Basics
Discussion about gcc -ansi -pedantic -Wall
man gcc to see what is the meaning of the flags
-ansi = -std=c90 disables several GNU extensions
(i.e. typeof, asm, inline etc.) also several extensions of c99
are not supported (like declarations in for loop's condition)
-pedantic makes gcc reject programs using forbidden extensions
and some non compliant to -ansi (but not all) and issue any
warnings accordingly
-Wall generate warnings about suspicious/questionable code (A well
written program should not print any warnings when using this flag)
-Wextra even more warnings
File permissions
An example of the output produced by ls -l is shown below.
drwx------ 2 richard staff 2048 Jan 2 1997 private
drwxrws--- 2 richard staff 2048 Jan 2 1997 admin
-rw-rw---- 2 richard staff 12040 Aug 20 1996 admin/userinfo
drwxr-xr-x 3 richard user 2048 May 13 09:27 public
Field 1: a set of ten permission flags.
Field 2: link count (don't worry about this)
Field 3: owner of the file
Field 4: associated group for the file
Field 5: size in bytes
Field 6-8: date of last modification (format varies, but always 3 fields)
Field 9: name of file (possibly with path, depending on how ls was called)
chmod
chmod changes the permissions of a file/directory
+-----------------------------+
| Permissions |
+-----------+--------+--------+
| execute | x | 1 |
+-----------+--------+--------+
| read | r | 4 |
+-----------+--------+--------+
| write | w | 2 |
+-----------+--------+--------+
+-----------------------------+
| Numerical permissions |
+---+-------------------+-----+
| # | Permission | rwx |
+---+-------------------+-----+
| 7 | full | 111 |
+---+-------------------+-----+
| 6 | read and write | 110 |
+---+-------------------+-----+
| 5 | read and execute | 101 |
+---+-------------------+-----+
| 4 | read only | 100 |
+---+-------------------+-----+
| 3 | write and execute | 011 |
+---+-------------------+-----+
| 2 | write only | 010 |
+---+-------------------+-----+
| 1 | execute only | 001 |
+---+-------------------+-----+
| 0 | none | 000 |
+---+-------------------+-----+
Examples
chmod g+w myfile
give group write permission to myfile, leaving all other permission
flags alone
chmod g-rw myfile
remove read and write access to myfile, leaving all other permission
flags alone
chmod g+rwxs mydir
give full group read/write access to directory mydir, also setting
the set-groupID flag so that directories created inside it inherit the
group
chmod u=rw,go= privatefile or chmod 600 privatefile
explicitly give user read/write access, and revoke all group and other
access, to file privatefile
chmod -R g+rw .
give group read write access to this directory, and everything inside
of it (-R = recursive)
Notes
Directories must always have execute permission.
Files with execute and without read can still be executed but not read.
The directories with your assignments should never be readable by other users.
References:
Authored by: Foivos S. Zakkak