Unix and Shell Programming

1. Unix and Shell Programming Department of Computer Science and Engineering

2. Unit-2 Shells- UNIX Session, standard streams, redirection, pipes tee Command, Command Execution andSubstitution, Command-Line Editing, job control, Aliases, Variable Types and options, Shell Customization. Filters and Pipes – related Commands. Commands for Translating Characters, Fileswith duplicate Lines, Counting characters, words and Lines and Comparing files

3. Process groups • A process group is a collection of (related) processes. Each group has a process group ID. • Each group has a group leader who pid = pgid • To get the group ID of a process: pid_t getpgrp(void) • A signal can be sent to the whole group of processes.

4. Process groups: • A process may joint an existing group, create a new group. int setpgid(pid_t, pid, pid_t, pgid) • A process can set group ID of itself or its children • _POSIX_JOB_CONTROL must be defined • Most shells with job control create new group for each line of command (job).

5. Sessions • A session is one or more process groups proc1 | proc2 & proc3 | proc4 | proc5 • results in a session with three groups, see ‘ps –j’ • A login shell is a session in general. proc1 proc3 Login shell proc2 proc4 proc5

6. Session • A session can have a single controlling terminal • Terminal device for a terminal login • Pseudo-terminal device for a network login • The I/O devices somewhat link to the window and keyboard. • The session leader that establishes the connection to the control terminal is called the controlling process.

7. Session • Only one I/O device for all processes (and process groups) in a session. Which process should get the input from the keyboard? • Foreground and background process • One foreground group • Many background groups • Input • Only foreground group • Terminals’ interrupt signals are only sent to the processes in the foreground group. • Output • Typically shared

8. Sessions • To establish a new session: pidsetsid(void); • Process become the session leader • Process become a new group leader of a new group • Process has no controlling terminal (break up the old one) • Each shell is a session. When a shell is created, a terminal must be setup. • Fails if the caller is a group leader. • A process can open file /dev/tty to talk to the controlling terminal regardless how standard IO are redirected. • A way to by pass I/O redirection, see example1.c

9. How to make a group foreground and background? • So that the terminal device driver knows where to send the terminal input and the terminal-generated signals. pid_t tcgetpgrp(int filedes); Return the process group ID of the foreground process group associated with fieldes. int tcsetpgrp(int filedes, pid_t pgrpid); • If the process has control terminal, set the foreground process group ID to pgrpid. • Pgrpid must be group ID in the same session.

10. Job control • Allows start multiple jobs from a single terminal and control which job can access the terminal. • Foreground jobs can access terminal • Background jobs may not: • When a backgound job try to read, SIGTTIN signal is sent • A background job must be able to output to the terminal (options may be set by the stty command) • See control.c for an example of switching terminal among groups.

11. Orphaned process group: • Parent of every member is either in the orphaned group or is not a member of the group’s session. • Happens when a process forks a child and then dies. • The child becomes a member of the orphaned group. • Can have problems: the child may transform from a foreground process to a background process automatically. • Remember in the control.c program, foreground group is set in both the parent and the child. • Can be in an inconsistent state. • If any IO is involved, strange things may happen.

12. How to make sure that a shell program handles terminal I/O and signals correctly • Create a new group for each job • Both parent and child do setpgid • For foreground job: • After fork, shell set tcsetpgrp to give foreground jobs control over terminal • Shell waits for all foreground processes in the foreground job to finish. After that, shell set tcsetpgrp to itself and print the prompt. • For background job: • Create a separate group so that processes in background jobs do not have access to terminal.

13. UNIX Processes A PROCESS is a program that is executing. • Processes have Input and Output • There are two types of Processes • Foreground Processes • Background Processes

14. Foreground Process When running a FOREGROUND process, the shell waits for the program to finish When the process is finished, you will be returned to the command prompt /export/home/morris07>

15. Background Process When running a BACKGROUND process, the shell starts the process, then leaves • This allows you to execute other commands at the command prompt NOTE: Handy for programs that take a long time to run and do not need to run interactively.

16. Running Processes in Background Type an ampersand (&) after the command to run it in the background > find . –name java > javaLocations.txt &

17. Suspending Processes You can SUSPEND a process by typing ^Z This will pause the process temporarily. The process can be resumed in the Foreground or moved to the Background: /export/home/morris07> fg [jobid] /export/home/morris07> bg [jobid]

18. Displaying List of Suspended Jobs /export/home/morris07> jobs … [1] Stopped vi document [2] Stopped elm -l option gives more information

19. Displaying Status of Processes Use ps to report the status of a process /export/home/morris07>ps … PID TTY TIME CMD 19834 pts/7 0:05 ksh 19954 pts/7 0:04 ps

20. Killing a Process Use kill to terminate a process running in the background /export/home/morris07>kill {process id} This sends a signal to the process to end.

21. That Process Won’t Die!! The kill command waits for the process to perform cleanup: write to files, close files,etc. If the process will not end, use the ‘-9’ or ‘-KILL’ to kill the process immediately > kill –9 19954 > kill –KILL 19954

22. terminal command standard output Standard Input and Output keyboard command standard input

23. terminal command standard output redirected output file command Redirection and Pipes Redirecting Standard Output to a File with: ‘>’, ‘>>’ ls –al > mylist(creates or overwrites file) ls –al >> mylist(appends or creates file)

24. terminal command standard output command command redirected output Redirection and Pipes Redirecting Standard Output to a Command with: ‘|’ ls –al | more(passes output to command)

25. redirected input Redirection and Pipes Redirecting Standard Input from a File with: ‘<’ cat < file(passes file to command) keyboard command standard input file command

26. Filters A FILTER is any program that reads from Standard Input and writes to Standard Output. grep uniq look spell sort wc

27. Filters: grep The grep command searches for the pattern specified and writes these lines to Standard Output. grep [-cilnvw]pattern [file…] >grep ‘Easy’ assignments.txt

28. Demonstration - grep

29. Filters: uniq The uniq command examines data, looking for consecutive, duplicate lines. uniq [-cdu][infile [outfile]] • uniq –d , retains one copy of all lines that are duplicated • uniq –u, retains only those lines that are not duplicated. • uniq –c, counts how many times each line is found. >uniq document.txt

30. Filters: look The look command searches data in alphabetical order and will find lines that begin with a specified pattern (alphabetical characters only). look [-df]pattern [file…] >look Amer * Access the dictionary of correctly spelled words. • Look is not really a filter and cannot be used within a pipeline. • File must be pre-sort file with –dfu options. I.e dictionary, fold, unique.

31. Filters: look

32. Filters: spell The spell command will read data and generate a list of all words that look as if they are misspelled. This is a very primitive spell checker. spell [file…] >spell document.txt

33. Filters: spell • For example, • Mispeel was not flagged • Adds standard prefixes and suffixes to words in dictionary

34. Filters: sort The sort command sorts data (using ASCII format). sort [-dfnru] [infile…] [-o outfile] >sort names -o sorted_names or >sort names > sorted_names

35. Filters: sort

36. Filters: sort

37. Filters: wc The wc command counts lines, words, and characters. wc [-lwc][file…] >wc -l document.txt

38. wc options -c Count bytes. -m Count characters. -C Same as -m. -l Count lines. -w Count words delimited by white space characters or new line characters.