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bash: Shell Parameters

 
 3.4 Shell Parameters
 ====================
 

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· Positional Parameters       The shell's command-line arguments.
· Special Parameters          Parameters denoted by special characters.
 
 A PARAMETER is an entity that stores values.  It can be a 'name', a
 number, or one of the special characters listed below.  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 Bash Builtins).
 
    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.
 
    A variable may be assigned to by a statement of the form
      NAME=[VALUE]
 If VALUE is not given, the variable is assigned the null string.  All
 VALUEs undergo tilde expansion, parameter and variable expansion,
 command substitution, arithmetic expansion, and quote removal (detailed
 below).  If the variable has its 'integer' attribute set, then VALUE is
 evaluated as an arithmetic expression even if the '$((...))' expansion
 is not used (Arithmetic Expansion).  Word splitting is not
 performed, with the exception of '"$@"' as explained below.  Filename
 expansion is not performed.  Assignment statements may also appear as
 arguments to the 'alias', 'declare', 'typeset', 'export', 'readonly',
 and 'local' builtin commands (DECLARATION commands).  When in POSIX mode
 (Bash POSIX Mode), these builtins may appear in a command after
 one or more instances of the 'command' builtin and retain these
 assignment statement properties.
 
    In the context where an assignment statement is assigning a value to
 a shell variable or array index (Arrays), the '+=' operator can
 be used to append to or add to the variable's previous value.  This
 includes arguments to builtin commands such as 'declare' that accept
 assignment statements (DECLARATION commands).  When '+=' is applied to a
 variable for which the INTEGER attribute has been set, VALUE is
 evaluated as an arithmetic expression and added to the variable's
 current value, which is also evaluated.  When '+=' is applied to an
 array variable using compound assignment (Arrays), the
 variable's value is not unset (as it is when using '='), and new values
 are appended to the array beginning at one greater than the array's
 maximum index (for indexed arrays), or added as additional key-value
 pairs in an associative array.  When applied to a string-valued
 variable, VALUE is expanded and appended to the variable's value.
 
    A variable can be assigned the NAMEREF attribute using the '-n'
 option to the 'declare' or 'local' builtin commands (Bash
 Builtins) to create a NAMEREF, or a reference to another variable.
 This allows variables to be manipulated indirectly.  Whenever the
 nameref variable is referenced, assigned to, unset, or has its
 attributes modified (other than using or changing the nameref attribute
 itself), the operation is actually performed on the variable specified
 by the nameref variable's value.  A nameref is commonly used within
 shell functions to refer to a variable whose name is passed as an
 argument to the function.  For instance, if a variable name is passed to
 a shell function as its first argument, running
      declare -n ref=$1
 inside the function creates a nameref variable REF whose value is the
 variable name passed as the first argument.  References and assignments
 to REF, and changes to its attributes, are treated as references,
 assignments, and attribute modifications to the variable whose name was
 passed as '$1'.
 
    If the control variable in a 'for' loop has the nameref attribute,
 the list of words can be a list of shell variables, and a name reference
 will be established for each word in the list, in turn, when the loop is
 executed.  Array variables cannot be given the nameref attribute.
 However, nameref variables can reference array variables and subscripted
 array variables.  Namerefs can be unset using the '-n' option to the
 'unset' builtin (Bourne Shell Builtins).  Otherwise, if 'unset'
 is executed with the name of a nameref variable as an argument, the
 variable referenced by the nameref variable will be unset.
 

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