Operating Systems Lecture 2 UNIX and Shell Scripts

1. Operating SystemsLecture 2UNIX and Shell Scripts

2. C++ Programs with Arguments A C++ program may be called with arguments (like parameters for the whole program. Arguments may be used for: 1. Options used for decisions in main( ) 2. Strings that change in output statements 3. Filenames for input and output files 4. Strings can be changed to values with atoi( ) Arguments have built-in names: argc: Argument count (the number of arguments given) argv: List of arguments (array of strings)

3. Calling a program with Arguments Example: > myProgram inputFile outputFile argc = 3 (myProgram counts as one of the arguments) argv holds: argv[0] = "myProgram" argv[1] = "inputFile" argv[2] = "outputFile"

4. C++ Code using argc and argv #include <iostream.h> #include <fstream.h> int main(int argc, char * argv[ ] ) { ifstream inFile; ofstream outFile; if (argc != 3) //Error checking cout << "Usage: myProgram inFile outFile " << endl; else { inFile.open(argv[1]) if (!inFile) //Error checking cout << argv[1] << " not opened." << endl; outFile.open(argv[2]) if (!outFile) //Error checking cout << argv[2] << " not opened." << endl; //Rest of code for input and output from files... } return 0; }

5. Using atoi All arguments are input as strings. If you want to use numeric values, you must convert from the string to a number: #include <stdlib.h> ... int number, square; ... number = atoi(argv[1]); square = number * number; ...

6. UNIX Commands Commands you should already know: ls, pwd, cd, mkdir, rmdir cp, mv, rm * = substitution character Commands you should learn: echo, cat, more, grep | = pipe that connects stdout with stdin e.g. ls -l | grep Aug Use man or apropos to find out about commands. A good UNIX tutorial can be found here: http://www.ee.surrey.ac.uk/Teaching/Unix/index.html

7. Kernel vs. Utilities The kernel is a process that is always executing. Utilitites reside on the disk (e.g. grep, lpr, etc.) The shell program is a utility. The shell knows some built-in commands that don't have to be read off the disk. (cd, pwd, etc)

8. Shells The shell is a command line interpreter. Its function is to get and execute the next statement. Common shells: Bourne shell (/bin/sh) Korn shell (/bin/ksh) C shell (/bin/csh) T shell (/bin/tcsh) GNU Bourne-Again Shell (/bin/bash) Z shell (/bin/zsh) The Bourne shell is standard. All information here is for the Bourne shell. The shell is just a program. Anyone can write their own custom taylored shell.

9. The Shell Environment • The shell environment is a list of associated strings in the shell: • PATH: path or paths used to look for programs or utilities. • e.g. /opt/local/bin • HOME: Location of home directory • e.g. /home/fac/croyden • SHELL: Current shell • e.g. /bin/csh • Type setenv to see a list of current environment variables. • Use setenv to change environment: • setenv TERM xterm • Change terminal type to xterm

10. Redirecting I/O Normally, the shell will use standard input and standard output for executing commands. You can redirect the standard input and output using < and > ls -l > filex Redirect the output of ls -l into the file named filex. Using >> allows you to append output to a file: ls -l >> filex Append the output of ls -l to the end of file named filex.

11. File Permissions Files have permissions for the user/owner, group and others. You can view the permissions by typing: ls -l -rwxr--r-- 1 croyden fac 81 Jan 3 21:53 myFile The information given is: file type (- = plain file) permissions for user (rwx), group (r--) and other (r--) number of hard links (1) user/owner name (croyden) group name (fac) size of file (81) date and time last modified (Jan 3 21:53) filename (myFile)

12. Changing Permissions File types: - = plain file, d = directory Permissions: r = read permission, w = write permission, x = execute permission. Use chmod to change permission for user, group, other or all: chmod a+r filename everyone gets read permission chmod g+x filename group gets execute permission chmod u+w filename user gets write permission chmod o-w filename others lose write permission chmod og-rw filename group and others lose read and write permission

13. Shell Programming • Shell scripts must be marked as executable: chmod a+x myScript 2. Use # to start a comment. Comments run from # to the end of the line. 3. All shell scripts begin with the interpreter you want to use: #!/bin/sh Example: #!/bin/sh who | grep croyden exit 0

14. Running a shell script To run a UNIX script: Type the script into a file. Change the file permission to executable. Execute it (by typing the filename at the prompt).

15. Shell Variables Shell variables are stored as strings: Example: #!/bin/sh x=1 # Note: No spaces in assignment. # If space after x, thinks x is a command echo The value of x is $x # $x prints the value of variable x echo The home directory is $HOME echo The current shell is $SHELL (Note: to debug, use -x: sh -x scriptFileName This will list the commands as they are executed.)

16. Using Quotes Single quote: Groups together characters until end quote. $ is not processed. Example: #!/bin/sh grep Constance Royden /etc/passwd #Tries to open Royden as a file grep 'Constance Royden' /etc/passwd #Searches for Constance Royden #in passwd file x=1 echo $x #echos 1 echo '$x' #echos $x exit 0

17. Double Quotes Double quotes act like single quotes, except the $ is processed: #!/bin/sh x=1 echo $x #echos the value of x echo "$x" #echos the value of x address="College of the Holy Cross" echo $address #echos College of the Holy Cross echo "$address" #ditto exit 0

18. More Quotes Backslash (\): Places a single quote around a character: \> is the same as '>' Back quote (`): Tells shell to execute the enclosed command and insert the output here: #!/bin/sh echo There are `who | wc -l` users logged on exit 0 Try these examples out for yourself!