I'm trying to understand the grammar/syntax of the shell command language (to be able to parse it) and I cannot seem to find where the syntax of the comparison operations is defined.
As an example:
x=10
while [ $x -gt 0 ]
do
#do something
x=$(($x-1))
done
The syntax for [ $x -gt 0 ] doesn't appear to be defined anywhere in the specification linked above. In fact, the syntax [...] doesn't appear in the grammar rules and ] is missing from the punctuation/operator tokens. The specification doesn't say much about this construction other than [] being used for regular expression brackets and that [, but not ], "may" need to be quoted in strings.
My questions:
[...] generally (whether in POSIX or elsewhere)?Are the comparison operators a part of the POSIX specification or a generally recognized extension?
There is no special syntax for comparison in the POSIX shell language. [ is the name of a standard utility command and $x -gt 0 ] are its arguments, the closing square bracket is there only for esthetics. The command [ $x -gt 0 ] can also be written as test $x -gt 0.
Where can I find the syntax for the shell comparison operators, or
[...]generally?