Difference between $HOME and '~' (tilde)?

Question

I had always thought that $HOME and ~ were exactly the same and thus could be used interchangeably. Today, when I tried to install pylibmc, a python binding to memcached, on my shared server the use of ~ gave me error but not $HOME. I would like to reason out why.

libmemcached is a requirement for pylibmc. I have libmemcached installed under my home directory because I have no root on the server. As a result, to install pylibmc, I need to make sure the installation script knows where to find libmemcached.

When executing python setup.py install --with-libmemcached=~, the installation script runs

 gcc -pthread -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall \   -Wstrict-prototypes -fPIC -DUSE_ZLIB -I~/include \   -I/usr/local/include/python2.7 -c _pylibmcmodule.c \   -o build/temp.linux-i686-2.7/_pylibmcmodule.o -fno-strict-aliasing 

which gives the errors that libmemcached can't be found.

If I change to --with-libmemcached=$HOME, the script runs

 gcc -pthread -fno-strict-aliasing -g -O2 -DNDEBUG -g -fwrapv -O3 -Wall \   -Wstrict-prototypes -fPIC -DUSE_ZLIB -I/home/waterbotte/include \   -I/usr/local/include/python2.7 -c _pylibmcmodule.c \   -o build/temp.linux-i686-2.7/_pylibmcmodule.o -fno-strict-aliasing 

without any problem. It looks like the problem is that tilde doesn't get resolved. But why?

Answer 1

The tilde is part of a shell expansion (like in bash, csh, zsh, etc). The $HOME variable is exportable and can be used independent of a specific shell.

Answer 2

The shell replaces ~ with the user's home directory (update: or perhaps by the home directory of some other user, if ~ is followed by something other than a /), but only if it's the first character of a word.

--with-libmemcached=~ has ~ not in the beginning, so the shell leaves it alone.