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Symbol attributes are as follows:
- Built-In
- Symbols that cannot be removed.
- Global
- Symbols that retain their value outside
of statement blocks. All built-in symbols are global.
- Local
- Symbols that have scope only within the
statement block in which they are used.
- Immutable
- Built-in symbols that cannot have
their value altered.
- Constant
- Global symbols that cannot have their
value altered by ordinary assignment. They can only have their
value altered using the
constant command.
Symbols can be of string, number or array type. They may be
of both string and number type simultaneously. There are two
arrays types: associative and data.
The print command always prints the string
representation of a symbol. The printf() and
sprintf() functions print the number or string
representation, depending on the given format specifications.
Arithmetic operators use the number value of a symbol. The
relational operators
> < <= >=
use the number value if both operands are of number type,
otherwise the string values are lexicographically compared.
If the string value of a symbol cannot be interpreted as a
number, its number value is zero.
A nonglobal symbol is automatically brought into existence
within a statement or statement block ({ ...
}) simply by using its name.
A symbol assigned a value at the top level (outside of any
curly brackets) is automatically made global.
The local command forces the scope of a symbol
to be within the current statement block and allows reuse of
global names within statement blocks. Names already used as
macro names may be used as local symbols within a statement
block, although that macro then becomes unavailable within that
block.
BUILT-IN SYMBOLS
spec's built-in symbols include the
following:
FRESH - is nonzero if spec
was started with the
-f (fresh) flag.
VERSION - a string containing the
spec version as in
"4.03.12".
PI - the numeric constant 3.14159...
USER - the user's name, from
/etc/passwd.
HOME - the user's home directory, from the
environment.
TERM - the user's terminal type, from the
environment.
GTERM - the user's graphics terminal type,
from the environment. May be changed while running.
DISPLAY - used by X Window clients to
figure out on which host to run. On startup, the value is taken
from the environment. If not set in the environment, the value
saved in the user's state file is used. The value can be changed
while running, and the current value will be exported to new
x11filt processes as they are spawned.
COLS - the number of columns on the screen.
Initially taken from the environment, may be changed by
assignment, unless the
TIOCGWINSZ
ioctl() is available, in which case the values via
that call will be used.
ROWS - the number of rows on the screen.
Obtained as above.
CWD - the current working directory.
SPEC - the name of the program.
SPECD - the auxiliary file directory.
DEBUG - the debugging level.
MOTORS - the number of motors from the
config file.
COUNTERS - the number of counters from the
config file.
A - used to communicate motor positions.
S - holds counter values.
Other built-in symbols are installed by site-dependent code.
In particular, the symbols G, Q,
Z and UB are used with most of the
standard geometry modules.
EXAMPLE
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