How to get a variables datatype in zsh

export in zsh is shorthand for typeset -gx, where the attribute g means “global” (as opposed to local to a function) and the attribute x means “exported” (i.e. in the environment). Thus:

typeset +x GREP_OPTIONS

This also works in ksh and bash.

If you never export GREP_OPTIONS in the first place, you don't need to unexport it.

You can also use the indirect, portable way: unsetting a variable unexports it. In ksh/bash/zsh, this doesn't work if the variable is read-only.

tmp=$GREP_OPTIONS
unset GREP_OPTIONS
GREP_OPTIONS=$tmp

Awk has 4 types: "number", "string", "numeric string" and "undefined". Here is a function to detect that:

function o_class(obj,   q, x, z) {
  q = CONVFMT
  CONVFMT = "% g"
    split(" " obj "\1" obj, x, "\1")
    x[1] = obj == x[1]
    x[2] = obj == x[2]
    x[3] = obj == 0
    x[4] = obj "" == +obj
  CONVFMT = q
  z["0001"] = z["1101"] = z["1111"] = "number"
  z["0100"] = z["0101"] = z["0111"] = "string"
  z["1100"] = z["1110"] = "strnum"
  z["0110"] = "undefined"
  return z[x[1] x[2] x[3] x[4]]
}

For the third argument of split, you need something that is not a space, and not part of obj or else it will be treated as a delimiter. I chose \1 based on Stéphane suggestion. The function does internal CONVFMT toggling, so it should return the correct result regardless of CONVFMT value at the time of the function call:

split("12345.6", q); print 1, o_class(q[1])
CONVFMT = "%.5g"; split("12345.6", q); print 2, o_class(q[1])
split("nan", q); print 3, o_class(q[1])
CONVFMT = "%.6G"; split("nan", q); print 4, o_class(q[1])

Result:

1 strnum
2 strnum
3 strnum
4 strnum

Full test suite:

print 1, o_class(0)
print 2, o_class(1)
print 3, o_class(123456.7)
print 4, o_class(1234567.8)
print 5, o_class(+"inf")
print 6, o_class(+"nan")
print 7, o_class("")
print 8, o_class("0")
print 9, o_class("1")
print 10, o_class("inf")
print 11, o_class("nan")
split("00", q); print 12, o_class(q[1])
split("01", q); print 13, o_class(q[1])
split("nan", q); print 14, o_class(q[1])
split("12345.6", q); print 15, o_class(q[1])
print 16, o_class()

Result:

1 number
2 number
3 number
4 number
5 number
6 number
7 string
8 string
9 string
10 string
11 string
12 strnum
13 strnum
14 strnum
15 strnum
16 undefined

The notable weakness is: if you provide "numeric string" of any of the following, the function will incorrectly return "number":

• integer
• inf
• -inf

For integers, this is explained:

A numeric value that is exactly equal to the value of an integer shall be converted to a string by the equivalent of a call to the sprintf function with the string %d as the fmt argument

However inf and -inf behave this way as well; that is to say that none of the above can be influenced by the CONVFMT variable:

CONVFMT = "% g"
print "" .1
print "" (+"nan")
print "" 1
print "" (+"inf")
print "" (+"-inf")

Result:

 0.1
 nan
1
inf
-inf

In practice this doesn’t really matter, see the Duck test.