File Permissions

file_permissions.txt

# This is well-known behavior, it's just interesting.
$ mkdir a
$ echo "hello!" > a/file.txt
$ cat a/file.txt
hello!
$ chmod 000 a/file.txt 
# Now I don't expect to be able to change a/file.txt...
$ echo "GOODBYE" > a/file.txt
bash: a/file.txt: Permission denied
# Okay, good, I can't modify the file directly.
# But... I can just delete it and create another file with the same name...
$ rm a/file.txt
rm: remove write-protected regular file ‘a/file.txt’? y
$ echo "GOODBYE" > a/file.txt
$ cat a/file.txt 
GOODBYE

This is possible because I have write access to the directory. If I chmod -w a then I would no longer be able to delete the file. This explains why ssh is so picky about the permissions of the .ssh directory and not not just the files inside.