1. 2000 copyright Danielle S. Lahmani UNIX Tools G22.2245-001, Fall 2000 Danielle S. Lahmani email: lahmani@cs.nyu.edu Lecture 2

2. 2000 copyright Danielle S. Lahmani Overview Review of file manipulation utilities UNIX process subsystem Overview of the UNIX shells csh/ksh Unix tools project description

3. 2000 copyright Danielle S. Lahmani File Attributes Stored in the file I-node File’s ownership: user and group file permissions: read, write, execute file modification times file type: regular, directory, link, symbolic link, special file

4. 2000 copyright Danielle S. Lahmani Utilities for Manipulating file attributes chmodchange file permissions chownchange file owner chgrpchange file group only owner or super-user can change file attributes upon creation, default permissions given to file modified by process umask value

5. 2000 copyright Danielle S. Lahmani File Permissions Three types of permissions: read, process may read contents of file write, process may write contents of file execute, process may execute file three sets of permisions: permissions for owner permissions for group permissions for other access checks made against process’s effective ids

6. 2000 copyright Danielle S. Lahmani Chmod command Symbolic access modes example: chmod +r file Octal access modes octalread writeexecute 0nonono 1nonoyes 2noyesno 3noyesyes 4yesnono 5yesnoyes 6yesyesno 7yesyesyes

7. 2000 copyright Danielle S. Lahmani Directory permissions Same types and sets of permissions as for files –read: means process may a read a dir (i.e., list files) –write: process add/rm files in dir –execute: process can “search”, access files, in dir or subdir

8. 2000 copyright Danielle S. Lahmani Common Utilities for Managing files and directories pwd print process current dir cat, ed, vi, emacs… create files ls list contents of directory rm remove file mv rename file cpcopy a file lncreate a hard link to a file mkdir and rmdir create and remove dir lp: print a file wc counts the words in a file

9. 2000 copyright Danielle S. Lahmani Unix Processes Definitions: program: collection of bytes and data stored in a file image: computer execution environment process: execution of an image multi-tasking: many processes can execute simultaneously in Unix.

10. 2000 copyright Danielle S. Lahmani Unix Process Groups process id: unique id assigned to process upon creation process group id: id of the group to which the process belongs to foreground process group: is the process group associated with a terminal at a time background process group: processes created by you not in the foreground group

11. 2000 copyright Danielle S. Lahmani Process Relationships A process spawns another process using the fork(2) system call. The creating process is the parent process The newly created process is the child process. fork() returns 0 to the child process fork() returns the process_id of the child to the parent process

12. 2000 copyright Danielle S. Lahmani Process Relationship (continued) exec(2) :To run a new program, the child, will issue the exec( ) system call and overwrites itself with the code and initial data of the new program, thus initiating the execution of the new program wait(2): a parent can suspend its execution until one or more child processes complete via a wait(2) system call

13. 2000 copyright Danielle S. Lahmani Process Relationships (continued) exit(2) :upon terminations, process can set an exit status available to parent. Code used –zero for success –non-zero for failure

14. 2000 copyright Danielle S. Lahmani Example : Program that creates a new process to copy files Reference: M.Bach, "The Unix Operating system", p 11. main(argc,argv) int(argcl char *argv[]; {/* assumes 2 args, source and target files */ if ( fork() == 0) { /* child process */ execl("cp"."cp",argv[1],argv[2],0); } /* parent process */ wait(int *) 0); printf("copy done\n"); }

15. 2000 copyright Danielle S. Lahmani Fork operation

16. 2000 copyright Danielle S. Lahmani After exec of prog2 in child (prog2 is cp in example)

17. 2000 copyright Danielle S. Lahmani Unix process genealogy

18. 2000 copyright Danielle S. Lahmani Process permissions real id and one of more real group id set at login. effective uid and effective group id determine process access to read/write/search/execute files or dir. umask() file mode creation mask, used when file or dir created by process

19. 2000 copyright Danielle S. Lahmani Signals Signal: mesg a process can send to a process or process group, if it has appropriate permissions. mesg number represented by a symbolic name for each signal, receiving process can: –explicitly ignore signal –specify action to be taken upron receipt (signal handler) –otherwise, default action takes place (usually process is killed)

20. 2000 copyright Danielle S. Lahmani Signals (continued) Example: When a child exists, it send a SIGCHLD signal to its parent. When the parent issues a wait, it tells the system it wants to catch the SIGCHLD signal When a parent does not issue a wait, it ignores the SIGCHLD signal

21. 2000 copyright Danielle S. Lahmani Inter-process Communication Related Processes signals read/write regular files pipes: when a process B tries to read from a pipe –returns data if process A has written to pipe –returns with EOF, if no other process has pipe open for writing –suspends execution until process A writes data to it child returns exit value to waiting parent process

22. 2000 copyright Danielle S. Lahmani Interprocess Communication Unrleated Processes –FIFO (named Pipes) –System V IPC msg queues semaphores shared memory –sockets (client/server model)

23. 2000 copyright Danielle S. Lahmani Process Environment includes: Process id and process group id open files current working directory real and effective user and group ids file creation mask (umask) resource limits signal action settings set of named local variables

24. 2000 copyright Danielle S. Lahmani File Descriptors each process associates a number or handle, called file descriptor, (fd) with each file it has opened. At login, three files associated with terminal –standard input: fd 0, open for reading –standard output: fd 1, open for writing –standard error: fd 2, open for reading,writing process inherits parent’s file descriptors unless specified (close-on-exec)

25. 2000 copyright Danielle S. Lahmani Process Subsystem utilities psmonitors status of processes kill terminate a process (by pid) wait parent process wait for one of its children to terminate nohupmakes a command immune to the hangup and terminate signal sleepsleep in seconds nice run processes at low priority

26. 2000 copyright Danielle S. Lahmani Setuid and Setgid Mechanisms Mechanism pattented process effective uids are different from its real uids when it executes a set-uid or set- gid program. the process effective uid and gid become that of the executable example: changing your passwd

27. 2000 copyright Danielle S. Lahmani Security Problems Permissions on the executable program and directory in which it is contained must be correct, otherwise easily replaced by Trojan Horse. Some systems remove setuid and setgid bits whenever files are modified as a security precaution.

28. 2000 copyright Danielle S. Lahmani Overview of the shell Command line interpreter and programming language between operating system and user user may select which shell to run: –/bin/csh Cshell –/bin/kshKorn shell –other shells shell scripts: files of UNIX and shell commands executed from a UNIX shell

29. 2000 copyright Danielle S. Lahmani Working with the shell Shell invoked automatically during a login session or manually at the prompt by user –1. Reads a special startup file for initialization –2. Displays prompt and waits for user command –3. Executes user command and goes to step 2, unless contrl D, then shell terminates

30. 2000 copyright Danielle S. Lahmani Redirection of input/ouput Redirection of output: >, >> –example :$ man ls > info.ls Redirection of input: < –example: $ cat <input.data using filters: pipes –example: $ cat file| wc -l; /* counts the number of line in file */

31. 2000 copyright Danielle S. Lahmani Shell Core Features Simple and complex commands redirection of input/output pipes wildcards command substitution background processes shell variables here documents built-in cmds programming constructs

32. 2000 copyright Danielle S. Lahmani Simple Commands supported simple command: sequence of non blanks arguments separated by blanks or tabs. 1 st argument (numbered zero) usually specifies the name of the command to be executed. Any remaining arguments (with a few exceptions, see meta-characters) –Are passed as arguments to that command. –Arguments may be filenames, pathnames, directories or special options

33. 2000 copyright Danielle S. Lahmani Complex commands Multiple commands Command groupings Conditional command execution

34. 2000 copyright Danielle S. Lahmani File name expansion Wildcards * matches any string of characters ?matches any single character [list] matches any character in list [lower-upper] matches any character in range lower-upper inclusive

35. 2000 copyright Danielle S. Lahmani Command substitution A command can be placed with grave accents ` ` to capture the output of command often used with shell variables

36. 2000 copyright Danielle S. Lahmani Shell Scripts A shell script is a regular text file that contains shell or UNIX commands Before running it, it must have execute permissions ( see chmod +x filename) Very useful for automating repetitive task and administrative tools and for storing commands for later execution

37. 2000 copyright Danielle S. Lahmani Shell Scripts (continued) When a script is run, kernel determines which shell it is written for by examining the first line of the script –If 1 st line is just #, then it is interpreted by a C shell –If 1 st line is of the form #!pathname, then the executable –Pathname is used to interpret the script –If neither rule 1 nor rule 2 applies, the script is interpreted by a Bourne shell.

38. 2000 copyright Danielle S. Lahmani Here Documents Shell provides alternative ways of supplying standard input to commands Shell allows in-line input redirection using << called here documents format command [arg(s)] << arbitrary-delimiter command input : arbitrary-delimiter arbitrary-delimiter should be a string that does not appear in text

39. 2000 copyright Danielle S. Lahmani Shell Variables Shell has several mechanisms for creating variables. A variable is a name Representing a string value –Shell variables can save time and reduce typing errors, variables Allow you to store and manipulate information two types : local and environmental –local are set by the user of by the shell itself –Positional parameters variables are normally set only on a command line

40. 2000 copyright Danielle S. Lahmani Environmental Variables NAMEMEANING $HOMEabsolute pathname of your home directory $PATHa list of directories to search for $MAILabsolute pathname to mailbox $USERyour user id $SHELLabsolute pathname of login shell $TERMtype of your terminal

41. 2000 copyright Danielle S. Lahmani Positional parameters when a shell procedure is invoked, the shell implicitly creates positional parameters. The name for a positional parameter is a number. Positional parameters are used mainly in scripts. –$0 is the argument in position zero on the command line –$1 is the first argument –$1.. $9 $n refers to the nth argument on the command line if applicable –$# the number of positional parameters, not counting 0 –$* the list of all arguments

42. 2000 copyright Danielle S. Lahmani QUOTING Quoting restores the literal meaning to characters that are processed specially by the shell. The literal quotes are not passed on to the command Single quotes ( ' ) inhibit wildcard replacement, variable substitution, and command substitution Double quotes ( " ) inhibit wildcard replacement only When quotes are nested, only the outer quotes have any effect

43. 2000 copyright Danielle S. Lahmani BUILT-IN commands commands that are internal to the shell Faster to execute and more efficient than other commands –Shell does not have to fork to execute the command –Trade-off: redirection of input/output not allowed for most of these

44. 2000 copyright Danielle S. Lahmani Built-in commands (continued) built-in commands common to the 3 shells: echoexec cdshift waitumask exiteval

45. 2000 copyright Danielle S. Lahmani Subshells When a parent shell forks a child to execute a command, the new child shell is sometimes called a subshell. This happens when: –a group command is executed ( $(cmd1; cmd2; cmd3) ) –a shell script is executed( $myscript ) –a background job is executed ( cmd1&) A shell inherits the parent's environment but not the parent's local variables.