1. CSC 4630
Meeting 4
January 29, 2007

2. Mysteries What do we need to resolve?
What issues did the Moby Dick homework raise?

3. What About Shells
What do you know about the UNIX shell?

4. The Shell A program, specifically a command interpreter
Special properties
Limited understand of regular expressions to denote sets of files
I/O redirection
User defined commands

5. Shell: Regular Expressions
* matches any string, including the empty string. Equivalent to .* for grep
[s] matches any single element of the set s. The set can include ASCII ranges, for example m-t designates the set
{m n o p q r s t}

6. Shell: I/O Redirection
Four symbols
< Take input from
> Send output to, overwriting what was previously there
>> Send output to, appending new information to the end of the file
| Use output from first command as input to second command

7. Shell: Command Interpreter
Single word commands
Names a file for execution, e.g. who means run the command file /bin/who
Command termination
newline
semi-colon ;
ampersand &

8. Shell: Command Interpreter (2)
Command grouping: compare
date; who (a sequence of commands on one line with
date; who | wc (Which of ; and | has higher precedence?) with
(date; who) | wc with
date; (who | wc)

9. Shell: Command Interpreter (3)
The ampersand means run the command in the background.
Example: (sleep 5 ; date) & date

10. Shell: Exercises
Compare ls junk and echo junk, where junk is the name of a file in the current directory.
Compare ls / and echo /
Compare ls and echo (given as commands with no arguments)
Compare ls * and echo *
Compare ls ‘*’ and echo ‘*’

11. Shell: Exercises (2) Explain the results of ls > ls.out
Explain the results of wc temp > temp
Explain the results of whom (given as a command without an argument)
Compare who | sort and who > sort
Compare
cat file | pr with
pr < file with
pr file

12. Shell: Exercises (3)
Explain the results of ls .*