Bash Parameters and Variables

Internal

• bash Concepts
• bash Built-In 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, with the #export keyword.

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 sub-shell. The sub-shell gets a copy of the variable, not a reference. The sub-shell has no access to the parent process's environment whatsoever. Modifying the variable from the sub-shell does not reflect into the value of the global variable maintained by the invoking shell. When the shell sub-process terminates any changes made to its environment are lost. There is no way to modify directly the parent's environment.

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.

Built-In Variables

Built-In Environment Variables

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.

Variable Expansion

http://tldp.org/LDP/abs/html/varassignment.html

Expand to a Default Value

http://wiki.bash-hackers.org/syntax/pe#use_a_default_value

${var:-alternative}

The ":" form: if the variable 'var' referred in the expression was not defined, or it is the empty string, the expression expands the alternative.

${var-alternative}

If the colon is omitted, the alternative value is only used when the variable is not defined, not when it was empty.

Assign a Default Value

http://wiki.bash-hackers.org/syntax/pe#assign_a_default_value

${var:=word}

${var=word}

Use an Alternate Value

http://wiki.bash-hackers.org/syntax/pe#use_an_alternate_value