expression1 || expression2
True if either expression1 or expression2 is true.
The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.
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3.2.6 Grouping Commands
Bash provides two ways to group a list of commands to be executed as a unit. When commands are grouped, redirections may be applied to the entire command list. For example, the output of all the commands in the list may be redirected to a single stream.
()
( list )
Placing a list of commands between parentheses causes a subshell to be created, and each of the commands in list to be executed in that subshell. Since the list is executed in a subshell, variable assignments do not remain in effect after the subshell completes.
{}
{ list; }
Placing a list of commands between curly braces causes the list to be executed in the current shell context. No subshell is created. The semicolon (or newline) following list is required.
In addition to the creation of a subshell, there is a subtle difference between these two constructs due to historical reasons. The braces are reserved words, so they must be separated from the list by blanks. The parentheses are operators, and are recognized as separate tokens by the shell even if they are not separated from the list by whitespace.
The exit status of both of these constructs is the exit status of list.
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3.3 Shell Functions
Shell functions are a way to group commands for later execution using a single name for the group. They are executed just like a "regular" command. When the name of a shell function is used as a simple command name, the list of commands associated with that function name is executed. Shell functions are executed in the current shell context; no new process is created to interpret them.
Functions are declared using this syntax: [ function ] name () { command-list; }
This defines a shell function named name. The reserved word function is optional. If the function reserved word is supplied, the parentheses are optional. The body of the function is the command-list between { and }. This list is executed whenever name is specified as the name of a command. The exit status of a function is the exit status of the last command executed in the body.
Note that for historical reasons, the curly braces that surround the body of the function must be separated from the body by blanks or newlines. This is because the braces are reserved words and are only recognized as such when they are separated by whitespace. Also, the command-list must be terminated with a semicolon or a newline.
When a function is executed, the arguments to the function become the positional parameters during its execution (see section 3.4.1 Positional Parameters). The special parameter `#' that expands to the number of positional parameters is updated to reflect the change. Positional parameter 0 is unchanged. The FUNCNAME variable is set to the name of the function while the function is executing.
If the builtin command return is executed in a function, the function completes and execution resumes with the next command after the function call. When a function completes, the values of the positional parameters and the special parameter `#' are restored to the values they had prior to the function's execution. If a numeric argument is given to return, that is the function's return status; otherwise the function's return status is the exit status of the last command executed before the return.
Variables local to the function may be declared with the local builtin. These variables are visible only to the function and the commands it invokes.
Functions may be recursive. No limit is placed on the number of recursive calls.
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3.4 Shell Parameters
3.4.1 Positional Parameters The shell's command-line arguments.
3.4.2 Special Parameters Parameters with special meanings.
A parameter is an entity that stores values. It can be a name, a number, or one of the special characters listed below. For the shell's purposes, a variable is a parameter denoted by a name. A variable has a value and zero or more attributes. Attributes are assigned using the declare builtin command (see the description of the declare builtin in 4.2 Bash Builtin Commands).
A parameter is set if it has been assigned a value. The null string is a valid value. Once a variable is set, it may be unset only by using the unset builtin command.
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