Parameter expansion

Sub: Coding

Parameter expansion is a powerful feature in bash (and other POSIX-compliant shells) that allows you to manipulate variables and their values. It's commonly used for tasks like providing default values, removing prefixes or suffixes, and performing simple text transformations.

Basic Syntax

The basic syntax of parameter expansion is ${parameter} or ${parameter:-word}.

${parameter}: Expands to the value of parameter.
${parameter:-word}: If parameter is unset or null, expands to word; otherwise, expands to the value of parameter.

Common Parameter Expansion Operators

Default Values:
You can provide a default value for a variable that hasn't been set. Here's the syntax:
```
${variable:-defaultValue}
```
If variable is not set or is null, this will return defaultValue.
Assign Default Value:

${parameter:=word}

Assigns word to parameter if parameter is unset or null; otherwise, retains the value of parameter.

Error if Null or Unset:
You can display an error message if a variable is null or not set.
```
${variable:?errorMessage}
```
If variable is not set or is null, this will display errorMessage.
Substring Expansion:
You can get a substring of a variable.
```
${variable:offset:length}
```
This will return a substring of variable starting at offset and of length length.
Length of a String:
You can get the length of a string stored in a variable.
```
${#variable}
```
This will return the length of the value stored in variable.
Replace Substring:
You can replace a substring in a variable.
```
${variable/pattern/replacement}
```
This will replace the first match of pattern with replacement in variable.
Replace All Substrings:
You can replace all occurrences of a substring in a variable.
```
${variable//pattern/replacement}
```
This will replace all matches of pattern with replacement in variable.
Use Alternate Value: ${parameter:+word}
- Expands to word if parameter is set and not null; otherwise, expands to null.
Remove Prefix: ${parameter#prefix}

Removes the shortest match of prefix from the beginning of parameter.

Remove Suffix: ${parameter%suffix}

Removes the shortest match of suffix from the end of parameter.

Substring: ${parameter:start:length}

Extracts a substring of length length from parameter, starting at position start. If length is omitted, extracts to the end of the string.

Examples

Let's illustrate these operators with examples:

# Example variables
unset var1
var2="value2"
var3=""

# Use default value
echo "${var1:-default}"   # Output: default
echo "${var2:-default}"   # Output: value2

# Assign default value
echo "${var1:=default}"   # Output: default (and sets var1 to "default")
echo "${var2:=default}"   # Output: value2 (var2 already has a value)

# Use alternate value
echo "${var1:+alt}"      # Output: (var1 is unset or null, so no output)
echo "${var2:+alt}"      # Output: alt

# Remove prefix
var="prefix_value"
echo "${var#prefix_}"    # Output: value

# Remove suffix
var="value_suffix"
echo "${var%suffix}"     # Output: value

# Substring
var="substring"
echo "${var:3:5}"        # Output: string

Practical Use Cases

Parameter expansion is commonly used in scripting for tasks such as:

Providing default values for variables.
Checking if variables are set or empty.
Manipulating strings by removing prefixes, suffixes, or extracting substrings.
Performing simple conditional logic based on variable values.

Summary

Parameter expansion is a versatile feature in bash that allows for dynamic manipulation of variables and their values. By mastering parameter expansion, you can write more concise and flexible shell scripts for a variety of tasks.