Otherwise, the file doesn't exist

What do I need to do to change the permissions for my file?

Ex: file.txt has userA as owner.
I would like to let staff and everyone permissions as they are, i.e. read only. But I want to add read and write permissions for userB.
It's like making a chmod 755 for userA, then another chmod 755 for userB, but all that with a terminal command in userA's session.

Thanks for your help.

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Step by step:

(1) Start the computer on single user mode:
cmd s

(2) Do the FSCK thing (type exactly the following and press enter/return):
/sbin/fsck -fy

If fsck displays the following message you are on the right way:
***** FILE SYSTEM WAS MODIFIED *****

(3) Mount the disk (the -uw is critical // type exactly the following and press enter/return):
/sbin/mount -uw/

(4) Change the permissions (type exactly the following and press enter/return):
chmod -R 755 /

(5) Boot (you will have to type it twice)
exit
*****Gives you a message*****

exit

(6) The system should start up fine. If not, force restart with the on/off button.

(7) Once back in the GUI or Graphic Interface, open DiskUtility from the Utilities folder inside Applications. Run the option Verify Permissions after selecting your harddrive from the menu on the left. After it finished finding problems everywhere, select Repair Permissions.

(8) Done!

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Other Mac OS boot options:

To use these boot shortcuts or commands, hold down the bolded keypress while your system is booting up. If it does not work, you probably did not start holding down the key (or key combination) soon enough.

shift – Safe Mode

Booting with the shift key depressed starts OS X in Safe Mode with Account preferences ignored and unessential kernal extensions / kexts disabled.

option – Startup Manager

Booting with the option key depressed starts OS X with the Startup Manager which allows the user to select which OS or partition from which to boot.

Mouse button – Eject Media

When the user boots with the mouse key depressed, any media in the drive such as a DVD or CD is ejected.

cmd v – Verbose Boot

If this key command is held down when booting, all the startup messages that are typically hidden will be displayed. This is an excellent method for debugging booting issues.

c – Boot from CD/DVD

Booting with the “C” key held will tell the system to boot directly from a CD/DVD in the drive.

t – Target Mode

If the “T” key is held during the bootup, the computer will be placed in target mode. This basically allows the computer to be accessed by other systems as a FireWire drive. Other computers can read, write, or mount the target mode system just like any other FireWire drive.

Other Bootup Shortcuts —

cmd s – Single User Mode

x – OS X Force boot

r – PowerBook Display Reset

n – Boot up from NetBoot Server

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More

Bypass startup drive and boot from external (or CD).... CMD-OPT-SHIFT-DELETE
Boot from CD (Most late model Apples) ................. C
Boot from a specific SCSI ID #.(#=SCSI ID number)...... CMD-OPT-SHIFT-DELETE-#
Zap PRAM .............................................. CMD-OPT-P-R
Disable Extensions .................................... SHIFT
Rebuild Desktop ....................................... CMD-OPT
Close finder windows.(hold just before finder starts).. OPT
Boot with Virtual Memory off........................... CMD
Trigger extension manager at boot-up................... SPACE
Force Quadra av machines to use TV as a monitor........ CMD-OPT-T-V
Boot from ROM (Mac Classic only)....................... CMD-OPT-X-O
Force PowerBooks to reset the screen................... R
Force an AV monitor to be recognized as one............ CMD-OPT-A-V
Eject Boot Floppy...................................... Hold Down Mouse Button
Select volume to start from............................ OPT
Start in Firewire drive mode........................... T
Startup in OSX if OS9 and OXS in boot partition........ X or CMD-X
Hold down until the 2nd chime, will boot into 9?....... CMD-OPT
OSX: Watch the status of the system load............... CMD-V
OSX: Enter single-user mode (shell-level mode)......... CMD-S

chmod -R means execute the chmod recursively through all enclosed folders
chmod -r means remove the ability to read for all (user, group, other) on whatever file is specified
chmod +r means add the ability to read for all (user, group, other) on whatever file is specified
chmod +R is not valid syntax

from the Terminal application, things become (mostly) case sensitive, as in most Unix systems. For more information on the correct syntax for chmod, type 'man chmod' (without the single quotes) from the command line and read the man page.

re: Comment 9424
changing permissions will *not* work on a file that has been locked. First one must unlock the file, then change the permissions. Files attributes, such as locked, invisible, etc., can be manipulated by the /usr/bin/SetFile command.