2000 copyright Danielle S. Lahmani UNIX Tools G22.2245-001, Fall 2000 Danielle S. Lahmani email: [email protected] Lecture 2

2000 copyright Danielle S. Lahmani

UNIX ToolsG22.2245-001, Fall 2000

Danielle S. Lahmani

email: [email protected]

Lecture 2


Overview

• Review of file manipulation utilities

• UNIX process subsystem

• Overview of the UNIX shells csh/ksh

• Unix tools project description


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


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


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


Chmod command• Symbolic access modes

• example: chmod +r file

• Octal access modesoctal read write execute0 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


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


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


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.


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


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


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


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


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");

}


Fork operation

A fte r fo rk op era tion

p aren t p rocess d a ta ch ild p rocess d a ta

sh ared text


After exec of prog2 in child

A fte r exec "p rog 2 " in ch ild

p rog text d a ta u n ch an g ed

p aren t p rocess d a ta

p rog 2 text d a ta

ch ild p rocess d a ta

(prog2 is cp in example)


Unix process genealogyP rocess g en era tion

g e tty

in itexecs

/b in /sh

log inexecs

g e ttyexecs

in itexecs

g e tty

In itexecs

In it p rocess 1fo rks in it p rocesses


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


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)


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


Inter-process CommunicationRelated 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


Interprocess Communication

Unrleated Processes– FIFO (named Pipes)– System V IPC

• msg queues• semaphores• shared memory

– sockets (client/server model)


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


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)


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


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


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.


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


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


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 */


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


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


Complex commands

• Multiple commands

• Command groupings

• Conditional command execution


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


Command substitution

• A command can be placed with grave accents ` ` to capture the output of command

• often used with shell variables


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


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.


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


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


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


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


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


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


Built-in commands (continued)

• built-in commands common to the 3 shells:echo exec

cd shift

wait umask

exit eval


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.