Lecture 2. What Is The Shell ?

1. Lecture 2. What Is The Shell ? Spring 2006 Course, Sungkyunkwan University Hyunseung Choo choo@ece.skku.ac.kr

2. WHAT IS THE SHELL? • The Kernel and the Utilities (1/2) • The “UNIX system” is itself logically divided into two pieces • The kernel • The heart of the UNIX system • resides in the computer’s memory from the time the computer is turned on and booted until the time it is shut down • The utilities • reside on the computer’s disk • only brought into memory as requested • virtually every command you know under the UNIX system is classified as a utility

3. UTILITIES UNIX SYSTEM KERNEL disks Memory WHAT IS THE SHELL? • The Kernel and the Utilities (2/2) • The Shell • A utility program • It is loaded into memory for execution whenever you log into the system [Fig. 1] The UNIX system

4. login: login: UNIX SYSTEM KERNEL getty getty login: login: getty login: login: WHAT IS THE SHELL? • Logging In (1/3) • A terminal is connected to a UNIX system through a direct wire, modem, or local area network • For each terminal port on a system, a program called getty will be active [Fig. 2] The getty process

5. WHAT IS THE SHELL? • Logging In (2/3) • The UNIX(init) system automatically starts up a getty program on each terminal port whenever the system is allowing users to log in login: login: sue Password: UNIX SYSTEM KERNEL login getty login: login: getty login: login: [Fig. 3] login started on sue’s terminal

6. WHAT IS THE SHELL? • Logging In (3/3) • /etc/passwd • After login checks the password you type in against the one stored in /etc/passwd, it then checks for the name of a program to execute sue: fkrIboLrjdkK.:15:47::/usr/sue: pat: MjvntQLddYop.:99:7::/usr/pat:/usr/lbin/ksh bob: j9GTYQzHGwIG.:13:100::/usr/data:/usr/data/bin/data_entry [Fig. 4] /etc/passwd

7. WHAT IS THE SHELL? • Typing Commands to the Shell (1/3) • When the shell starts up, it displays a command prompt and then waits for you type in a command (Steps 1 & 2) • Each time you type in a command and press the Enter key (Step 3) • The shell analyzes the line you typed and then proceeds to carry out your request (Step 4) • If you ask it to execute a particular program, then the shell searches the disk until it finds the named program

8. WHAT IS THE SHELL? • Typing Commands to the Shell (2/3) • Once found, the shell asks the kernel to initiate the program’s execution and then the shell “goes to sleep” until the program has finished (Step 5) • If the program reads input from standard input, then it will wait for you to type in input unless redirected from a file or piped from another command (Step 6) • When the command has finished execution, control once again returns to the shell which awaits your next command (Step 7 & 8)

9. shell shell ls shell $ $ ls $ ls foo rje foo rje $ login: login: login: login: WHAT IS THE SHELL? • Typing Commands to the Shell (3/3) [Fig. 5] Command cycle

10. WHAT IS THE SHELL? • Log-in cycle • When you log off the system, execution of the shell then terminates and the UNIX system start up a new getty at the terminal that waits for someone else to log in init sh getty login [Fig. 6] Log-in cycle

11. program execution interpreted programming language variable and file name substitution SHELL environment control I/O redirection pipeline hookup WHAT IS THE SHELL? • The Shell’s Responsibilities [Fig. 7] The shell’s responsibilities

12. WHAT IS THE SHELL? • Program Execution (1/2) • The shell is responsible for the execution of all programs that you request from your terminal • The shell scans this command line and determines the name of the program to be executed and what arguments to pass to the program • Multiple occurrences of whitespace characters are simply ignored by the shell program-name argument(s)

13. WHAT IS THE SHELL? • Program Execution (2/2) mv rje/mazewars games [Fig. 8] Execution of mv with two arguments echo when do we eat? [Fig. 9] Execution of echo with four argements

14. WHAT IS THE SHELL? • Variable and File Name Substitution • Like any other programming language, the shell lets you assign values to variables • The shell also performs file name substitution on the command line • e.g.) *, ?, […] $ ls mrs.todd prog1 shortcut sweeney $ $ echo * mrs.todd prog1 shortcut sweeney $ [Fig. 10] Execution of echo

15. WHAT IS THE SHELL? • I/O Redirection (1/2) • The shell scans the command line for the occurrence of the special redirection characters <, >, or >> • Before the shell starts execution of the desired program, it redirects the standard output of the program to the indicated file echo Remember to tape Miami Vice > reminder

16. WHAT IS THE SHELL? • I/O Redirection (2/2) $ wc –l users 5 users $ wc –l < users 5 $ [Fig. 11] Execution of wc –l users [Fig. 12] Execution of wc –l < users

17. WHAT IS THE SHELL? • Pipeline Hookup • Just as the shell scans the command line looking for redirection characters, it also looks for the pipe character | • For each such character that it finds, it connects the standard output from the command preceding the | to the standard input of the one following the | who | wc -l

18. WHAT IS THE SHELL? • Interpreted Programming Language • The shell has its own built-in programming language • The shell analyzes each statement in the language one line at a time and the executes it