Bash Parameters and Variables

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Internal

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.

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/.*=//'