introduction to unix-like systems (part i-iv)

A Unix-like system is one that behaves in a manner similar to a Unix system.

Examples of Unix-like systems are:

�

�

Introduction

1.1 What is a Unix-like system?

HP-UX,Solaris (mixed open source/proprietary),Mac OS X,

BSD (FreeBSD, OpenBSD etc.), andGNU/Linux (Ubuntu, Debian, Fedora etc.)

��

��

PROPRIETARY

FREE

Introduction

The Open Group, an industry standards consortium, owns the UNIX trademark.

Dennis Ritchie, one of the original creators of Unix, expressed his opinion that Unix-like systems such as Linux are de facto Unix systems.

�

�

1.1 What is a Unix-like system? (cont’d)

Unix filesystem and shell (I)

2.1 Unix system structure

A computer system consists of hardware and systems software (OS) that together provide a working environment for running general purpose application programs.

Hardware

Kernel

Shell & utilities

User space

Different UNIX variants have different implementations of different subsystems.


2.2 Kernel subsystems

Process management

Memory management

Filesystem

Network stack (TCP/IP)

�

�

�

A process is a running instance of a program.

At any given point in time, the CPU is operating within the execution context of a specific process.

The process manager is responsible for process scheduling, context-switching, inter-process communication etc.

Process management


2.2 Kernel subsystems (cont’d)

�

�

Memory manager



Responsible for controlling process access to the hardware memory resources.

Memory virtualization separates logical memory from physical memory.

Provides each process with a uniform address space.Protects the address space of each process from corruptionby other processes.

��

�

�

�

Manages access to data and metadata of the files, and organizes the available space of the device(s) which contain it.

A Unix file is an information container structured as a sequence of bytes.

Most Unix-like systems support several filesystems.

Filesystem



Network stack (TCP/IP)



Unix-like operating systems are known for rich Ethernet networking support.

Unix implements the TCP/IP protocol stack.

The Berkeley sockets API provides a high level interface to the TCP/IP stack.

�

�

�


2.3 The shell

A command line interface to the operating system.

Used to execute other programs, manage files and processes.

Users typically interact with a Unix shell using a terminal emulator, e.g., xterm or OS X Terminal.

Graphical user interfaces for Unix are sometimes referred to as graphical shells.

�

�

�

�


2.4 Simple command examples

pwd

ls

adduser

top

mkdir

ps

who

df

cd

Print name of current/working directory

List directory contents

Add a user to the system

Display processes

Make directories

Report a snapshot of the current processes

Show who is logged on

Report file system disk space usage

Change directory


2.5 Shell scripting

A shell script is a regular text file that conatains shell or Unix commands.

Useful for automating tasks and storing command sequences for later execution. Makes life easier for the user.

Some shell scripts run at system startup or shutdown.

The /etc/init.d directory contains shell scripts used to start, stop, or restart system daemons.

�

�

�

�


2.6 Options and parameters

$ ls -l /bin

Parameters are pieces of information (e.g., a file name) that the command needs in order to do its job.

Flags (or options) are typically composed using a hyphen (-) followed by one or more letters. A flag instructs the shell to perform the command in a particular way.

man:

whatis:

Display entries from Unix online documentation.

example: man mkdir

Displays a single line information about a command.


2.7 Getting help

$ whatis ifconfig

ifconfig (8) - configure a network interface


2.7 Getting help (cont’d)

$ man mkdir

MKDIR(1) USER COMMANDS MKDIR(1)

NAME mkdir - make a directory

SYNOPSIS mkdir [ -p ] dirname...

DESCRIPTION mkdir creates directories. Standard entries,`.',for the directory itself, and `..' for its parent, are made automat- ically.

The -p flag allows missing parent directories to be created as needed.

With the exception of the set-gid bit, the current umask(2V) setting determines the mode in which directories are created. The new directory inherits the set-gid bit of the parent directory. Modes may be modified after creation by using chmod(1V).

mkdir requires write permission in the parent directory.

SEE ALSO chmod(1V), rm(1), mkdir(2v), umask(2V)

Each user owns some quota of the total disk space for storage of personal files, mail etc. This portion of the filesystem is visible only to its owner.

Users are identified by a unique number, called user id.

�

�

Unix filesystem and shell (II)

3.1 Security fundamentals

A multi-user system is one that allows concurrent access by multiple users of a computer.


3.1 Security fundamentals (cont’d)

�

�

�

A set of users can form a group.

Each user is a member of one or more groups.

Groups enable users to share material with other users. Like users, every group has a unique identifier, called group id.

Groups



�

�

Any Unix-like operating system has a special user called root (or sometimes superuser).

The root user can access every file on the system and interfere with the activity of running user programs.

The root user

Can a given file be read or written to by a particular user?

What users can run a specific program? Can a user terminate a running process?

�

�



Users and groups are used to determine if file or process operations can be performed.


3.2 Unix file hierarchy

Files and directories are organized in a hierarchical tree structure.

In Unix, all files and directories appear under the root directory "/".

/

(root)

/bin /home /usr

/home

/bin /lib

/ls /local

/lc


3.3 Working directory

Every running process has a current working directory.

A nickname for the current working directory of a process is “.” (a single period).

The pwd (present working directory) command displays your current working directory.

�

�

�


3.4 File path (relative vs. absolute paths)

To identify a specific file, the operating system uses a pathname, which consists of slashes alternating with a sequence of directory names that lead to the file.

If the first item in the pathname is a slash, the pathname is said to be absolute. Its starting point is the root directory.

If the first item is a directory name or filename, the path is said to be relative. Its starting point is the current directory.

�

�

�


3.5 The file abstraction

Unix follows the mantra: “everything is a file.” As mentioned earlier, a Unix file is simply a sequence of bytes.

Unix files may have one of the following types:

Regular fileDirectorySymbolic linkBlock-oriented device fileCharacter-oriented device filePipe and named pipe (also called FIFO)Socket

��


3.6 Directories

A directory is a list of files with associated information.

Every user has a home directory. (/home/<username>)

�

�

Creation dateAttributesetc.

��


3.7 Tilde expansion

Most shells support the ~ operator (called tilde).

~ expands to my home directory

~user expands to user’s home directory

Useful since home directory locations vary between different machines.

�

�

�

�

~/file /home/<username>/file

~alice/file2 /home/alice/file2


cat

3.8 The cat, more and less commands

Copies the contents of a file to the terminal. When invoked with a list of file names, it concatenates them. Use the -n flag to show line numbers.

more/less

more shows file contents, one page at a time. less allows both forward and backward navigation. Programs of this sort are called pagers.


3.9 Common utilities

pwd

ls

adduser

top

mkdir

rmdir

ps

who

df

cd

vi

nano

rm

mv

cp

touch

wc

file

mount

umount

date

echo

tar

passwd

kill

nice

&

Unix filesystem and shell (III)

4.1 File permissions

Unix file permissions

01

02

04

01

02

04

01

02

04suid

sgid

sticky

read

write

execute

user group others

Example:


4.1 File permissions (cont’d)

read

write

execute

user group others

01

0404

0101

02

04

557

A common method for representing Unix permissions is using octal (base-8) notation.

With three-digit octal notation, each numeral represents a different component of the permission set: user, group, and others respectively.


4.2 Directory permissions

Similar to ordinary file permissions.

Read means that the process may read directory contents.

Write means that the process can add/remove files.

Execute permission on a directory means you can cd to that directory and access its files.

�

�

�

�


4.3 Reading ls output

-rwxr--r-- 1 root root 4096 Sep 10 21:11 foo

Permissions for User (Owner) of file

Directory flag; 'd' if a directory, '-' if a normal file etc.

Permissions for Group

Permissions for Other

owner group filenamelink count file size

The ls command shows the permissions and group associated with files when used with the -l option:


4.4 Manipulating file attributes

chown

� Used to change file owner.

chgrp

� Used to change file group.

chmod

� Used to change file permissions.


4.4 Manipulating file attributes (cont’d)

Symbolic access modes

� chmod u, g, o +/- r, w, x

$ ls -la

-rw-r--r-- 1 root root 0 foo

$ chmod +x foo

-rwxr-xr-x 1 root root 0 foo

$ chmod g-x foo

-rwxr--r-x 1 root root 0 foo


4.5 File system internals

Process table

�

�

The process table is a data structure in the computer’s main memory that holds information about the processes currently handled by the operating system.

Each entry in the process table contains a table of open file descriptors for that process.


4.5 File system internals (cont’d)

Open file table

�

�

The open-file table contains an entry for each open file for every process.

File descriptors are unique to a process – a file that has been opened by several processes may be assigned different descriptors for each process.


4.5 File system internals (cont’d)

pid #10 fd #0fd #1fd #2fd #3fd #4

statusoffsetetc.

pid #11

pid #12

statusoffsetetc.

statusoffsetetc.P

roce

ss t

able

fd #0fd #1fd #2fd #3fd #4fd #5fd #6

fd #0fd #1fd #2fd #3fd #4 memory disk or other device

Open-file table

Actual file entriesin v-node table

File

File


4.6 Commands for file/process information

Shows a report of all open files and the processes that opened them.

Selects the listing of files for the processes whose ID numbers are in the comma-separated set s.

List information about every process now running.

lsof

lsof -p s

ps -e

Unix filesystem and shell (IV)

5.1 The standard in/out/err

Entry 0: Standard input is data (often text) going into a program.

Entry 1: Standard output is the stream where a program writes its output.

Entry 2: Standard error is typically used by programs to output error messages or diagnostics.

The first three entries in the file descriptor table are preset:

�

�

�


5.2 Devices

In Unix, devices are treated just like ordinary files.

Entries in the /dev file system are device driver interfaces. Applications interact with the device using standard I/O system calls.

/dev/tty represents the terminal for the current process.

Some special devices, called pseudo-devices do not correspond to physical devices (e.g. /dev/null).

�

�

�

�


5.3 Redirection

Before a command is executed, the input and output can be changed (redirected) from the terminal to a file:

Append output:

$ ls > file

$ date >> file

$ cat < file

Redirect standard out

Redirect standard in


5.4 Mounting file systems

When the system starts, the directory tree corresponds to the file system located on a single disk called the root device.

File systems created on other devices are attached to the original directory hierarchy using the mount mechanism.

The commands mount and umount are used to mount and unmount devices.

�

�

�


5.4 Mounting file systems (cont’d)

/

a

b

/

/

a

a

b

b

a

b

/

/a/b

Device Mount point

root device external device


5.5 Links

A directory is a list of files and directories.

Each directory entry links to a file on the disk.

Two different directory entries can link to the same actual file.

Moving a file (changing its location) does not actually move any data around.

�

�

�

�

Creates a link in new location.Deletes link in old location.

��

In same directory or across different directories.�

5.5 Links (cont’d)

The ln command is used to create a regular link, often called hard link.

Symbolic links (soft links) are created using ln -s.

The main differences are that:

�

�

�Hard links don’t work across file system boundaries.Hard links only work for regular files, not directories.

��

$ ln filename linkname



5.5 Links (cont’d)

Hard vs. symbolic links

directoryentry

file contents

directoryentry

directoryentry

hard linkhard link

symboliclink

filecontents

The Unix find command is used to locate files.

This will search the whole system for any files named foo and display their pathnames.

�

$ find / -name foo

You can specify as many places to search as you wish.

$ find /tmp /var/tmp . -name foo


5.6 Find utility

syntax: find where-to-look what-to-do

Prints an entry for a directory called './src/misc' (if one exists).

find . -path './sr*sc'

-path pattern�

Find every file under the directory /home owned by the user joe.

find /home -user joe

-user name�


5.6 Find utility (cont’d)

Search for files which permissions matches the provided expression.

find . -perm 664

-perm p�

Same as previous except that this will ignore presence of any extra permission bits. This will match a file which has mode 0777, for example.

find . -perm -664



find . -mtime 0 # find files modified between now and 1 day ago

# (i.e., within the past 24 hours)

find . -mtime -1 # find files modified less than 1 day ago

# (i.e., within the past 24 hours, as before)

find . -mtime 1 # find files modified between 24 and 48 hours ago

find . -mtime +1 # find files modified more than 48 hours ago

find . -mmin +5 -mmin -10 # find files modified between

# 6 and 9 minutes ago

-mmin n -mtime n�



introduction to unix-like systems (part i-iv)

Technology

unix shell

shell i2

unix onlinedocumentation

examples of unix

originalcreators of

shell ii3

shell scriptinga shell

dierent unix variants