What is parameter expansion in bash and how to use it

Parameter expansion in Bash allows you to manipulate and expand the values of variables. It provides a way to perform string manipulation, substitution, or extraction operations on variable values.

Here are some commonly used parameter expansion techniques in Bash:

  1. Variable substitution: You can substitute the value of a variable using the ${variable} syntax. If the variable is unset or empty, you can provide a default value.Example:

echo "Hello, ${name}!"   # Output: Hello, John!

In this example, the value of the name variable is substituted into the string using ${name}.

  1. Default value substitution: If a variable is unset or empty, you can specify a default value to be used instead.Example:
echo "Age: ${age:-Unknown}"   # Output: Age: Unknown

In this example, the value of the age variable is empty. The ${age:-Unknown} syntax substitutes the default value “Unknown” since age is unset or empty.

3. Length of a string: You can determine the length of a string using ${#variable}.Example:

message="Hello, world!"
echo "Length: ${#message}"   # Output: Length: 13

In this example, ${#message} returns the length of the string stored in the message variable.

4. String manipulation: You can perform various string manipulation operations, such as substring extraction, pattern matching, and replacement, using different parameter expansion techniques. Example:

greeting="Hello, world!"
echo "Substring: ${greeting:7:5}"         # Output: Substring: world
echo "Pattern match: ${greeting#Hello, }" # Output: Pattern match: world!
echo "Replacement: ${greeting/world/John}" # Output: Replacement: Hello, John!

In this example, different parameter expansion forms are used to extract a substring, perform a pattern match, and perform a replacement operation on the greeting variable.

These are just a few examples of parameter expansion in Bash. Bash provides a rich set of expansion options and techniques to manipulate and extract information from variable values. You can refer to the Bash documentation for a comprehensive list of available expansions and their usage.

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