UNIX Admin Tools

1. UNIX Admin Tools

2. Overview • Review of file manipulation utilities • UNIX process subsystem • Overview of the UNIX shells csh/ksh

3. 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. Utilities for Manipulating file attributes • chmod change file permissions • chown change file owner • chgrp change file group • only owner or super-user can change file attributes • upon creation, default permissions given to file modified by process umask value

5. 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. Chmod command • Symbolic access modes • example: chmod +r file • Octal access modes octal read write execute 0 no no no 1 no no yes 2 no yes no 3 no yes yes 4 yes no no 5 yes no yes 6 yes yes no 7 yes yes yes

7. 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. 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 • cp copy a file • ln create 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. 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. 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. 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. 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. 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. Example: Program that creates a new process to copy files 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. Fork operation

16. (prog2 is cp in example) After exec of prog2 in child

17. Unix process genealogy

18. 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. 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. 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. 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. Interprocess Communication Unrleated Processes • FIFO (named Pipes) • System V IPC • msg queues • semaphores • shared memory • sockets (client/server model)

23. 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. 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. Process Subsystem utilities • ps monitors status of processes • kill terminate a process (by pid) • wait parent process wait for one of its children to terminate • nohup makes a command immune to the hangup and terminate signal • sleep sleep in seconds • nice run processes at low priority

26. 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. 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. 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/ksh Korn shell • other shells • shell scripts: files of UNIX and shell commands executed from a UNIX shell

29. 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. 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. 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. Simple Commands supported • simple command: sequence of non blanks arguments separated by blanks or tabs. • 1st 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. Complex commands • Multiple commands • Command groupings • Conditional command execution

34. 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. 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

36. 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 1st line is just #, then it is interpreted by a C shell • If 1st 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.

37. Here Documents • Shell provides alternative ways of supplying standard input to commands • Shell allows in-line input redirection using << calledhere documents • format command [arg(s)] << arbitrary-delimiter command input : : arbitrary-delimiter • arbitrary-delimiter should be a string that does not appear in text

38. 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

39. Environmental Variables NAME MEANING $HOME absolute pathname of your home directory $PATH a list of directories to search for $MAIL absolute pathname to mailbox $USER your user id $SHELL absolute pathname of login shell $TERM type of your terminal

40. 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

41. 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

42. 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

43. Built-in commands (continued) • built-in commands common to the 3 shells: echo exec cd shift wait umask exit eval

44. End of Lecture • Questions?

45. 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.