Bash Parameter Expansion: A Short Guide
Bash parameter expansion is a powerful feature that allows you to manipulate strings in shell scripts. It can be used to remove or replace substrings, handle error cases, and more.
Here’s a quick overview of the most commonly used parameter expansion syntaxes:
Basic Syntax
The basic syntax of parameter expansion is ${var}. This expands to the value of the variable var. If var is unset or null, it expands to an empty string. You can also use ${var:-default} to expand to the value of var, or default if var is unset or null.
Example:
name="John Doe"
echo "Hello, ${name:-there}" # Output: "Hello, John Doe"
unset name
echo "Hello, ${name:-there}" # Output: "Hello, there"
Removing Substrings
You can remove substrings from the beginning or end of a variable using ${var#pattern}, ${var##pattern}, ${var%pattern}, or ${var%%pattern}. The # and % characters remove the shortest match, while the ## and %% characters remove the longest match.
Example:
path="/path/to/some/file.txt"
echo ${path#/} # Output: "path/to/some/file.txt"
echo ${path##/} # Output: "file.txt"
echo ${path%.txt} # Output: "/path/to/some/file"
echo ${path%%/*} # Output: ""
Replacing Substrings
You can replace substrings in a variable using ${var/pattern/replacement} or ${var//pattern/replacement}. The first syntax replaces the first match of pattern with replacement, while the second syntax replaces all matches.
Example:
greeting="Hello, world!"
echo ${greeting/world/universe} # Output: "Hello, universe!"
echo ${greeting//o/0} # Output: "Hell0, w0rld!"
echo ${greeting/#Hello/Hi} # Output: "Hi, world!"
echo ${greeting/%!/... goodbye!} # Output: "Hello, world... goodbye!"
Error Handling
You can use ${var:?error message} to exit the script with an error message if var is unset or null.
Example:
name=
echo "Hello, ${name:?Please provide a name}"
This will output an error message and exit the script if name is unset or null.
Substring Length
You can use ${#var} to get the length of a variable’s value.
Example:
word="hello"
echo ${#word} # Output: 5
Default Value
You can use ${var:-default} to provide a default value if var is unset or null. If var is set, ${var:-default} expands to var.
Example:
echo ${missing:-default} # Output: "default"
name="John Doe"
echo ${name:-default} # Output: "John Doe"
Final Thoughts
Bash parameter expansion is a powerful tool that can help you write more efficient and concise shell scripts. By mastering the basic syntax and some of the most common patterns, you’ll be able to manipulate strings with ease and handle error cases more gracefully.