A variable may be assigned to by a statement of the form
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 (see
EXPANSION below). If the variable has its integer attribute set, then value is evaluated as an
arithmetic expression even if the $((...)) expansion is not used (see Arithmetic Expansion below). Word
splitting is not performed, with the exception of "$@" as explained below under Special Parameters.
Pathname expansion is not performed. Assignment statements may also appear as arguments to the alias,
declare, typeset, export, readonly, and local builtin commands.
In the context where an assignment statement is assigning a value to a shell variable or array index, the
+= operator can be used to append to or add to the variable's previous value. 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 (see Arrays below), 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.
Before a command is executed, its input and output may be redirected using a special notation interpreted
by the shell. Redirection may also be used to open and close files for the current shell execution
environment. The following redirection operators may precede or appear anywhere within a simple command
or may follow a command. Redirections are processed in the order they appear, from left to right.
Redirection of output causes the file whose name results from the expansion of word to be opened for
writing on file descriptor n, or the standard output (file descriptor 1) if n is not specified. If the
file does not exist it is created; if it does exist it is truncated to zero size.
The general format for redirecting output is:
If the redirection operator is >, and the noclobber option to the set builtin has been enabled, the
redirection will fail if the file whose name results from the expansion of word exists and is a regular
file. If the redirection operator is >|, or the redirection operator is > and the noclobber option to
the set builtin command is not enabled, the redirection is attempted even if the file named by word