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.sh NAME
= (equals) \*- shell assignment command
.sh SYNOPSIS
.bd =
letter
[ arg1 [ arg2 ] ]
.sh DESCRIPTION
The ``='' command provides shell string variables.
The 26
.it letter
variables (`a'\-`z') are referenced in later commands
in the manner of shell arguments, i.e.:
.it "$a, ..., $z."
If no arguments are given, the standard input is read to newline
or EOT for the value.
The exit code is set to 0 if a
newline is found in the input;
it is set to 1 otherwise, thus providing and end-of-file indicator.
If
.it arg1
is the only argument,
or if two non-null arguments are given,
the variable is set to
.it arg1,
and the exit code set to 0.
If two arguments are given,
and if
.it arg1
is a null string,
the value of
.it arg2
is assigned to the variable,
and the exit code is set to 1,
permitting a convenient default mechanism:
.s3
.in +5n
= a "$1" "default value" && shift
.in -5n
.s3
The ``='' command works either at the terminal, or in
shell command files.
The variables can be assigned repeatedly.  Storage
is assigned as needed, but there is no recovery.
.s3
Some
.it letter
variables have predefined meanings and are initialized once at
the time the Shell begins execution:
.s3
.lp +5 5
$n	The argument count.
``sh file arg1 arg2 arg3'' has the value 3.
The shift command does not change the value of $n.
.s3
.lp +5 5
$p	This variable holds the shell search sequence
of pathname prefixes for command execution.
Alternatives are separated by ``:''.
The default initial value is:
.s3
		= p ":/bin:/usr/bin"
.s3
which prepends successively
.nf
		the null pathname (execute from current dir.),
		/bin,
		/usr/bin.
.fi
.s3
Using the same type of specification, users may choose their own
sequence by storing it in a file named ``.path'' in
their login directory.
The ``.path'' information passes to successive shells
(and other commands like
.it time\^\c
(I)
or
.it nohup\^\c
(I));
the $p value does not.
In any case, no prepending occurs when a command name contains a `/'.
.s3
.lp +5 5
$r	exit(status) of the most recent command executed
by the Shell.
The value is ASCII numeric, and is initially `0'.
At end-of-file the shell exits with the value of
.it $r.
.s3
.lp +5 5
$s	Name of login (starting) directory.
.s3
.lp +5 5
$t	Terminal identification letter or number:
/dev/tty$t is a file name for the terminal.
.s3
.lp +5 5
$w	First component in $s pathname, i.e., file system name (such as /usr).
.s3
.lp +5 5
$z	Is the name of the Shell.
Its default value is `/bin/sh',
but this can be overridden by supplying a name as
the second line of the `.path' file.
.s3
.i0
Note that variables (`a' \*- `m')
are guaranteed to be initialized to null strings and usable in any way
desired.
Variables (`n' \*- `z') may acquire special uses in the future.
The values of $n, $s, $t, and $w may reasonably be modified;
it is catastrophic to change $p;
it is possible, but useless to modify $r.
.s3
The
.bd"``=''"
command
is executed within the shell.
Note that it is commonly used to read the first line of output from a
pipe or a line from the terminal, for example:
.s3
.in +5n
.nf
grep \*-c string file \*v = a
.ti -5n
or:
.br
= a </dev/tty
.fi
.in -5n
.sh "EXIT CODES"
0 \*- normal read, or first of two arguments is not null.
.br
1 \*- end-of-file, or first of two arguments is null.
.br
.sh "SEE ALSO"
expr(I), sh(I)