chmod (short for change mode) is a Linux / Unix command used to change permissions of files and directories.
chmod accepts either human readable notation of the octal bitwise mask. The bitwise mask often has the three digits, specifying (from left to right) permissions for the world, group and the owner of the file. The bits (left to right) are read, write, and execute. For instance,
chmod 740 x.sh
makes x.sh viewable, editable and executable for the current owner. The group can view but not change or execute, and the world has no access. This can be verified with ls -l x.sh:
-rw-r--r-- 1 me 11 2013-01-25 09:53 x.sh
Permission flags can also be specified as letters (r - read, w - write, x - execute), using + or - sign to turn them on or off, for all users. For instance
chmod +r-x x.sh
with make x.sh readable for possible users but no longer executable, even for the owner. The write permission that has not been mentioned in the command, will not be revoked form the owner:
-rw-r--r-- 1 me 11 2013-01-25 09:53 x.sh
Chmod also accepts the forth (actually first) digit that sets (left to right) setUID, setGUI and sticky flags. If not specified, it is assumed 0 (no such flags).
If chmod parameter is less than 3 digits, the first owner and then group permissions are assumed zero. The following example sets (probably in an unexpected way) full permissions for the world and no permissions for the user or group:
chmod 7 x.sh
cat x.sh
cat: x.sh: Permission denied