Bash Functions

Function declaration, local variables, return codes, argument handling, and function libraries.

Overview

Overview Bash functions allow code reuse and organization within scripts. They behave like mini-scripts within your script, with their own positional parameters but sharing the script's variable scope by default.

Declaration syntax: Two equivalent forms exist: function name { body; } and name() { body; }. The name() form is slightly more POSIX-compatible. Functions must be defined before they are called (bash is interpreted top-to-bottom).

local keyword: Critical for preventing variable pollution. Without local, any variable set inside a function is global within the script. local var=value creates a variable that only exists within the function and its children. Always use local for function-internal variables.

Arguments: Inside a function, $1 $2 $@ $# refer to the function's arguments, shadowing the script's positional params. $0 still refers to the script name.

Return values — the key concept: The return statement in bash only returns an exit code (0-255 integer). It cannot return strings or complex data. To "return" a string, use echo inside the function and capture with command substitution: result=$(my_function). For large data, write to a file or use a global variable (named explicitly, not local).

$FUNCNAME: An array containing the call stack. ${FUNCNAME[0]} is the current function, ${FUNCNAME[1]} is the caller. Useful for error messages: echo "Error in ${FUNCNAME[0]}: message" >&2.

Sourcing libraries: source /path/to/lib.sh or . /path/to/lib.sh loads functions and variables from another file into the current shell. Use this to build reusable function libraries. Source relative paths: source "$(dirname "$0")/lib.sh".

Key concepts

local keyword: local var=value restricts variable to the function scope. Without local, variables set inside a function are globally visible in the script. Always use local for internal function variables.
return vs echo for values: return N only returns an exit code 0-255. To "return" a string, echo it and capture: result=$(my_func). For large data, write to a temp file or use a named global.
$FUNCNAME array: ${FUNCNAME[0]} = current function name. ${FUNCNAME[1]} = caller. Useful for error messages and call stack tracing. Array automatically maintained by bash.
Sourcing function libraries: source /path/lib.sh or . /path/lib.sh loads functions into current shell. Use "$(dirname "$0")/lib.sh" for relative paths. All sourced functions share the current shell environment.

When to use it

Organizing complex scripts into reusable components
Building shared function libraries sourced by multiple scripts
Reducing duplication in deployment and automation scripts
Creating wrapper functions for common operations with error handling

Interview questions

How do you return a string value from a bash function? Why can't you use the return statement?
Explain the difference between local and global variables in bash functions with an example.

Bash Functions

Overview

Key concepts

When to use it

Interview questions

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