Seeing file permission/ownership ls -l (Shows the long listing of a file/directory, which allows you to see file permission & ownership)
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1 Objective Manage file permission and ownership by Christine Bresnahan: Christine-Bresnahan.com Manage access permissions on regular and special files as well as directories. Use access modes such as SUID, SGID, and the sticky bit to maintain security. Know how to change the file creation mask. Use the group field to grant file access to group members. Commands to Know Seeing file permission/ownership ls -l (Shows the long listing of a file/directory, which allows you to see file permission & ownership) Change file owner or group chown chgrp Change file permission chmod Control default permissions umask Change file attributes chattr ========================================== Understanding file permission/ownership Permissions: r = read w = write x = execute - = none Permission fields have three placeholders: Placeholder #1 can be r or - (read or none) Placeholder #2 can be w or - (write or none) Placeholder #3 can be x or - (execute or none) Examples: Read, Write and Execute permission is written: rwx Read, and Execute permission is written: r-x Default directory permission: Default file permission: (rwxrwxrwx) (rw-rw-rw-) See a file's permissions (and more) by using the ls -l command Note: that option is a lowercase "L" and NOT a one. Continued on next page... 1
2 Understanding file permission/ownership continued... ls -l my_file displays: Field Numbers: Field Descriptions: 1 - File Type (- =file, d=directory, b=block device file, c= character device file, l= symbolically linked file, n=named pipe, s=socket) 2 - User Permissions (applies to file's owner) 3 - Group Permissions (applies to file's group) 4 - Other Permissions (applies to every other user that is not in the group nor is the file's owner) 5 - File's owner (user name) 6 - File's group (group name) 7 - File's size (in bytes) 8. - File's most recent modification date/time File's name Permissions can be referred to either alphabetically (rwx) or numerically (777): The four permissions (r= Read, w=write, x=execute, - = none) R W X The permissions numeric equivalent. The numeric value of different combinations of permissions X 1 -W- 2 -WX 3 R-- 4 R-X 5 RW- 6 RWX 7 Example of permissions, their groupings, and their numeric equivalents: Alphabetically (r-x-wxrw-) Numeric Equivalent = 536 Default directory permission: Default file permission: 777 or (rwxrwxrwx) 666 or (rw-rw-rw-) Continued on next page... 2
3 Change file owner or group chown - changes the owner of a file chown Sally my_file - rw-r--r-- Sally HR 2048 Apr 08 11:22 my_file Note: You can use the -R option (Recursively) to change an entire directory of files' owners. ======= chown can also change the owner and group at the same time! chown Sally.Admin my_file - rw-r--r-- Sally Admin 2048 Apr 08 11:22 my_file Note: If a plain ole' user changes the ownership of his/her file to another user, the plain user cannot change the ownership back! The new owner of the file has to change ownership of the file. ====== chgrp - changes the group of a file chgrp Admin my_file - rw-r--r-- Bob Admin 2048 Apr 08 11:22 my_file Continued on next page... 3
4 Change file permission chmod - changes permissions (aka mode) of a file Method #1 - Using alphabetical characters Codes used in Method #1: u=user, g=group, o=other, a= all r=read,w=write, x=execute - to subtract a permission + to add a permission = to set an exact permission Examples: chmod g+w my_file - rw-rw-r-- Bob HR 2048 Apr 08 11:22 my_file chmod g-w my_file - rw-r--r-- Bob HR 2048 Apr 08 11:23 my_file chmod u-r, g+wx, o-r my_file - -w-rwx--- Bob HR 2048 Apr 08 11:24 my_file chmod a=rwx my_file - rwxrwxrwx Bob HR 2048 Apr 08 11:25 my_file chmod g=r,o=r my_file - rwxr--r-- Bob HR 2048 Apr 08 11:26 my_file Method #2 - Using numbers Codes used in Method #2 : See permission numeric equivalents on previous page 2 of study guide. Examples: chmod 664 my_file - rw-rw-r-- Bob HR 2048 Apr 08 11:22 my_file chmod 644 my_file - rw-r--r-- Bob HR 2048 Apr 08 11:23 my_file chmod 777 my_file - rwxrwxrwx Bob HR 2048 Apr 08 11:24 my_file 4
5 Control default permissions umask - "takes away" permissions To see current setting: umask To set: umask #### where #### is s four digit number. first digit applies to Special Permissions (see below) last three digits apply to file permissions. If umask is set to 0022: Remember files are created with a default permission of 666. Thus, the file would have a permission of 666 (rw-rw-rw-) minus 022 (----w--w-), which = 644 (rw-r--r--). If umask is set to 0024: The file would have a permission of 666 (rw-rw-rw-) (----w-r--)= 642 (rw--w-r--) =================== Special Permissions Special permissions are designated in the first digit of a four digit permission number. SUID - designated by a 4 or an s in the User permissions. rwsrwxrw- (4474) SGID - designated by a 2 or an s in the Group permissions. rwxrwsrw- (2744) Sticky Bit - designated by a 1 or a t in the Other permissions. rw-rw-rwt (1444) Note: If letter is capitalized that means the file has execute privilege too. rw-rwsrw- (2474) What do they do? 1) Set User ID (SUID) set on a file allows user to become temporary owner of file (with owner's access rights), while it is executing set on a directory - useless 2) Set Group ID (SGID) set on a file allows user to become temporary member of file's group, while it is executing set on a directory allows a file created in the directory to be owned by creator, but sets the file's group to the directory's group 3) Stick Bit set on a file - useless set on a directory allows files o To be created in a directory by anyone (Note: write access must be granted on directory.) o Only to be deleted by the files' owner =================== Continued on next page... 5
6 Change file attributes chattr - change additional file attributes (besides permissions, owner, etc) General syntax: chattr add or subtract option filename Add or Subtract: Use a + to add an option or use a - to remove an option Option (a few): a Set file to append data only. i Set file to be immutable, so it cannot be changed (can't be written to, can't be deleted, etc) j Set file to have all data written to it journaled. (This is done automatically on some filesystems). 6
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