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.
while list-1; do list-2; done
until list-1; do list-2; done
The while command continuously executes the list list-2 as long as the last command in the list
list-1 returns an exit status of zero. The until command is identical to the while command,
except that the test is negated; list-2 is executed as long as the last command in list-1 returns
a non-zero exit status. The exit status of the while and until commands is the exit status of the
last command executed in list-2, or zero if none was executed.
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 input causes the file whose name results from the expansion of word to be opened for
reading on file descriptor n, or the standard input (file descriptor 0) if n is not specified.
The general format for redirecting input is: