What does the command "PWD=/proc/self/cwd another_executable " mean?

When building AOSP 9.0.0_r30 source with these commands:

$ make clean $ make showcommands -j8 > ~/BuildAndroid.txt 

there are commands line like this in ~/BuildAndoird.txt:

[ 58% 62374/106553] PWD=/proc/self/cwd prebuilts/clang/host/linux-x86/clang-4691093/bin/clang++ ........ 

I suppose this is a bash command,but why there is "PWD=/proc/self/cwd" and a space(not ; or &&) before "prebuilts/clang/host/linux-x86/clang-4691093/bin/clang++"?What does this mean,I don't think it is legal for bash because space means the following text are parameters instead of command.

Another problem is,what is "/proc/self/cwd",is that a standard proc node?

Some experiment:

$ VAR=123 $ VAR=456 echo $VAR 123 $ echo $VAR 123 

So this is really legal,but what is the meaning?

2 Answers

It executes the command

prebuilts/clang/host/linux-x86/clang-4691093/bin/clang++ 

but when setting up the environment for this command, it sets the environment variable PWD to the value /proc/self/cwd. Note that this affects only the environment of the clang++ process, not the environment/variable space of the shell script.

Try again:

VAR=123 VAR=456 echo $VAR 123 

Ok, but:

VAR=456 /bin/sh -c 'echo $VAR' 456 

POSIX variable $PWD

$PWD will point to current directory

echo $PWD; ls -l /proc/self/cwd /home/user lrwxrwxrwx 1 user user 0 mar 26 09:18 /proc/self/cwd -> /home/user 

Doing so will ensure $PWD to be set, as android don't require them at all.