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Conditional Expressions

Conditionals are how your scripts make decisions: run different code based on a true/false outcome.

In Bash, [[ is your go-to for testing files, strings, and numbers.

Let's start with a basic example:

touch conditional_test.sh

Open conditional_test.sh and add:

#!/usr/bin/env bash

my_var="data"

if [[ -n "$my_var" ]]; then # If variable is not empty
  echo "Variable contains data."
fi

Save, make executable, and run:

chmod +x conditional_test.sh
./conditional_test.sh

Output: Variable contains data.

Now, for your full reference:

File Tests

These check file attributes. Use "${file}" to handle spaces in names.

• [[ -e "${file}" ]] - True if file exists.
• [[ -f "${file}" ]] - True if file exists and is a regular file.
• [[ -d "${file}" ]] - True if file exists and is a directory.
• [[ -s "${file}" ]] - True if file exists and is not empty (size > 0).
• [[ -r "${file}" ]] - True if file exists and is readable.
• [[ -w "${file}" ]] - True if file exists and is writable.
• [[ -x "${file}" ]] - True if file exists and is executable.
• [[ -L "${file}" ]] - True if file is a symbolic link (-h is also used).
• [[ -b "${file}" ]] - True if file exists and is a block device.
• [[ -c "${file}" ]] - True if file exists and is a character device.

String Tests

Compare string properties and values.

• [[ -v "${varname}" ]] - True if varname is set (has a value).
• [[ -z "${string}" ]] - True if string has zero length (empty).
• [[ -n "${string}" ]] - True if string has non-zero length (not empty).
• [[ "${string1}" == "${string2}" ]] - True if strings are equal. (== allows pattern matching inside [[.)
• [[ "${string1}" != "${string2}" ]] - True if strings are not equal.
• [[ "${string1}" < "${string2}" ]] - True if string1 sorts before string2 lexicographically.
• [[ "${string1}" > "${string2}" ]] - True if string1 sorts after string2 lexicographically.

Numeric Comparisons

Compare integer values.

• [[ ${arg1} -eq ${arg2} ]] - True if arg1 equals arg2.
• [[ ${arg1} -ne ${arg2} ]] - True if arg1 not equals arg2.
• [[ ${arg1} -lt ${arg2} ]] - True if arg1 less than arg2.
• [[ ${arg1} -le ${arg2} ]] - True if arg1 less than or equal to arg2.
• [[ ${arg1} -gt ${arg2} ]] - True if arg1 greater than arg2.
• [[ ${arg1} -ge ${arg2} ]] - True if arg1 greater than or equal to arg2.

Logical Operators

Combine multiple conditions.

• AND (&&): Both conditions must be true.

  # Example: file exists AND is not empty
  if [[ -f "my_file.txt" && -s "my_file.txt" ]]; then
      echo "File is present and has content."
  fi
  

• OR (||): At least one condition must be true.

  # Example: either file or directory exists
  if [[ -f "another_file.txt" || -d "another_dir" ]]; then
      echo "Found a file or a directory."
  fi
  

Exit Status Checks

$?: The exit status of the last executed command. 0 means success. Any non-zero value indicates an error.

• Success:

  command_to_run
  if [[ $? -eq 0 ]]; then
    echo "Command succeeded."
  fi
  

• Failure:

  command_to_run
  if [[ $? -ne 0 ]]; then
    echo "Command failed."
  fi