Speaking UNIX, Part 4: UNIX ownership and permissions provide for privacy and participation

Share and share alike

Learn how to manipulate file permissions to protect your files, or share them with others.

Martin Streicher (martin.streicher@gmail.com), Chief Technology Officer, McClatchy Interactive

Photo of Martin StreicherMartin Streicher is a freelance Ruby on Rails developer and the former Editor-in-Chief of Linux Magazine. Martin holds a Masters of Science degree in computer science from Purdue University and has programmed UNIX-like systems since 1986. He collects art and toys. You can reach Martin at martin.streicher@gmail.com.



17 October 2006

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From the dawn of the electronic age a scant 50 years ago until the launch of the Apple computer in 1977, computing hardware was scarce and extremely expensive to acquire and operate. What cycles could be eked out of (the relatively) primitive systems of the day were precious commodities, grudgingly allocated to only the most vexing problems. Projects vied to get computer time, and the earliest information technology (IT) managers worked to keep systems busy 24 hours per day, seven days per week. After all, an idle tick was money wasted.

Ideally, those lost ticks would be allocated on demand and automatically to any ready task awaiting computation. And indeed, that's the idea (first proposed by Robert Berner as early as 1957) behind time-sharing. As realized in Multics, RSTS/E, and later UNIX® and its many modern variants, time-sharing, or multitasking, divides a computer's resources (the CPU, input and output, and memory) among awaiting jobs, creating the illusion that every job has exclusive use of the machine. Attach several dumb terminals to a multitasking mainframe and each terminal appears to be a personal computer.

Nowadays, it's quite likely that you have your own UNIX computer and share an even more powerful multi-processor system with others. Yet in both cases -- your laptop and the UNIX behemoth in the corporate machine room -- simultaneous access remains fundamental. UNIX provides robust tools and infrastructure so that you can both protect and share information.

This article looks at user privileges and, in particular, examines how to manipulate file permissions to restrict or share your directories and files with others. Understanding permissions is crucial if you want to speak UNIX fluently.

The id, the ego, and the UID

To start, let's explore just who you are. At a command prompt, type whoami:

$ whoami
strike

My computer answers strike, which is my user name (the same name I use to log in). Your whoami should return your login name.

As it turns out, your user name is really a pseudonym for your user ID (UID). To see your UID, type id -u:

$ id -u
501

Here, my UID is 501.

In general, user names are used whenever possible instead of UIDs simply because they're human-readable and easy to remember. For example, if you run the ps uxf command to see a list of your running processes, ps displays your user name as the owner of your processes.

$ ps uxf
USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
strike   32346  0.0  0.1  6496 1832 ?        S    19:39   0:00 sshd: strike@pts/0
strike   32347  0.0  0.1  2592 1476 pts/0    Ss   19:39   0:00  \_ -bash
strike   32358  0.0  0.0  2476  820 pts/0    R+   19:39   0:00      \_ ps uxf

The output shows three running processes: an ssh login, which spawned a bash shell, which launched the ps command itself. Similarly, if you run ls -alFG in your home directory to view the owners of your files, your user name is displayed instead of your UID.

In general, only you can interrupt your tasks. (Of course, the superuser, root, can control and manipulate all tasks.) For example, joe, another user sharing the same system, cannot terminate my running shell, which is process 32347 in the list above:

$ whoami
joe
$ kill -INT 32347
-bash: kill: (32347) - Operation not permitted

Here, 32347 is the process ID of my shell, which joe can find by running ps auxf. However, because I own the process, joe cannot terminate it. In contrast, I can certainly kill any of my own tasks, as shown in Listing 1.

Listing 1. Kill your own processes
$ ps uxf
...
strike   32347  0.0  0.1  2592 1488 pts/0    Ss   19:39   0:00  \_ -bash
strike   32733 39.5  0.0  1480  356 pts/0    R    19:50   0:01      \_ yes
$ kill -INT 32733
$ ps uxf
...
strike   32347  0.0  0.1  2592 1488 pts/0    Ss   19:39   0:00  \_ -bash

Upon running kill -INT 32733, process 32733 (the yes command) is terminated.

While a relatively simple concept, strong ownership rights and per-user provisioning are two features that make UNIX far more secure than, say, Microsoft® Windows®.


Membership has its privileges

Like the jobs you launch, you also own the directories and files you create. For example, run the ls -alFG command on your home directory and $HOME to see what you own, as shown in Listing 2.

Listing 2. List what you own in the home directory
$ ls -alFG $HOME
...
-rw-------   1 strike     6175 Aug 25 07:03 .bash_history
-rw-------   1 strike      567 Apr 20  2005 .bash_profile
-rw-------   1 strike     1834 Apr 20  2005 .bashrc
drwx------   2 strike     4096 Mar  8 10:54 .ssh/
-rw-------   1 strike     9516 Aug 22 16:42 .viminfo
-rw-r--r--   1 strike  1529617 Jul 19 07:00 Archive.zip
drwxrwx---   3 strike     4096 Aug 24 04:04 IBM/
drwxr-xrwx   3 strike     4096 Jun 14 06:06 backups/
...

The trouble with UNIX groups

Changing the group owner of a file is a convenient way to share the file with all the members of that group. For example, if you, jane, and joe compose the science UNIX group and you want to share the sensitive nuclear.csv data file with both users, simply change the group owner of that file to science.

But what if sam and bertha also need access to that file and neither are in the science group? You could certainly add both users to science, but that would allow them to access all the files that the science group owns.

This is an unfortunate limitation of the UNIX group implementation: A file can have one and only one group owner, and implementing complex sharing schemes typically leads to an explosion of groups. (You could create a new group, say, jjsb, just for joe, jane, sam, and bertha. But what happens when donald needs access, too?)

If you require complex access control schemes, consider using access control lists (ACLs), which many UNIX implementations offer. An ACL extends the standard UNIX ownership model to grant (or exclude) specific rights per user or per group. An ACL is like keeping a bouncer at the door: You can let your buddies in plus certain celebrities and other hand-picked guests, but keep the paparazzi and the rest of the riff-raff out.

As you can see, I own all the directories and files in my home directory. As the owner, I can delete, rename, and move any of my files and directories, and I can edit any of my files. You have the same rights for your files (files means files and directories collectively, unless a specific exception is made).

In addition, you can elect to share your files with others. Indeed, because UNIX was designed as a multi-user system, sharing files is another principle tenet of the operating system.

In addition to a specific user owner, every file and directory also has a group owner. A UNIX group is simply a collection of users, and you can be a member of one or more groups. Use id to discover your memberships:

$ id
uid=501(strike) gid=501(strike) groups=501(strike),
  81(appserveradm), 79(appserverusr), 80(admin)

On my system, my primary group is group id (GID) 501, or the group named strike. I also belong to three other groups:

  • appserveradm
  • appserveruser
  • admin

Typically and by default, the group owner of a file that you create is your primary group, but you can subsequently change the group owner to any of the groups that you belong to. Run ls -laF on your home directory to reveal more about your files, as shown in Listing 3.

Listing 3. Discover more about your files
-rw-------   1 strike strike     6118 Aug 27 21:59 .bash_history
-rw-r--r--   1 strike strike      567 Apr 20  2005 .bash_profile
-rw-r--r--   1 strike strike     1834 Apr 20  2005 .bashrc
drwx------   2 strike strike     4096 Mar  8 10:54 .ssh/
-rw-------   1 strike strike     9516 Aug 22 16:42 .viminfo
-rw-r--r--   1 strike strike  1529617 Jul 19 07:00 Archive.zip
drwxr-xr-x   3 strike strike     4096 Aug 24 04:04 IBM/
drwxrwxr-x   3 strike admin      4096 Jun 14 06:06 backups/

Note: Listing 3 displays the typical output of ls -l. Listing 2 looks different because the group owner was intentionally hidden to simplify the earlier discussion. You can hide the group owner with -G

The directory named backups has group owner admin, which extends certain privileges to all the members of that group. Meanwhile, the group owner of the rest of my files is strike. Typically, a user is the only member of his or her eponymous group, which effectively limits access solely to the user.


The fine details

If you glance back at the output of ls above, you'll likely notice a sequence of 10 characters at the beginning of each line. Each character is an off or on setting, or bit, representing a specific right for one of three constituencies: you, one of your groups, and others. Figure 1 shows the uses of each bit.

Figure 1. The permission bits of a UNIX file
UNIX permissions

In Figure 1:

  • The initial bit indicates whether the file is a directory. (In general, the initial bit indicates whether a file is special. If the file is special, the initial character is d for directory and l for symbolic link, among others.) This setting is immutable.
  • The next three bits (shown in blue) represent your right to read, write, and execute the file, respectively. You might disable your write bit, for example, to prevent the file from being deleted. (Yes, you need write permission to delete a file.)
  • The next three bits (in green) represent a group's right to read, write, and execute the file.
  • The final three bits (in orange) represent the rights of every other user (that is, all users, excluding yourself and the members of your group).

Use the ls -laF output found above as examples:

  • The files .bash_history, .bash_profile, .bashrc, and .viminfo are readable and writable only by me. I can view, edit, and delete these files.
  • The .ssh directory is accessible only by me. The first bit indicates that it's a special file, and the d stands for directory. I can display the contents of the directory because its user-read bit is set. I can add and remove files from the directory, because its user-write bit is set. You might be wondering why the directory is user-executable, as well. A directory cannot be traversed (entered and cataloged) unless this bit is set. (By the way, as mentioned in Part 3 of this series, your .ssh directory must be private to you, or your public key access won't work.)
  • I can read and write the file Archive.zip, and others can read the file. (Of course, the strike group can also read the file, but if I'm the only member of that group, the permission is somewhat moot.)
  • I can catalog, read, and write files in IBM, another directory, and others can catalog its contents.
  • Finally, the members of the admin group and I can catalog, read, and write files in backups, and everyone else can catalog and read.

Make a directory sticky

If you want to ensure that a directory persists and isn't removed by accident, set the sticky bit by using chmod +T.

$ ls -lF drwxrwxrwx 2 strike strike 68 Aug 28 06:21 dropbox/ drwx------ 2 strike strike 68 Aug 28 06:21 mine/ $ chmod +t dropbox $ ls -lF drwxrwxrwt 2 strike strike 68 Aug 28 06:21 dropbox/ drwx------ 2 strike strike 68 Aug 28 06:21 mine/

With the sticky bit set and with permissions set to all can read, write, and execute, everyone can place files in my drop box, but I am the only user who can remove files in the directory or remove the directory itself.

If you run ls -ld /tmp, chances are that your system-wide temporary directory, used widely as application scratch space, is sticky.

You modify permissions (except for the directory bit) by using the chmod (change mode) command. You manipulate a file's group assignment by using the chgrp (change group) command. (The superuser, root, can also change file ownership by using the chown, or change owner, command.)

Here are example applications of chmod:

  • chmod u+x script.sh: If you write a shell script and want to execute it, enable its execute bit. Here, the phrase u+x means for the user owner (u) to enable (+) the execute bit (x). The general form of chmod is chmod, none (which implies user), one, or more of u, g (for group), or o (for others), a + or -, and one or more of r, w, and x.
  • chmod go+rx IBM: This command enables the read and execute privileges for group and others.
  • chmod a+rx script.sh: In addition to u, g, and o, you can use the modifier a for all or user, group, and others. Thus, this command enables read and execute for all three constituencies.
  • chgrp admin backups: This command changes the group owner to admin.

If you add permissions with +, it adds the specified permissions but leaves other permissions untouched. Similarly, if you retract permissions with - (minus), it disables certain permissions but otherwise leaves the status quo. If you want to set all the permissions at once, use numeric file modes. (You can also use the chmod = operator. See the man page for details.)

Note: A numeric file mode is a octal digit from 0 to 7, or a three-bit number, one bit for each of read, write, and execute. Because there are three constituencies for each file, a fully specified file mode is three digits, such as 400, 644, or 777. Here are some examples:

  • To make a directory private, you enable rights for yourself but retract the rights for a group and others:
    $ mkdir example
    $ ls -l
    drwxr-xr-x    2 strike  strike       68 Aug 28 11:27 example
    $ chmod 700 example
    $ ls -l
    drwx------    2 strike  strike       68 Aug 28 11:27 example

    Mode 700 translates to enable read, write, and execute for user (the leading 7), but disable all other permissions (the two latter zeroes).
  • If you want to let everyone else on your computer read one of your files, enable the read permission for all three constituencies:
    $ ls -l .aliases
    -rw-------   1 mstreich  mstreich  79 Jul 26 17:08 .aliases
    $ chmod 644 .alias
    $ ls -l
    -rw-r--r--  1 mstreich  mstreich  79 Jul 26 17:08 .aliases

    644 is a shorthand for set, read, and write permissions for me (6), and read for group (4) and others (4). Of course, if the file .aliases was in your home directory, the permissions for $HOME would have to permit cataloging and reading.

The chart in Table 1 presents numeric values and the associated result for user owner, group owner, and others. Simply add the values in each section to find the proper value to set.

Table 1. Numeric values and associated results
WhoValueResult
User0400Allow read by owner.
0200Allow write by owner.
0100For files, allow execute by owner; for directories, allow the owner to search in the directory.
Group0040Allow read by group members.
0020Allow write by group members.
0010For files, allow execute by owner; for directories, allow group members to search in the directory.
Others0004Allow read by others.
0002Allow write by others.
0001For files, allow execution by others; for directories allow others to search in the directory.

For example, to set read and write privileges for others, add 0004 and 0002 to yield 0006. Do something similar for user and group and add all three sums to yield a fully qualified numeric mode.


Permissions are fundamental

Setting and managing permissions comes up again and again on UNIX systems. You often need proper permissions on specific files and directories to run daemons; directories, such as /tmp, work only if correct permissions are set; and, of course, to share files with others -- or to protect your files -- you must be able to set, change, and read permissions.

The next article in this series continues with files and explores utilities and techniques to manage hundreds or thousands of files on multiple computers.

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