# The ‹chmod› command serves to change permission bits of files
# (well, i-nodes, so that covers directories, specials and so on).
# Let's start by creating an empty file:

echo creating file.txt...
touch file.txt
echo hello > file.txt # this also works
cat file.txt # this works as expected

# We have checked that we can read the file and write into it --
# this is normal and expected. However, we can change the
# permissions (using ‹chmod›) to remove either of those access
# rights (or both). First, we check what the initial permissions
# were:

ls -l file.txt

# Let's look at ‹chmod› then. What it does is manipulate the
# individual bits, but using a somewhat different syntax. The
# formatting we see in the output of ‹ls› is optimized for output,
# but writing out the permissions in this form could be somewhat
# tedious, especially if all we want is to flip one of the bits. For
# this reason, ‹chmod› gives names to each of the triplet: ‹u› for
# user (owner), ‹g› for owning group and ‹o› for others. Beware that
# ‹o› does «not» mean the owner!

# What we can do now is tell ‹chmod› which bits we are interested in
# and what to do with them. We can either set bits with ‹+› or
# remove them with ‹-›. For instance, ‹g+w› would mean ‘add the ‹w›
# bit to the group triplet’, or in other words, let the users in the
# owning group write into this file. To remove the write permission
# from ourselves (the owner), we will use ‹u-w›, like this:

echo
echo removing the write permission and testing...
chmod u-w file.txt
ls -l file.txt
echo world >> file.txt

# Likewise, we can remove the read permission from ourselves, using
# ‹u-r›. Then we won't be able to read from the file either:

echo
echo removing the read permission and testing...
chmod u-r file.txt
ls -l file.txt
cat file.txt # permission error

# Multiple bits can be specified in a single group. Let's restore
# the original permissions:

echo
echo "restoring permissions (and testing again)"
chmod u+rw file.txt
ls -l file.txt
echo world >> file.txt
cat file.txt

# If you want to specify multiple groups in a single ‹chmod›
# command, that can be done too: separate the groups with a comma,
# like this: ‹g+r,o-w›. You can also set the permissions
# directly, using assignment, or combine the two alternatives:
# ‹chmod u=rw,g+x›).

# With that out of the way, let's try an exercise (in ‹1.txt›).