111 lines
3.4 KiB
Markdown
111 lines
3.4 KiB
Markdown
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# Conditional Expressions
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Conditionals are how your scripts make decisions: run different code based on a true/false outcome.
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In Bash, `[[` is your go-to for testing files, strings, and numbers.
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Let's start with a basic example:
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```bash
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touch conditional_test.sh
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```
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Open `conditional_test.sh` and add:
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```bash
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#!/usr/bin/env bash
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my_var="data"
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if [[ -n "$my_var" ]]; then # If variable is not empty
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echo "Variable contains data."
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fi
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```
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Save, make executable, and run:
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```bash
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chmod +x conditional_test.sh
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./conditional_test.sh
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```
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Output: `Variable contains data.`
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Now, for your full reference:
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## File Tests
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These check file attributes. Use `"${file}"` to handle spaces in names.
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* `[[ -e "${file}" ]]` - True if `file` exists.
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* `[[ -f "${file}" ]]` - True if `file` exists and is a regular file.
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* `[[ -d "${file}" ]]` - True if `file` exists and is a directory.
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* `[[ -s "${file}" ]]` - True if `file` exists and is not empty (size > 0).
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* `[[ -r "${file}" ]]` - True if `file` exists and is readable.
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* `[[ -w "${file}" ]]` - True if `file` exists and is writable.
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* `[[ -x "${file}" ]]` - True if `file` exists and is executable.
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* `[[ -L "${file}" ]]` - True if `file` is a symbolic link (`-h` is also used).
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* `[[ -b "${file}" ]]` - True if `file` exists and is a block device.
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* `[[ -c "${file}" ]]` - True if `file` exists and is a character device.
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## String Tests
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Compare string properties and values.
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* `[[ -v "${varname}" ]]` - True if `varname` is set (has a value).
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* `[[ -z "${string}" ]]` - True if `string` has zero length (empty).
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* `[[ -n "${string}" ]]` - True if `string` has non-zero length (not empty).
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* `[[ "${string1}" == "${string2}" ]]` - True if strings are equal. (`==` allows pattern matching inside `[[`.)
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* `[[ "${string1}" != "${string2}" ]]` - True if strings are not equal.
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* `[[ "${string1}" < "${string2}" ]]` - True if `string1` sorts before `string2` lexicographically.
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* `[[ "${string1}" > "${string2}" ]]` - True if `string1` sorts after `string2` lexicographically.
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## Numeric Comparisons
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Compare integer values.
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* `[[ ${arg1} -eq ${arg2} ]]` - True if `arg1` equals `arg2`.
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* `[[ ${arg1} -ne ${arg2} ]]` - True if `arg1` not equals `arg2`.
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* `[[ ${arg1} -lt ${arg2} ]]` - True if `arg1` less than `arg2`.
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* `[[ ${arg1} -le ${arg2} ]]` - True if `arg1` less than or equal to `arg2`.
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* `[[ ${arg1} -gt ${arg2} ]]` - True if `arg1` greater than `arg2`.
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* `[[ ${arg1} -ge ${arg2} ]]` - True if `arg1` greater than or equal to `arg2`.
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## Logical Operators
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Combine multiple conditions.
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* **AND (`&&`)**: Both conditions must be true.
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```bash
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# Example: file exists AND is not empty
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if [[ -f "my_file.txt" && -s "my_file.txt" ]]; then
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echo "File is present and has content."
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fi
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```
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* **OR (`||`)**: At least one condition must be true.
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```bash
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# Example: either file or directory exists
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if [[ -f "another_file.txt" || -d "another_dir" ]]; then
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echo "Found a file or a directory."
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fi
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```
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## Exit Status Checks
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`$?:` The exit status of the *last* executed command. `0` means success. Any non-zero value indicates an error.
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* **Success:**
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```bash
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command_to_run
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if [[ $? -eq 0 ]]; then
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echo "Command succeeded."
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fi
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```
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* **Failure:**
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```bash
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command_to_run
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if [[ $? -ne 0 ]]; then
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echo "Command failed."
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fi
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```
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