Lab46 Wiki

Comm

“comm” is a UNIX command which can be used to compare two sorted files. The input for comm is two files, and the output is two separate columns. The command syntax for comm is

comm [OPTION]... FILE1 FILE2

comm has three main options, -1, -2, and -3.

The -1 option will print the lines present in FILE1 in the left hand column, and all common lines in the right hand column, whereas the -2 option will do the opposite (print lines present in FILE2 on the left, and common lines on the right). The -3 option, however, will only print lines unique to both files, with FILE1's unique lines being on the left, and FILE2's unique lines on the right.

• man comm!

Identification of chosen keyword: umask

The command umask is used to determine file modes at creation. Using umask on a file will determine the permissions of that file and all child processes resulting from that file. umask was used on Super Puzzle Box 2 Turbo (vim setup.exe). Where chmod changes permissions on a file, umask sets the initial permissions. In Super Puzzle Box 2 Turbo (the best Puzzle Box this side of the Danube), when the datafile is created, umask is used to set permissions to (octal) 777, ensuring that permissions will not be an issue when working with the file.

lab46:~$ umask u=rwx,g=,o=
lab46:~$ mkdir goop; cd goop
lab46:~/goop$ touch trees
lab46:~/goop$ mkdir pain
lab46:~/goop$ ls -l
total 0
drwx------ 2 dsherbur lab46 6 Nov 14 15:19 pain
-rw------- 1 dsherbur lab46 0 Nov 14 15:18 trees

Setting the umask values to what would be octal 700, which gives all permissions to the user and owner and no permissions to group or other, I have made private files. Changing umask again changes the permissions of newly created files.

lab46:~$ umask u=rwx,g=rwx,o=rwx
lab46:~$ cd goop
lab46:~/goop$ ls -l
total 0
drwx------ 2 dsherbur lab46 6 Nov 14 15:19 pain
-rw------- 1 dsherbur lab46 0 Nov 14 15:18 trees
lab46:~/goop$ touch park
lab46:~/goop$ ls -l
total 0
drwx------ 2 dsherbur lab46 6 Nov 14 15:19 pain
-rw-rw-rw- 1 dsherbur lab46 0 Nov 14 15:22 park
-rw------- 1 dsherbur lab46 0 Nov 14 15:18 trees

Note the difference in the umask line from the first block to the second block. We can use this to prank people who leave their terminals open by setting the umask vales to 0 across the board, making files that they create themselves unaccessible. However, umask can be changed and the files created under the permissionless umask and be changed via chmod to be accessible.

lab46:~/goop$ umask u=,g=,o=
lab46:~/goop$ touch jello
touch: setting times of `jello': Permission denied
lab46:~/goop$ ls -l
total 0
---------- 1 dsherbur lab46 0 Nov 14 15:26 jello
drwx------ 2 dsherbur lab46 6 Nov 14 15:19 pain
-rw-rw-rw- 1 dsherbur lab46 0 Nov 14 15:22 park
-rw------- 1 dsherbur lab46 0 Nov 14 15:18 trees
lab46:~/goop$ vim jello
lab46:~/goop$ chmod 777 jello; ls -l
total 0
-rwxrwxrwx 1 dsherbur lab46 0 Nov 14 15:26 jello
drwx------ 2 dsherbur lab46 6 Nov 14 15:19 pain
-rw-rw-rw- 1 dsherbur lab46 0 Nov 14 15:22 park
-rw------- 1 dsherbur lab46 0 Nov 14 15:18 trees

When I vim'd jello, i was greeted with a permission denied screen and I was unable to modify or write to the file, verifying the initial permissions of the file.

List any sites, books, or sources utilized when researching information on this topic. (Remove any filler text).

• Reference 1
• Reference 2
• Reference 3

umask

umask is a command in Linux/UNIX systems that can be used to determine a the default permissions granted to any file upon creation of the file.

• man umask
• http://linuxzoo.net/page/sec_umask.html

For my demonstration, I'll be creating a file, then changing the umask, and creating another file.

The first file was created with the default umask.

lab46:~/umask$ ls -la
total 4
drwxr-xr-x  2 smeas lab46    6 Dec 13 16:54 .
drwx-----x 28 smeas lab46 4096 Dec 13 16:53 ..
lab46:~/umask$ vi test
lab46:~/umask$ ls -la
total 4
drwxr-xr-x  2 smeas lab46   17 Dec 13 16:57 .
drwx-----x 28 smeas lab46 4096 Dec 13 16:57 ..
-rw-r--r--  1 smeas lab46    0 Dec 13 16:57 test
lab46:~/umask$

Unlike normally when you chmod a file, the umask settings are the opposite (in chmod, 0 is no permissions, 7 is all permissions). Here is a list of the permissions granted by umask.

Octal value : Permission
0 : read, write and execute
1 : read and write
2 : read and execute
3 : read only
4 : write and execute
5 : write only
6 : execute only
7 : no permissions

I then changed the umask using the command “umask 111” and created a second file. This file was created with read and write permissions for everyone.

lab46:~/umask$ umask 111
lab46:~/umask$ vi test2
lab46:~/umask$ ls -la
total 4
drwxr-xr-x  2 smeas lab46   29 Dec 13 17:02 .
drwx-----x 28 smeas lab46 4096 Dec 13 17:02 ..
-rw-r--r--  1 smeas lab46    0 Dec 13 16:57 test
-rw-rw-rw-  1 smeas lab46    0 Dec 13 17:02 test2
lab46:~/umask$