How do I check if an environment variable is set in cmake

Question

I set an environment variable in my bash profile, so I can see it in the terminal just fine . .

blah/builds$ echo $THING

thingy

How do I display it in a cmake message and check if it is set? I have tried the following, but it just displays thing as blank and skips the body of the if statement

message("THING:" $ENV{THING})
if(DEFINED ENV{THING})
    message(STATUS "THING environment variable defined")
    # some more commands
endif()

Answer 1

Knowing perfectly well what export does and how environment works in general, I've still got a mouthful of WTFs with this form:

IF(DEFINED $ENV{THING})

but it worked fine in this form:

IF(DEFINED ENV{THING})

Notice the $ omission.

---

N.B. you can quickly test this using cmake -P:

[~] cat > test-env.cmake << 'EOF'
IF(DEFINED ENV{FOOBAR})
    MESSAGE(STATUS "FOOBAR env seen: --[$ENV{FOOBAR}]--")
ELSE()
    MESSAGE(STATUS "WTF")
ENDIF()
EOF
[~]
[~] FOOBAR=test cmake -P test-env.cmake
-- FOOBAR env seen: --[test]--

---

The reason it behaves so weirdly, is legacy, as usual. IF used to do insane stuff in older CMake; they sort-of fixed this in CMake 3.1 — in a backward-compatible manner — with CMP0054, which you have to enable explicitly:

CMAKE_MINIMUM_REQUIRED(VERSION 3.1)
PROJECT(...)

CMAKE_POLICY(SET CMP0054 NEW) #-- fixes IF() with quoted args
CMAKE_POLICY(SET CMP0057 NEW) #-- enables IF(.. IN_LIST ..)

Answer 2

You CMake code is correct. The problem is most likely that you only set the environment variable in your shell but did not export it. Run the following before invoking cmake:

export THING

Answer 3

Just to be clear: as https://cmake.org/cmake/help/latest/command/if.html says:

if (DEFINED ENV{THING})
  # do stuff
endif()

is the right syntax, no $ anywhere.