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landing/docs/Bash-Scripts/009-bash-conditional-expressions.md

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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:
+
+```bash
+touch conditional_test.sh
+```
+
+Open `conditional_test.sh` and add:
+
+```bash
+#!/usr/bin/env bash
+
+my_var="data"
+
+if [[ -n "$my_var" ]]; then # If variable is not empty
+  echo "Variable contains data."
+fi
+```
+
+Save, make executable, and run:
+
+```bash
+chmod +x conditional_test.sh
+./conditional_test.sh
+```
+
+Output: `Variable contains data.`
+
+Now, for your full reference:
+
+## File Tests
+
+These check file attributes. Use `"${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 ${arg2} ]]` - True if `arg1` greater than or equal to `arg2`.
+
+## Logical Operators
+
+Combine multiple conditions.
+
+* **AND (`&&`)**: Both conditions must be true.
+    ```bash
+    # Example: file exists AND is not empty
+    if [[ -f "my_file.txt" && -s "my_file.txt" ]]; then
+        echo "File is present and has content."
+    fi
+    ```
+* **OR (`||`)**: At least one condition must be true.
+    ```bash
+    # Example: either file or directory exists
+    if [[ -f "another_file.txt" || -d "another_dir" ]]; then
+        echo "Found a file or a directory."
+    fi
+    ```
+
+## Exit Status Checks
+
+`$?:` The exit status of the *last* executed command. `0` means success. Any non-zero value indicates an error.
+
+* **Success:**
+    ```bash
+    command_to_run
+    if [[ $? -eq 0 ]]; then
+      echo "Command succeeded."
+    fi
+    ```
+* **Failure:**
+    ```bash
+    command_to_run
+    if [[ $? -ne 0 ]]; then
+      echo "Command failed."
+    fi
+    ```