Permissions

This is the first place at the terminal where people lose heart. Permission denied.

This chapter is about reading the -rw-r--r-- sitting to the left in ls -l, and changing it with chmod.

It looks like a mysterious row of symbols, but what is inside is very simple. Can read (r), can write (w), can execute (x) — those three, laid out in order for the owner, the group and everyone else. Read the nine characters three at a time.

There are two ways of changing them.

• write it in symbols — chmod u+x run.sh (give the owner execute)
• write it in numbers — chmod 640 secret.txt (three digits, each the sum of r=4, w=2, x=1)

Both do the same thing, so learn to read both. Numbers are what you meet more often in the field.

Once the mechanism makes sense, you need never go around putting 777 on things and hoping. That one means "anyone in the world may rewrite this", and it is far too strong a medicine for 99% of what you want to fix.

A directory's x does not mean "execute" but "can pass through". That is the one exception, so it comes at the end of the chapter.