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504.

Evaluate arithmetic expressions.

Evaluate each ARG as an arithmetic expression. Evaluation is done in
fixed-width integers with no check for overflow, though division by 0
is trapped and flagged as an error. The following list of operators is
grouped into levels of equal-precedence operators. The levels are listed
in order of decreasing precedence.

[tab]id++, id--[tab]variable post-increment, post-decrement
[tab]++id, --id[tab]variable pre-increment, pre-decrement
[tab]-, +[tab][tab]unary minus, plus
[tab]!, ~[tab][tab]logical and bitwise negation
[tab]**[tab][tab]exponentiation
[tab]*, /, %[tab][tab]multiplication, division, remainder
[tab]+, -[tab][tab]addition, subtraction
[tab]<<, >>[tab][tab]left and right bitwise shifts
[tab]<=, >=, <, >[tab]comparison
[tab]==, !=[tab][tab]equality, inequality
[tab]&[tab][tab]bitwise AND
[tab]^[tab][tab]bitwise XOR
[tab]|[tab][tab]bitwise OR
[tab]&&[tab][tab]logical AND
[tab]||[tab][tab]logical OR
[tab]expr ? expr : expr
[tab][tab][tab]conditional operator
[tab]=, *=, /=, %=,
[tab]+=, -=, <<=, >>=,
[tab]&=, ^=, |=[tab]assignment

Shell variables are allowed as operands. The name of the variable
is replaced by its value (coerced to a fixed-width integer) within
an expression. The variable need not have its integer attribute
turned on to be used in an expression.

Operators are evaluated in order of precedence. Sub-expressions in
parentheses are evaluated first and may override the precedence
rules above.

Exit Status:
If the last ARG evaluates to 0, let returns 1; let returns 0 otherwise.

505.

Read a line from the standard input and split it into fields.

Reads a single line from the standard input, or from file descriptor FD
if the -u option is supplied. The line is split into fields as with word
splitting, and the first word is assigned to the first NAME, the second
word to the second NAME, and so on, with any leftover words assigned to
the last NAME. Only the characters found in $IFS are recognized as word
delimiters.

If no NAMEs are supplied, the line read is stored in the REPLY variable.

Options:
-a array[tab]assign the words read to sequential indices of the array
[tab][tab]variable ARRAY, starting at zero
-d delim[tab]continue until the first character of DELIM is read, rather
[tab][tab]than newline
-e[tab][tab]use Readline to obtain the line in an interactive shell
-i text[tab]Use TEXT as the initial text for Readline
-n nchars[tab]return after reading NCHARS characters rather than waiting
[tab][tab]for a newline, but honor a delimiter if fewer than NCHARS
[tab][tab]characters are read before the delimiter
-N nchars[tab]return only after reading exactly NCHARS characters, unless
[tab][tab]EOF is encountered or read times out, ignoring any delimiter
-p prompt[tab]output the string PROMPT without a trailing newline before
[tab][tab]attempting to read
-r[tab][tab]do not allow backslashes to escape any characters
-s[tab][tab]do not echo input coming from a terminal
-t timeout[tab]time out and return failure if a complete line of input is
[tab][tab]not read withint TIMEOUT seconds. The value of the TMOUT
[tab][tab]variable is the default timeout. TIMEOUT may be a
[tab][tab]fractional number. If TIMEOUT is 0, read returns success only
[tab][tab]if input is available on the specified file descriptor. The
[tab][tab]exit status is greater than 128 if the timeout is exceeded
-u fd[tab][tab]read from file descriptor FD instead of the standard input

Exit Status:
The return code is zero, unless end-of-file is encountered, read times out,
or an invalid file descriptor is supplied as the argument to -u.

506.

Return from a shell function.

Causes a function or sourced script to exit with the return value
specified by N. If N is omitted, the return status is that of the
last command executed within the function or script.

Exit Status:
Returns N, or failure if the shell is not executing a function or script.

507.

Set or unset values of shell options and positional parameters.

Change the value of shell attributes and positional parameters, or
display the names and values of shell variables.

Options:
-a Mark variables which are modified or created for export.
-b Notify of job termination immediately.
-e Exit immediately if a command exits with a non-zero status.
-f Disable file name generation (globbing).
-h Remember the location of commands as they are looked up.
-k All assignment arguments are placed in the environment for a
command, not just those that precede the command name.
-m Job control is enabled.
-n Read commands but do not execute them.
-o option-name
Set the variable corresponding to option-name:
allexport same as -a
braceexpand same as -B
emacs use an emacs-style line editing interface
errexit same as -e
errtrace same as -E
functrace same as -T
hashall same as -h
histexpand same as -H
history enable command history
ignoreeof the shell will not exit upon reading EOF
interactive-comments
allow comments to appear in interactive commands
keyword same as -k
monitor same as -m
noclobber same as -C
noexec same as -n
noglob same as -f
nolog currently accepted but ignored
notify same as -b
nounset same as -u
onecmd same as -t
physical same as -P
pipefail the return value of a pipeline is the status of
the last command to exit with a non-zero status,
or zero if no command exited with a non-zero status
posix change the behavior of bash where the default
operation differs from the Posix standard to
match the standard
privileged same as -p
verbose same as -v
vi use a vi-style line editing interface
xtrace same as -x
-p Turned on whenever the real and effective user ids do not match.
Disables processing of the $ENV file and importing of shell
functions. Turning this option off causes the effective uid and
gid to be set to the real uid and gid.
-t Exit after reading and executing one command.
-u Treat unset variables as an error when substituting.
-v Print shell input lines as they are read.
-x Print commands and their arguments as they are executed.
-B the shell will perform brace expansion
-C If set, disallow existing regular files to be overwritten
by redirection of output.
-E If set, the ERR trap is inherited by shell functions.
-H Enable ! style history substitution. This flag is on
by default when the shell is interactive.
-P If set, do not follow symbolic links when executing commands
such as cd which change the current directory.
-T If set, the DEBUG trap is inherited by shell functions.
-- Assign any remaining arguments to the positional parameters.
If there are no remaining arguments, the positional parameters
are unset.
- Assign any remaining arguments to the positional parameters.
The -x and -v options are turned off.

Using + rather than - causes these flags to be turned off. The
flags can also be used upon invocation of the shell. The current
set of flags may be found in $-. The remaining n ARGs are positional
parameters and are assigned, in order, to $1, $2, .. $n. If no
ARGs are given, all shell variables are printed.

Exit Status:
Returns success unless an invalid option is given.

508.

Unset values and attributes of shell variables and functions.

For each NAME, remove the corresponding variable or function.

Options:
-f[tab]treat each NAME as a shell function
-v[tab]treat each NAME as a shell variable

Without options, unset first tries to unset a variable, and if that fails,
tries to unset a function.

Some variables cannot be unset; also see `readonly'.

Exit Status:
Returns success unless an invalid option is given or a NAME is read-only.

509.

Set export attribute for shell variables.

Marks each NAME for automatic export to the environment of subsequently
executed commands. If VALUE is supplied, assign VALUE before exporting.

Options:
-f[tab]refer to shell functions
-n[tab]remove the export property from each NAME
-p[tab]display a list of all exported variables and functions

An argument of `--' disables further option processing.

Exit Status:
Returns success unless an invalid option is given or NAME is invalid.

510.

Mark shell variables as unchangeable.

Mark each NAME as read-only; the values of these NAMEs may not be
changed by subsequent assignment. If VALUE is supplied, assign VALUE
before marking as read-only.

Options:
-a[tab]refer to indexed array variables
-A[tab]refer to associative array variables
-f[tab]refer to shell functions
-p[tab]display a list of all readonly variables and functions

An argument of `--' disables further option processing.

Exit Status:
Returns success unless an invalid option is given or NAME is invalid.

511.

Shift positional parameters.

Rename the positional parameters $N+1,$N+2 ... to $1,$2 ... If N is
not given, it is assumed to be 1.

Exit Status:
Returns success unless N is negative or greater than $#.

512.

Execute commands from a file in the current shell.

Read and execute commands from FILENAME in the current shell. The
entries in $PATH are used to find the directory containing FILENAME.
If any ARGUMENTS are supplied, they become the positional parameters
when FILENAME is executed.

Exit Status:
Returns the status of the last command executed in FILENAME; fails if
FILENAME cannot be read.

513.

Suspend shell execution.

Suspend the execution of this shell until it receives a SIGCONT signal.
Unless forced, login shells cannot be suspended.

Options:
-f[tab]force the suspend, even if the shell is a login shell

Exit Status:
Returns success unless job control is not enabled or an error occurs.