# Unfortunately, creating files and then changing their permissions
# is a race condition: some other user could come at the wrong
# moment, and open the file while it had incorrect permissions
# (between creation and ‹chmod›). In most cases, it is not a huge
# risk, but it is a risk and in certain circumstances can lead to
# serious security problems.

# There are multiple mechanisms that can be used to close this
# loophole, one of them being ‹umask›. The other is specifying the
# permissions at the time the file is being created, but this is
# mainly an option for C programs. On the user side of things, not
# many tools allow the user to specify the permissions they want for
# a particular new file.

# This is where ‹umask› comes in: the operating system maintains a
# per-process «permission mask» (called ‹umask›) which is subtracted
# from permissions of each newly created file. Most shells allow the
# user to manipulate this mask, and all programs executed by the
# shell inherit the mask from the shell (since it is inherited
# across ‹fork›).

# When a file is created, all the bits that are set in the ‹umask›
# are cleared:

echo umask 777
umask 777
touch umask.txt
ls -l umask.txt
rm -f umask.txt

# The clearing of permission bits from ‹umask› happens atomically: a
# file with the permission bits set does not exist, not even
# temporarily. Of course, 777 makes little sense as an actual
# ‹umask›, but it does demonstrate what is going on. A more
# reasonable (though also somewhat paranoid) option would be 077,
# which means new files are only accessible to the owner (i.e. the
# creator of the file, i.e. us):

echo
echo umask 077
umask 077
touch umask.txt
ls -l umask.txt
rm -f umask.txt

# Let's try to create a file that is marked executable by the tool
# that creates it. A compiler is a good example:

echo
echo umask 077 with cc
echo 'int main() {}' > empty.c
cc empty.c
ls -l a.out
rm -f a.out

# A somewhat less reasonable ‹umask› (again only useful for
# demonstration purposes) would deny any executable bits:

echo
echo umask 133 with cc
umask 133
cc empty.c
ls -l a.out
rm -f a.out

# And finally, a fairly common default would be 022: this prevents
# anyone but the owner from writing into new files, but anyone else
# can read and/or execute them if they wish so.

echo
echo umask 022
umask 022
touch file.txt
cc empty.c
ls -l a.out file.txt
rm -f a.out file.txt

# Possibly useful: if you run ‹umask› without arguments, it will
# print the currently active ‹umask›. That said, let's look at
# creating files in C now, and specifically setting their
# permissions upon creation. See ‹perm.c›.