Welcome to UNIX

Last modified Tuesday, 26-Aug-2025 12:37:59 UTC.

Users to Processes to Hardware
- Shell
  - Provides a text interface
  - Provides direct access to
    - Process control
    - File system
    - UNIX tools
  - Provides many options, techniques, configurations
  - Suitable for Computer Science
- Application/windowing layer
  - Hides the text interface
  - Hides functionality
  - Not suitable for Computer Science (woos button pushers)
  - Is useful for providing improved text interfaces and running graphical applications, but not for OS access.
- Accessing the computer's resources
  - Users cannot touch the hardware, not even root
  - Users can ask the OS to touch the hardware
  - Done through library routines that make requests to the kernel.
Online Documentation
- Traditionally available on every UNIX system, quite terse
- Usage: man [section] <command>
- Sections (may vary by UNIX flavour):
  1. User commands
  2. System calls
  3. C library routines
  4. Device drivers and network interfaces
  5. File formats
  6. Games and demos
  7. Misc. (ASCII, macro packages, tables, etc)
  8. Commands for system administration
  9. Locally installed manual pages. (i.e. X11)
- man -k <term> gives you a list of pages relevant to <term>
- GNU info files
  - Available with most Linux systems and most GNU packages
  - Usage: info <command>, then browse interactively
  - You can also use the info reader build into GNU Emacs
Users
- Each line of the /etc/passwd file identifies a user (Mac users - this has been mangled in OS-X.)
```
root:*:0:0:Charlie &:/root:/bin/csh
toor:*:0:0:Bourne-again Superuser:/root:
daemon:*:1:1:Owner of many system processes:/root:/usr/sbin/nologin
news:*:8:8:News Subsystem:/:/usr/sbin/nologin
man:*:9:9:Mister Man Pages:/usr/share/man:/usr/sbin/nologin
sshd:*:22:22:Secure Shell Daemon:/var/empty:/usr/sbin/nologin
mailnull:*:26:26:Sendmail Default User:/var/spool/mqueue:/usr/sbin/nologin
www:*:80:80:World Wide Web Owner:/nonexistent:/usr/sbin/nologin
nobody:*:65534:65534:Unprivileged user:/nonexistent:/usr/sbin/nologin
tptp:*:641:3001:TPTP Project:/home/tptp:/bin/tcsh
urban:*:1025:15:Josef Urban:/home/urban:/bin/tcsh
duncan:*:646:15:Christian A. Duncan,Ungar 441,284-2254:/home/duncan:/bin/tcsh
schulz:*:1003:15:Stephan Schulz:/home/schulz:/bin/tcsh
jennifer:*:1023:15:Jennifer Hwang:/home/jennifer:/bin/tcsh
julie:*:1028:15:Julie Garcia:/home/julie:/bin/tcsh
esacks:*:1018:15:Elisha Sacks:/home/esacks:/bin/tcsh
secretary:*:1000:15:CS secretary:/home/secretary:/bin/tcsh
irina:*:556:15:Irina Kaliman:/home/irina:/bin/tcsh
marcus:*:1001:15:Joe Clarke:/home/marcus:/bin/tcsh
hk:*:1004:15:Huseyin Kocak:/home/hk:/bin/tcsh
uttam:*:1027:15:Uttam Sarkar:/home/uttam:/bin/tcsh
ogihara:*:1007:15:Mitsunori Ogihara:/home/ogihara:/bin/tcsh
ntsinoremas:*:1014:15:Nicholas Tsinoremas:/home/ntsinoremas:/bin/tcsh
sarkar:*:510:15:Dilip Sarkar:/home/sarkar:/bin/csh
bora:*:1021:15:Borivoje Djokic:/home/bora:/bin/tcsh
burt:*:542:15:Burt Rosenberg:/home/burt:/bin/tcsh
jef:*:634:15:jef moskot:/home/jef:/bin/csh
vjm:*:548:15:Victor Milenkovic:/home/vjm:/bin/csh
strac:*:1017:15:Steven Trac:/home/strac:/bin/tcsh
dimitris:*:1009:15:Dimitris Papamichail:/home/dimitris:/bin/tcsh
geoff:*:501:15:Geoff Sutcliffe,445,305 2842158:/home/geoff:/bin/tcsh 
```
- Each user has
  - Name
  - Password
  - UID
  - GID (primary)
  - Personal information
  - Home directory
  - Shell program
- Super-users
  - Always have numerical user id 0
  - Normally have user name root (but there can be more than one user name associated with UID 0)
  - Have unlimited access to all files and resources
  - Can seriously damage the system!
- Normal users
  - Can only access files if they have the appropriate permissions
  - Can belong to one or more groups. Users within a group can share files
  - Usually cannot damage the system or other users files!
- User name and password
  - The user name is used to look up a line in /etc/passwd, to obtain the user information.
  - The password field contains either the encrypted password, or an x/* in a shadowed password system where the encrypted passwords are kept in a file accessible by only priviledged users. This eliminates brute force password attacks.
  - passwd changes your password
  - Password encryption with salt, and checking on login
  - On NIS served systems the password file is stored centrally
    - yppasswd change your password
    - ypcat passwd shows the password information for systems running NIS (revealing that the shadow password system is rendered useless in some systems running NIS, e.g., Linux but not FreeBSD).
- User ids and user groups
  - Every user has a unique id, as given by the third field of the /etc/passwd.
  - The user id, not the user name, is the identifier for users.
  - Groups are used in UNIX to give a group of users the ability to access a common resource, e.g., a file, or a device.
  - Every user belongs to a primary group, and possible many other secondary groups.
  - id shows the user id and which groups a user belongs to
  - groups shows which groups a user belongs to
- Shell and home directory
  - These are used when starting the users' shell - see below.
- who lists users currectly logged in
  whoami tells who this shell belongs to
  finger tells you information about a user (name)
Processes
- Process components
  - Text segment
  - Data segment
  - Stack and heap
  - Process control block
- Processes and the UNIX kernel
  - UNIX processes talk to the UNIX kernel using system calls.
  - UNIX processes can only "do the math" by themselves. Everything else requires a system call to the kernel.
- UNIX is a multi-tasking system, i.e. it can run multiple programs at once. A running program (with its data) is called a process.
  - ps lists processes
    - ps has some useful options ... read the man page
  - top is useful for watching processes
- Process creation
  - Unless the system is being bootstrapped a process can only come into existance as the child of another process.
  - Every UNIX process has a unique PID, starting from 0 and recycled.
  - The first process created is "hand tooled" by the boot process:
    - Power on and run BIOS
    - Read the boot loader, e.g., GRUB, from the boot block (block 0) of the root file system, to select a kernel.
    - GRUB loads a kernel
    - Kernel mounts the root file system, and creates process 0.
    - Note: There may be more layers of bootstrapping.
    - Note: Process 0 does not show in top/ps on some UNIX variants.
  - Process 0 creates the systemd (commonly known as the init, also known as launchd in iOS) process, which is the ancestor of all further processes. It always has PID 1.
  - Process 0 then becomes the sched (Sys V) or swapper (BSD) process, which is responsible for bringing processes into memory.
  - init forks off processes that run getty, which monitors terminal lines and allows users to log in.
  - Upon login, the command shell replaces the getty program.. The command shell for a given user is specified in the /etc/passwd file.
  - When the user logs out the shell process ends. init notices that and starts a new getty.
  - The processes form a tree, and every process has a PPID. Strangely, the PPID of process 0 is also 0.
- Process termination
  - The process tells the OS it's done, and passes the OS an exit code.
  - The process becomes a zombie.
  - The parent has to clean up the zombie (the shell does this automatically), at which point it can collect the exit code.
  - Orphans (including zombies) are adopted by init
- Process ownership
  - Every process has a real user ID (the UID), a real user group ID (the GID), an effective user ID (the EUID), and an effective user group ID (the EGID).
  - The real IDs are used for accounting and user-user communication, while the effective IDs are used to control access to files and control signal sending.
  - The user ID and primary group ID are assigned to the user's shell as its initial IDs.
  - IDs are inherited by child processes.
- Every process has a current working directory in the file system.
  - The initial working directory of the command shell is the home directory specified in the /etc/passwd file.
  - pwd tells the current working directory of the shell
  - cd changes the current working directory of the shell
    - cd directory_name changes to that directory.
    - cd by itself changes to the user's home directory.
  - The current working directory is inherited by child processes.
- The Environment of a Process
  - The environment of a process includes details of the home directory, path names to be searched for the program, the type of terminal being used, the user name, the shell being run, etc.
  - Common environment variables include:
    USER HOME PATH TERM SHELL
    EMAIL EDITOR PRINTER
  - printenv lists the current environment variables
    setenv sets environment variables
    unsetenv clears environment variables
  - Environment variables are inherited by child processes
- Signals
  - kill can be used to (have the OS) send a signal to a process
  - The effect depends on the signal and the process. Examples are to terminate or suspend the process.
  - kill -l lists the signals, and
    man 7 signal gives details.
  - ^C is a shortcut for
    kill -INT
    which normally terminates the process
  - ^Z is a shortcut for
    kill -TSTP
    which normally suspends the process (kill -CONT to restart).
  - kill -KILL really means it

Exercises

Make sure you can answer these questions
- Give a reason for and against having super-user access on a UNIX system.
- What is your UID and GID on your computer?
- How many primary user groups does a user have?
- How many groups can a user belong to?
- Have you used the top command yet? If not, why not?
- What is a zombie process?
- Which process adopts orphans?
- What kind of UNIX users use a GUI interface?
- How does an application program make the hardware do something?
- For what kill parameters are ^C and ^Z shortcuts?
- What would the command kill -9 1 do?

Exam Style Questions

How is the information about each UNIX user stored?
What seven items of information are stored about each user of a UNIX system?
What file contains the information about users in a non-NIS based UNIX system?
What command lists the users in an NIS based UNIX system?
What UID does the super-user have?
What does the who command do?
What command shows the groups of the owner of the process?
Define a process.
How is the first process on a UNIX system created, and what is it (eventually) called?
What is the name of the important process whos PID is 1?
What do the getty processes do?
What does the pwd command do?
What does the cd command do?
Explain what the UID and EUID of a process are used for.
What does the id command tell?
What would the command kill -9 90210 do?
What are environment variables?
What commands display, set, and remove environment variables?
What does the command man man provide?