Bash Parameters and Variables

Internal

• bash Concepts
• bash Environment Variables

Global Variable

Global variables are only maintained within the context of the current shell. Once the shell exits, the global variables are discarded.

The global variables declared in a shell are visible inside the functions executed within that shell and inside functions invoked from function invoked from the shell, recursively.

The value of a global variable is not available to sub-shells, unless the variable is exported.

export

export VAR=VALUE

Declaring a global variable to be exported causes the variable and its value to be copied in the environment of a invoked sub-shell. The sub-shell gets a copy of the variable, not a reference. Modifying the variable from the sub-shell does not reflect into the value of the global variable maintained by the invoking shell.

Variables exported by Sub-Shells

If a sub-shell invoked from another shell exports a global variable, once the sub-shell exits, the exported variables are discarded, and the invoking shell does not sees them.

Local Variable

If a variable is declared inside a function, without any qualifier, it automatically becomes a global variable, and it is visible to the entire shell after the function execution, even after the function exits.

In order to prevent a variable declared inside of a function to become global, it must be declared as local: the local keyword designates a local variable. Local variables can only be declared inside a function.

Example:

local a=b

Local Variable Assignment and Failure

Do not do a local variable assignment from a function that may return non-zero and expect to use the return value to exit

Do not do something like this:

local a=$(do_something) || exit 1 # hoping to exit if 'do_something' returns a non-zero value

Even if do_something returns a non-zero error code, that is not detected and exit is not executed.

Do this instead:

local a;
a=$(do_something) || exit

Listing Declared Variables

Use declare or typeset:

declare -p

A way of obtaining the value for a specific variable is:

local var_name="HISTSIZE"
declare -p | grep "declare .. ${var_name}" | sed -e 's/.*=//'

Booleans

Recommended usage pattern:

Declaration:

some_var=true

Usage:

${some_var} && echo "this will execute"

if ${some_var}; then
   echo "this will execute"
fi

Do not use

if [ ${some_var} ]; then ...

This will always evaluate to "true" regardless of the some_var value.