks if a file exists and is readable.
• -w file: Checks if a file exists and is writable.
• -x file: Checks if a file exists and is executable.
• -s file: Checks if a file exists and has a size greater than zero.

Code Examples

Let’s illustrate these operators with practical examples.

String Comparisons:

#!/bin/bash

string1="hello"
string2="world"

if [ "$string1" = "$string2" ]; then
  echo "Strings are equal"
else
  echo "Strings are not equal"
fi

if [ -z "$string3" ]; then
  echo "string3 is empty"
fi

Numerical Comparisons:

#!/bin/bash

num1=10
num2=5

if [ "$num1" -gt "$num2" ]; then
  echo "$num1 is greater than $num2"
fi

File Tests:

#!/bin/bash

file="/tmp/myfile.txt"

if [ -e "$file" ]; then
  echo "File exists"
  if [ -f "$file" ]; then
    echo "It's a regular file"
  fi
else
  echo "File does not exist"
fi

if [ -d "/tmp" ]; then
  echo "/tmp is a directory"
fi

Combining Tests:

You can combine multiple tests using logical operators:

• -a: Logical AND
• -o: Logical OR
• !: Logical NOT

#!/bin/bash

if [ -f "/tmp/myfile.txt" -a -r "/tmp/myfile.txt" ]; then
  echo "File exists and is readable"
fi

These examples showcase the versatility of the test command. Remember to always quote your variables to prevent word splitting and globbing issues. Mastering this built-in is essential for creating effective shell scripts.