Overview
This lecture explains Bash parameter expansion, highlighting its syntax and practical uses for reading, modifying, and substituting variable values in shell scripts.
What is Bash Parameter Expansion?
- Parameter expansion retrieves or modifies the value of a shell variable using
${variable} syntax.
- Curly brackets
{} are used for complex expansions, especially when modifying parameter values.
Parameter Expansion vs. Substitution
- Parameter expansion allows setting default values if variables are undefined; substitution only retrieves values without defaults.
- Example:
${language:-Bash} prints "Bash" if language is unset; ${language} prints the set value.
Conditional Parameter Expansion
${var:-default} prints the default if var is unset or empty, but does not assign it.${var:+alt} prints alt if var is set, otherwise prints nothing.${var:=default} prints and assigns default if var is unset or empty.
Substring Operations
${var:offset:length} extracts part of a string; offset is start index, length is optional.- Example:
${var:0:4} gets the first four characters.
String Substitution
${var/pattern/replacement} replaces the first match of pattern with replacement.${var//pattern/replacement} replaces all matches.
Regex and Removal
- Use patterns like
${var/%pattern/replacement} to match and replace endings.
- Example: Remove numbers at string’s end and append another value.
Splitting Strings
- You can split strings on a delimiter into arrays using parameter expansion and loops.
- Example:
${course//:/ } splits a colon-separated string into parts.
Cheat Sheet Highlights
- Print variable:
${parameter}
- Print with default:
${parameter:-default}
- Assign default if unset:
${parameter:=default}
- Alternate if set:
${parameter:+alt}
- Length of var:
${#parameter}
- Substring:
${parameter:offset:length}
- Search/replace:
${parameter/search/replace} or ${parameter//search/replace}
- Indirect reference:
${!parameter}
Key Terms & Definitions
- Parameter Expansion — Method for retrieving or modifying variable values in Bash with
${} syntax.
- Default Value — Value assigned or printed if variable is unset, using
:- or :=.
- Substring Extraction — Pulling part of a string using start index and optional length.
- Substitution — Replacing part or all occurrences of a substring or pattern.
- Indirection — Accessing the value of a variable whose name is stored in another variable with
${!parameter}.
Action Items / Next Steps
- Practice writing scripts using different forms of parameter expansion.
- Review and memorize the common expansion patterns from the cheat sheet.