gnuplot documentation

20 SUBSTITUTION AND COMMAND LINE gnuplot MACROS 4.3 37
20 Substitution and Command line macros
When a command line to gnuplot is first read, i.e. before it is interpreted or executed, two forms of
lexical substitution are performed. These are triggered by the presence of text in backquotes (ascii
character 96) or preceded by @ (ascii character 64).
20.1 Substitution of system commands in backquotes
Command-line substitution is specified by a system command enclosed in backquotes. This command
is spawned and the output it produces replaces the backquoted text on the command line. Some
implementations also support pipes; see plot datafile special-filenames (p. 72).
Command-line substitution can be used anywhere on the gnuplot command line, except inside strings
delimited by single quotes.
Example:
This will run the program leastsq and replace leastsq (including backquotes) on the command line
with its output:
or, in VMS
f(x) = ‘leastsq‘
f(x) = ‘run leastsq‘
These will generate labels with the current time and userid:
set label "generated on ‘date +%Y-%m-%d‘ by ‘whoami‘" at 1,1
set timestamp "generated on %Y-%m-%d by ‘whoami‘"
20.2 Substitution of string variables as macros
Substitution of command line macros is disabled by default, but may be enabled using the set macros
command. If macro substitution is enabled, the character @ is used to trigger substitution of the current
value of a string variable into the command line. The text in the string variable may contain any
number of lexical elements. This allows string variables to be used as command line macros. Only string
constants may be expanded using this mechanism, not string-valued expressions. For example:
set macros
style1 = "lines lt 4 lw 2"
style2 = "points lt 3 pt 5 ps 2"
range1 = "using 1:3"
range2 = "using 1:5"
plot "foo" @range1 with @style1, "bar" @range2 with @style2
The line containing @ symbols is expanded on input, so that by the time it is executed the effect is
identical to having typed in full
plot "foo" using 1:3 with lines lt 4 lw 2, \
"bar" using 1:5 with points lt 3 pt 5 ps 2
The function exists() may be useful in connection with macro evaluation. The following example checks
that C can safely be expanded as the name of a user-defined variable:
C = "pi"
if (exists(C)) print C," = ", @C
Macro expansion does not occur inside either single or double quotes. However macro expansion does
occur inside backquotes.