Working with the Unix OS

Example:
drwxr-xr-- 2 fred student 1024 Mar 21 19:35.
0 = "d" its a directory
123 = "rwx", the owner has full permission
456 = "r-x" the student group members can list this directory and access its files
789 = "r--" everyone else can list the directory but not touch its files
or "--x"
access directory files but only if file names are known, can't list directory
! In Summary:
To access an existing file, a user needs:
- search "x" permissions to each directory in the path name
- access "r" permission to the file
Example:
cat ../dir1/dir2/prog.c
- need "x" search permissions in:
..
../dir1
../dir1/dir2
- and "r" read permission
../dir1/dir2/prog.c
Introduction to Unix
Warning:
- don't allow others "w" permissions to your directories, as files can then be deleted
- don't allow "w" permissions to your .profile "rm -r *" in your .profile would zap all your files next time you
logged in chown, chgrp & chmod
By the way...
- chown user file
-changes file ownership
-can donate a file to another user
-restricted to root on most systems
- chgrp group file
-changes the group of a file
-only group members need apply
- chmod mode file
-changes file mode (permission bits)
-can modify existing permissions or explicitly set new permissions
- chmod u+rw myfile
-adds user read & write permissions
- chmod og-x myprog
-removes group & others execute permission
! Mode bits:
File permissions are stored in one number as a set of bits called the mode
User | Group | Other
r w x r w x r w x
4 4 4 read
2 2 2 write
1 1 1 execute
Example
chmod 644 myfile
chmod 751 myprog
sets "rw-r--r--" permissions
sets "rwxr-x--x" permissions
! Unmask command:
The "umask" command is used to set the default creation mask for new files and directories (can be set in .profile).
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