Unix introduction

Introduction to UNIX

Copyright Department of Computer Science,

Northern Illinois University, 2004 01-1

System Consultant1

UNIX Power User Training

Unix Courseware

Version 1.2

2





3

What is an Operating System?

• Software that manages (allocates and de-allocates)

system resources in an efficient and secure manner

• System resources consist of hardware (e.g.

terminals, printers, storage device, etc) and

software (e.g. application programs, language

libraries, etc.)

4

System Resources

System

Resources

Hardware

ComponentsSoftware

System

Software

Application

Software




5

Different Types of Operating Systems

• Single-user, single-process operating systems:

allow only one user at a time to use the computer

system. The user can execute/run only one

process at a time.

– Examples: DOS, Windows 3.1

• Single-user, multi-process operating systems:

allow a single user to use the computer system;

however, the user can run multiple processes at

the same time.

– Example: OS/2

6

Different Types of Operating Systems

• Multi-user, multi-process operating systems:

allow multiple users to use the computer system

simultaneously. Each user can run multiple

processes at the same time.

– Examples: UNIX, Windows XP




7

UNIX Operating System

• UNIX is a multi-user, multi-processing, portable(?)

operating system.

• UNIX is designed to facilitate programming, text

processing and communication

8

Components of UNIX: perspective 1




9

Components of UNIX: perspective 2

10

History of UNIX

• Invented by Ken Thompson at AT&T in 1969

• First version written in assembly language– single user system, no network capability

• Thompson, Dennis Ritchie, Brian Kernighan– rewrote Unix in C: processor/architecture independent

• Unix evolution:I. Bell Labs, USL, Novell, SCO

II. AIX, Ultrix, Irix, Solaris, …

III. BSD, FreeBSD, Mach, OS X

IV. Linux, Redhat, Suse, …




11

What Can UNIX Do for You?

• User’s Support Tools

– Text processing (vi, sed, awk)

– Filing system

– E-mail and networking

– Electronic databases

• Programmer’s Support Tools

– Programming languages & compilers (C, C++)

– Shell scripts

– Programming Workbench

• Source Code Control System (SCCS)

• Revision Control System (RCS)

12

Some Standard UNIX Shells




13

Command Source and Destination

14

Command Line Structure

mp% command [-options] [arguments]

Command

prompt

Command

name

Arguments can be:

1. More information

2. Object identifiers

3. Names of files

Notes:

•UNIX is case sensitive!!!!

•Example: the command “ls –l” is not the same as “LS –L”

• Must be a space between the command, options and arguments

• No space between the plus or minus sign and the option letter

• Option letters must be typed exactly as they are indicated,

uppercase or lowercase

•Fields enclosed in [ ] are optional

•Must press [Return] after you have completed entry of a command

Command modifier;

usually one character

preceded by + or - sign




15

Correcting Mistakes

Note: UNIX is case sensitive (use lowercase)

Key Pressed Result

Backspace, Ctrl-h Back up & erase last character

Ctrl-c Terminates the current command

Ctrl-r Redraws the current command line

Ctrl-s Stops scrolling of output on screen

(Ctrl-q to resume/start scrolling)

Ctrl-w Erases a word on command line

Ctrl-u Erases/deletes entire command line

16

Command Line Structure

• How do the results of the following commands

differ?

• According to the synopsis of the “sort” command,

what parts of the “sort” command are required?

sort list

sort –f list

sort –o sorted list

Command

name

Command

option

Option

argument

Command

argument




17

The UNIX File System

18

A Directory Hierarchy




19

Sample UNIX File Structure (NIU, CS)

UNIX supports “tree-like” directory structure

usretcdev home

uxmptty null skel bin local ucb

z036473

.cshrc .logoutcsci330

Root Directory

Home Directory

Root subdirectories

z036473

subdirectory

Files in the

z036473 directory

20

Directory Types

• Root Directory: /

– The first directory in any UNIX file structure

– Always begin with the forward slash (/)

• Home Directory: $HOME or ~

• Created by system administrator

• This is where you are when you first log in!

• Under $HOME, you may create your own directory structure

• Type: cd [Return] takes you $HOME

• Current Working Directory: .

– The Directory you are currently working in

– Also called Current Working Directory (cwd)

• Parent Directory: ..

– The directory immediately above your current working directory.




21

File Types

- Root (/)

- Home (~)

- Working (.)

- Parent (..)

- Text

- Binary

22

Paths and Pathnames

Two ways of locating a file or a directory:

• By Using Absolute Pathname– Full pathname

– Traces a path from root to a file or a directory

– Always begins with the root (/) directory!

– Example: /home/ux/krush/unix/assignments/assign1.sp04

• By Using Relative Pathname– Traces a path from the ‘cwd’ to a file or a directory

– No initial forward slash (/)

– Two dots (..) goes up one level on file structure

– Dot (.) points to current working directory (cwd)

– Example: unix/assignments/assign1.sp04




23

Relative Pathnames for file3

Absolute Pathname:

/usr/staff/joan/file3

24

Directory Operations




25

Display Current Directory’s Full Pathname

• To determine the full pathname of the current

working directory, use the command named

“pwd”

• pwd stands for print working directory

Example: To display the full pathname of the current

working directory

ux% pwd

/home/ux/krush/unix

26

The ls Command




27

Long List Option

28

List ALL Contents of Current Directory

ux% ls -la

total 126

drwxr-xr-x 13 krush csci 1024 Apr 26 15:49 .

drwxr-xr-x 15 root root 512 Apr 24 15:18 ..

-rwx------ 1 krush csci 1120 Apr 12 13:11 .cshrc

-rwxr--r-- 1 krush csci 885 Dec 2 13:07 .login

-rw-r--r-- 1 krush csci 141 Mar 14 13:42 .logout

--rwx------ 1 krush csci 436 Apr 12 11:59 .profile

drwx------ 7 krush csci 512 May 17 14:11 330

drwx------ 3 krush csci 512 Mar 19 13:31 467

drwx------ 2 krush csci 512 Mar 31 10:16 Data

-rw-r--r-- 1 krush csci 80 Feb 27 12:23 crontab.cron

List contents of the current

directory in long format. Points to cwd,

.. Points to parent’s

dir.

File names

begins with

a dot (.) are

hidden files

Directories

Regular

text file




29

List Contents of a Specific Directory

ux% ls -l unix/grades

total 10

-rwxr-xr-x 3 krush csci 72 Jan 19 19:12 330assign-graderun

-rwxr-xr-x 1 krush csci 70 Jan 19 19:13 330exam-graderun

-rwxr-xr-x 2 krush csci 70 Jan 19 19:12 330quiz-graderun

-r-x------ 1 krush csci 468 Feb 1 11:55 test-330grade

-r-x------ 1 krush csci 664 Feb 1 11:55 test-330grade,v

Listing contents of a subdirectory named

“unix/grades”

30

File Name Expansion & Wildcards

Allows you to select files that satisfy a particular name pattern (wildcards)

Character Description Example

* Match zero or more char. ls *.c

? Match any single character ls conf.?

[list] Match any single character in list ls conf.[co]

[lower-upper] Match any character in range ls lib-id[3-7].o

str{str1,str2,…} Expand str with contents of { } ls c*.{700,300}




31

The mkdir Command

32

Creating a New Directory

• To create a directory, use the command named

“mkdir”

Example: To create two new directories called “csci330”

and “test-data”

ux% mkdir csci330 test-data

ux% mkdir /home/ux/krush/unix/demo

must already exist




33

Directory Names

• Use the following characters:

– Uppercase letters (A-Z)

– Lowercase letters (a-z)

– Numbers (0-9)

– Underscore ( _ )

– Period/dot ( . )

34

Directory Names

• When naming a directory, avoid the following

characters:

& * \ | [ ] {}

$ <> () # ? /

“ ‘ ; ^ ! ~

Space Tab




35

Example: Create a Directory Creation

Create a directory called Data under csci330

a) Using Absolute Pathname:

b) Using Relative Pathname:

usretcdev home


z036473


You are here

Data

Temp

mkdir /home/mp/z036473/csci330/Data

mkdir csci330/Data

36

The cd Command




37

Changing Directory

In the Data directory, go to $HOME directory

a) Using Absolute Pathname:

b) Using Relative Pathname:

usretcdev home


z036473


You are here Data

Temp

cd /home/mp/z036473

cd $home cd cd ~ cd ~z036473cd ../..

38

Remove Directories

• To remove an empty directory – a directory that does not contain user-created files, use the command named “rmdir”Example: To remove a directory called “test”, which

does not contain user-created files.

ux% rmdir test

• To remove a non-empty directory, use the command named “rm –r”Example: To remove a non-empty directory called “old-

data”

ux% rm –r old-data




39

Operations Common to Directories and

Regular Files

40

Copying Files

• To copy a file, use the command named “cp”

• Syntax: cp source-file new-file

• Commonly used options:

-i if “new-file” exists, the command cp prompts for

confirmation before overwriting

-p preserve permissions and modification times

-r recursively copy files and subdirectories




41

Copying Files

• “source-file” must have read permission.

• The directory that contains “source-file” must have

execute permission.

• The directory that contains “new-file” must have

write and execute permissions.

• Note that if “new-file” exists, you do not need the

write permission to the directory that contains it,

but you must have the write permission to “new-

file”.

42

Moving Files

• To move files from one directory to another

directory, or to re-name a file, use the command

named “mv”.

• The directory that contains the source file and the

destination directory must have write and execute

access permissions.




43

Moving Files

• Syntax: mv source-file destination-file

• If the destination file exists, “mv” will not

overwrite exiting file.

Example: Move “assign1.txt” a different directory and

rename it to “assign1.save”

ux% mv assign1.txt $HOME/archive/assign1.save

ux% mv assign1.txt $HOME/archive

44

Moving a File




45

Rename Directories

• To change the name of an existing directory, use

the command named “mv”

Example: To rename the file called “unix” to

“csci330”

ux% mv unix csci330

• For the above example, what happens if “csci330”

already exists in the current directory and it is the

name of a directory?

46

The mv Command




47

Removing/Deleting Files

• You should remove un-needed files to free up disk space.

• To remove/delete files, use the command named “rm”.

• Syntax: rm file-list

• Commonly used options:-f force remove regardless of permissions for “file-list”

-i prompt for confirmation before removing

-r removes everything under the indicated directory

48

Removing/Deleting Files

• If “file-list” contains pathname, the directory

components of the pathname must have execute

permission.

• The last directory that contains the file to be

deleted must have execute and write permissions.

Example: Remove the file named “old-assign”

ux% rm unix/assign/old-assign




49

Recap: Common Operations on Files

50

Finding Files

• The command named “find” can be used to locate

a file or a directory.

• Syntax: find pathname-list expression

• “find” recursively descends through pathname-list

and applies expression to every file.

• For syntax of expression, see Course Notes pp. 13-

9




51

Finding Files

Example 1: Find all files, in your directory hierarchy, that

have a name ending with “.bak”.

ux% find $home –name “*.bak” –print

Example 2: Find all files, in your directory hierarchy, that

were modified yesterday.

ux% find $home –mtime –1 -print

52

The “ln” command

• Allows file to listed in multiple directories

• 2 types:

– Hard link

– Symbolic link

• First: understand Unix file storage and organization




53

Unix file organization

• Computer has one or more physical hard drives

• Hard drive is divided into partitions

• Partition holds file system

– File system is set of data blocks

– Data blocks contain

• general information

• actual file data

• directory information

54

Blocks in a file system




55

inode

• Index (or information) node: one inode per file

• Each inode has unique number

• contents:– File type, access permissions, link count

– UID, GID

– Date and time of the file’s last• Data access (read and execute)

• Data modification (written)

• I-node modification (permission change)

– Data blocks assigned to the file

56

Inodes in a filesystem




57

inode Contents: where is the file data ?

Inode may store:

– 10 addresses of data blocks that belong to file

– 1 address of a block that contains data block addresses

– 1 address of a block that contains addresses of blocks

that contain data block addresses

– 1 address of a block that contains addresses of blocks

that contain addresses of blocks that contain data

block addresses

58

I-node Structure

Access, Links, and

other information

1

2

.

.

9

10

11

12

13

I-node blocks

triple indirectblock

indirectblock

blocksblocks

blocks

double indirectblock

blocks

blocks

blocks




59

Directory representation

Directory is a file:

– Has inode like regular file, but different file type

– Data blocks of directory contains simple table:

Name Inode number

60

Example structure

. 1076

.. 2083

myfile 2764

Contents of dir12763

2764

2765

.

.

.

Data blocks

on disk

I-node

list




61

Example: user view vs. system view

62

Output: ls -li

ux% ls -li crontab.cron118282 -rw-r--r-- 1 krush csci 80 Feb 27 12:23 crontab.cron

I-node




63

Howto share Files ?

• Duplicate shared files.

• Create common login name for members of the

team.

• Set appropriate access permissions on shared files.

• Create common group for members of the team.

• Share files via links.

64

Duplicate Shared Files

• Make copies of shared files and give them to all

members of the team.

• The simplest way to share files.

• Does not work well if members of the team work

on the files at the same time.




65

Create Common Login Name

• All team members use a common login that gives

them access to shared files.

• This is a simple solution that works well if the size

of the team is small and stable.

• A disadvantage is team members have to use a

separate account, not their regular account.

66

Set Appropriate Access Permissions

• Team members put all shared files in one place

and set access permissions so all team members

can access them.

• This scheme works well if all team members are

in the same group (group permissions can be

used).

• A disadvantage is if the group used has other users

in it, they will also have access to the files.




67

Create Common Group

• This solution is similar to setting appropriate

permissions except a new group is created that

contains just the members of the team.

• All team members have individual logins.

• This is an effective solution, especially if it is with

version control.

68

Linking Files

• To share a single file with multiple users, a link can

be used.

• A link is:

– A reference to a file stored elsewhere on the system.

– A way to establish a connection to a file to be shared.

• Two types:

– Hard link

– Symbolic link (a.k.a. “soft link”)




69

Hard Link

Advantages Disadvantages

Allow access to original file name

via the file name or the I-node

number

The original file continues to exist

as long as at least one directory

contains its I-node

Checks for the existence of the

original file

Cannot link to a file in a different

file system

Prevents owner from truly deleting

it, and it counts against his/her disk

quota

70

Hard Link

home

z036473

dir3

dir1 dir2

aa

bb

From dir3, link to the file ‘aa’ in dir1

name it ‘bb’:

% ln /home/z036473/dir1/aa bb

Syntax: ln shared-file link-name




71

The ln Command

72

A Hard Link




73

2406

2407

2408

.

.

.

Hard Link

home

z036473

dir3

dir1 dir2

aa

bb

. 1076

.. 2083

aa 2407

Contents of dir1

. 1070

.. 2050

bb 2407

Contents of dir3

74

Symbolic Link

Advantages Disadvantages

Allow access to original file name

Can use either relative or absolute

path to access the original file

Can cross partition and drives

Allows the creation of a link to a

directory

Created without checking the

existence of the shared file

Cannot access the shared file if its

path has restricted permissions

Can be circular linked to another

symbolic linked file




75

Symbolic Link

• A hard link may not be created for a file on a different file system

• Use symbolic link

• The linked files do not share the same I-node number

Syntax: ln –s shared-file link-name

Also called source-file Also called target-file

76

Symbolic Links to Different File Systems




77

User’s Disk Quota

• A disk quota is set for each user account

• The command: quota –vdisplays the user’s disk usage and limits

• 2 kinds of limits:– Soft limit: ex. 3MB

• Maybe exceeded for one week

• System will nag

– Hard limit: ex. 4MB• Cannot be exceeded

78

Operations Common to Directories and

Regular Files




79

Operations Unique to Regular Files

Create EditDisplay

ContentsPrint Others

80

Creating New Files

Create Regular

Files

vi picocat

Redirect

Command

Output

See Text Editors

Section 03

See the C shell

Section 06




81

Creating A File With cat

Example:

mp% cat > myfile

This is line 1 of input

Line 2 of input

^d

mp%

82

Editing Text Files

Editing Text

Files

pico sedvi awk

See Text Editors

Section 03

See course notes

section 10

See course notes

section 08




83

Displaying Contents of Text Files

Display Text

File contents

cat more less pg head tail

84

The cat Command




85

Viewing Contents of Text Files

• The command named “cat” can be used to

display/concatenate one or more files, displaying

the output all at once.

Example: Display the contents of a file called

“assign1.txt”.

ux% cat assign1.txt

86


• The commands named “more” and “less” can be used to display the contents of one or more files one page at a time.Space bar – to advance to next page

b – to go back a page

Enter Key – to advance to next line

Example: Display the contents of a file called “assign1.txt” one page at a time.

ux% more assign1.txt




87


• The command named “head” can be used to

display the beginning portion of indicated file(s);

the default head size is 10 lines.

Example: Display the first 20 lines of a file called

“assign1.txt”.

ux% head –20 assign1.txt

88


• The command named “tail” can be used to display

the ending portion of indicated file(s); the default

tail size is 10 lines.

Example: Display the last 10 lines of a file called

“assign1.txt”.

ux% tail assign1.txt

ux% tail –10 assign1.txt




89

Other Operations Unique to Text Files

Other File

Operations

Combine

contents

Extract

contents

Compare

contents

File

size

Compress

contentssort

Unique

lines

Encrypt/

decrypt

90

Combining Contents of Files

• Method 1: To vertically concatenate the contents

of two or more files, use the command named

“cat” with output redirection (>).

• Syntax: cat file-1 file-2 file-3 > all-file

• “all-file” will contain the combined contents of

file-1, file-2, and file-3 in top-down (vertical)

fashion

• See demo




91

Combining Contents of Files

• Method 2: To horizontally concatenate contents

(columns/fields) of two or more files, use the

command named “paste”.

• Syntax: paste file-1 file-2

• See demo

92

Extracting Contents of Files

• To extract one or more fields in a file, use the

command named “cut”.

Example: Extract the month and year fields from the output

of the “date” command.

ux% date

Mon Feb 2 20:37:38 CST 2004

ux% date | cut -d' ' -f2,6

Feb CST




93

The cut Command

94

Comparing Files: comm

• The command named “comm” can be used to

compare lines that are common in two sorted files.

• Syntax: comm [options] file-1 file-2

• The output contains three columns:

– Column1 contains lines unique to file-1

– Column 2 contains lines unique to file-2

– Column 3 contains lines common to both files

• See Demo




95

Comparing Files: diff

• The command “diff” can be used to compare two

files line by line.

• Syntax: diff [options] file-1 file-2

• If file-1 and file-2 are the same, no output is

produced.

• If file-1 and file-2 are not the same, diff reports a

series of commands that can be used to convert the

first file to the second file.

• See Demo

96

Determining File Size

• Recall: The “ls” command with the option –l gives the file size in bytes.

• Use the command named “wc” to display the size of files as number of lines, words, and characters.

• Syntax: wc file-list

• Commonly used options:-l display the number of lines

-w display the number of words

-c display the number of characters

• See demo




97

The wc Command

98

Compress File Contents

• The command named “compress” can be used to

reduce the size of one or more files.

• Syntax: compress file-name

• To display compression percentage and the names

of compressed files, use the option named –v.

• The file extension .Z is automatically appended to

file-name.

• See Demo




99

Viewing a Compressed File

• The command named “zcat” can be used to

display the contents of a compressed file in a

readable format.

Example: Display the contents of a compressed file called

“customer-data.Z”.

ux% zcat customer-data.Z

100

Un-compress File Contents

• To uncompress one or more compressed files,

used the command named “uncompress”.

• Syntax: uncompress file-list

Example: uncompress a compressed file called

“customer-data.Z”.

ux% uncompress customer-data.Z




101

Sorting Files

• To sort a text file in ascending or descending order, use the command named “sort”.

• Syntax: sort [options] file-name

• Commonly used options:-r sort in reverse order

-n numeric sort

+x [-y] specify a field as the sort key; skipping x fields and start sorting with field y

-f consider lowercase and uppercase to be equivalent

102

The sort Command




103

Fields Within A Line

104

Field Specifier Examples




105

Global and Local Options

106

Removing Repeated Lines

• The command named “uniq” can be used to

remove repetitious lines from a sorted input file,

sending unique (unrepeated) lines to standard

output.

• Syntax: uniq sorted-file-name

• Commonly used options:

-c place a count of repeated lines at beginning of each

output line

-d display the repeated lines

-u display the lines that are not repeated




107

The unique Command

108

Encrypting Files

• Encryption is a process that transforms a file to an unreadable form.

• The transformed file is called an “encrypted” file.

• You encrypt files to prevent other users from reading their contents.

• The command named “crypt” can be used to encrypt files.

• Syntax: crypt key < original-file > encrypted file

• See Demo




109

Decrypting Files

• The process of transforming an encrypted file to its

original format is called “decryption”.

• To decrypt an encrypted file, use the command named

“crypt”.

• Syntax: crypt key < encrypted-file > original-file

• See Demo

110

UNIX Text Editors




111

Editor Concepts

• Editing – creating a new file and modifying an existing text file.

• An editor – a utility that makes the editing task possible.

• A text editor differs from a word processor in that it does not perform text formatting, such as bold, center, underline, etc.

• A line editor – a utility that applies changes to a line or group of lines; two common line editors: ex and sed.

• A screen editor – shows a whole screen of text at a time; we can move cursor or select part of text, search for text, etc.

112

The Vi Editor

• A screen editor available on most UNIX systems.

• When invoked, it copies the contents of a file to a memory space know as a work buffer.

• All editing are applied to the contents in the work buffer.

• If the file does not exist, an empty buffer is created.

• When we exit vi, the work buffer is erased.

• At exit time, we can do two things:1. Quit without saving – the original contents remain

unchanged.

2. Save the file – the original contents are replaced by the new version in the work buffer.




113

Vi Buffers

• Work Buffer– This is where vi performs all editing

– Vi makes a copy of your file in the work buffer

– Makes all changes to the copied version

– Replaces original copy with edited copy when you save (:w)

• General Purpose Buffer– Stores recent deleted, or copied text

– Used by the ‘undo’ command to restore text

• Name Buffers– 26 named buffers, each named by a letter of the

alphabet

– Used to store a different block of text to recall later

114

The UNIX vi Text Editor

CommandMode

InputMode

Last-LineMode

Insert (i, I)Append (a, A),Open (o, O)Change (c),

Replace (r, R)

Esc

Return:




115

Vi Mode

• In vi, to find out what mode you are in, you need

to create a vi start-up file (.exrc)

• In your home directory or in your cwd directory,

create “.exrc” file with contents:

set showmode

116

File Names

• When naming a file, avoid the following

characters:

& * \ | [] {}

$ <> () # ? /

“ ‘ ; ^ ! ~

Space Tab




117

Commonly Used File Extensions

Extension File Type

.a An archive or library

.c C program source

.cc C++ program source

.csh C shell script

.f FORTRAN program source

.sh Bourne shell script

.bsh Bash shell script

.ksh Korn Shell script

118

Commonly Used File Extensions

Extension File Type

.o Object file of compiled program

.ps Postscript source

.shar Shell archive

.tar Tar archive

.txt ASCII text file

.Z Compressed file




119

Recovering Text After a Crash

• If the system crashes while you are editing a file

using vi, you may be able to recover your text

• The system sends you an e-mail telling you how to

recover your file

• To recover a file after a system crash:

% vi –r filename

120

Vi Demo

Demo




121

Access Permissions

122

Terminology

• A user – any one who has Unix account on the

system.

• Unix recognizes a user by a number called user id.

• A super user:

– has the maximum set of privileges in the system

– also know as system administrator

– can change the system

– must have a lot of experience and training

• Users can be organized into groups.

• One or more users can belong to multiple groups.




123

Users

124

Terminology

• To find out group information, use the command

named: groups user-id

• Example: To find out what groups the user

z036473 belongs to.

ux% groups z036473

student csci467a csci330c

• Information about groups is stored in the Network

Information Service (NIS) file named

group.org_dir.




125

Security Levels

• There are three levels of security in UNIX:

system, directory and file.

• System security – controlled by the system

administrator, a super user.

• Directory and file – controlled by the user who

owned them.

126

Security Levels




127

Access Permission Code

• The protection on a file is referred to as its file modes

• File modes are set with the “chmod” command

• UNIX supports three types of access permissions:r read

w write

x execute

- permission denied

128

Directory and File Permissions




129

Access Types

Access Type Meaning on File Meaning on Dir.

r (read) View file contents

(open, read)

List directory contents

w (write) Change file contents - Change directory contents

- Be careful !!!

x (execute) Run executable file - Make it your cwd

- Access files (by name) in it

- Permission denied Permission denied

130

Checking Permissions

• To check the permissions of an existing file or an

existing directory, use the command: ls –l

• Example:

ux% ls –l unix

total 387drwxr--r-- 1 z036473 student 862 Feb 7 19:22 unixgrades

-rw-r--r-- 1 z036473 student 0 Jun 24 2003 uv.nawk

-rw-r--r-- 1 z036473 student 0 Jun 24 2003 wx.nawk

-rw-r--r-- 1 z036473 student 0 Jun 24 2003 yz.nawk




131

Changing Permissions

132

The chmod Command




133

Changing Permissions: Symbolic Mode

134


mp% chmod who operation permissions filename

u for user

g for group

o for others

a for all

+ for add

- for remove

= for assign

r for read

w for write

x for execute




135

ux% ls -li sort.c

118283 -rw-r--r-- 1 krush csci 80 Feb 27 12:23 sort.c


Example 1: To change the permissions on the file “sort.c” using

Symbolic mode, so that:

a) Everyone may read and execute it

b) Only the owner and group may write to it.

rwx|rwx|r-xWe want this using

symbolic mode:

Answer:chmod ug=rwx,o=rx sort.c

chmod ugo+rx,go+w sort.c

136

The chmod Command: Octal Mode




137

Changing Permissions: Octal Mode

ux% ls -li sort.c


Example 2: Ignoring the original permission settings, change the

permissions on the file “sort.c” using octal mode, so that:

a) Everyone may read and execute it

b) Only the owner and group may write to it.

rwx|rwx|r-xWe want this using

octal mode:

Answer: chmod 775 sort.c

138

Changing Permissions: Octal Mode

Step Perform… Settings

1 List the desired setting

2 Assign binary:

1 for access; 0 for no access

3 List octal values for the

corresponding binary 1’s

4 Convert the octal values to a 3-

digit number

5 Write the command

ux% ls -li sort.c


rwx|rwx|r-x

111|111|101

421|421|401

775

chmod 775 sort.c




139

Changing Permissions: exercise

Ignoring the original permission settings on a file

called “myfile”, assign:

a) Read, write, and execute permissions to owner

b) Read and execute permissions to group

c) Execute permission to others

1) Using Symbolic Mode: __________________________________

2) Using Octal Mode: __________________________________

We want: rwx|r-x|--x

chmod u=rwx,g=rx,o=x myfile

chmod 751 myfile

140

User Masks (default permissions)• The default permissions are initially set for a file or directory

using a three-digit octal system variable called user mask

(mask).

• This user mask was defined initially by the system

administrator when your account is created.

• Initially set by system administrator in a start-up file:

$HOME/.cshrc

• The user mask contains the octal settings for the permissions

to be removed from the default when a directory or file is

created.

• The default permissions are:

– 777 for a directory

– 666 for a file




141

The umask Command

142

User MaskUser mask

Value

Directory

Default: 777

File (666)

Default: 666

000 777 (rwx rwx rwx) 666 (rw- rw- rw-)

111 666 (rw- rw- rw-) 666 (rw- rw- rw-)

222 555 (r-x r-x r-x) 444 (r- - r- - r- -)

333 444 (r- - r- - r- -) 444 (r- - r- - r- -)

444 333 (-wx –wx –rx) 222 (-w- -w- -w-)

555 222 (-w- -w- -w-) 222 (-w- -w- -w-)

666 111 (- -x - -x - -x) 000 (--- --- --- )

777 000 (--- --- --- ) 000 (--- --- --- )




143

The command: umask

% umask 000

For regular files: rw-rw-rw- 666

For directories: rwxrwxrwx 777

In $HOME/.cshrc file: umask 022

For regular files: rw-r- - r- - 644

For directories: rwxr-xr-x 755

144

Default Access Permissions

• To prevent cheating, do not grant read and write

access permissions to group and others!

• Set default permissions for all files and directories

created in the future, so that only you/owner can

rw to it (rw- --- ---)

• In your $HOME/.cshrc file: umask 077




145

Default Access Permissions: exercise

Assume your $HOME/.cshrc file contains the

command: umask 002.

a) What permissions would be set for newly created

text files?

b) What permissions would be set for newly created

directories?

146

Special Permissions

• The regular file permissions (rwx) are used to

assign security to files and directories.

• Three additional special permissions can be

optionally used on files and directories.

– Set User Id (SUID)

– Set Group ID (SGID)

– Sticky bit




147

Special Permissions: SUID

• SUID enables regular users to execute a program file and become the owner of the file for the duration of execution.

• Example: The “passwd” command is used to change a user’s password. The file “/usr/bin/passwd” (owned by “root”) has this special permission set: r-sr-sr-x

• In theory, only the root user has the ability to change passwords. Since “/usr/bin/passwd” has the SUID permission on, when a regular user executes the “passwd” command, the user temporarily becomes the “root” user while the “passwd” command is executing.

148

Special Permissions: GUID

• Like SUID, SGID enables regular users to execute

a program file and become a member of the group

that is attached to the file.

• If a file is owned by the system group and also has

the SGID permission, then any user who executes

that file will be a member of the system group

during the execution.




149

Special Permissions: Sticky Bit

• Sticky bit performs a useful function on

directories

• Recall: Write permission applied to a directory

enables you to add and remove any files to or from

that directory.

• If you had “write” permission to a certain

directory but no permissions for files within it,

you could delete all of those files.

150


Scenario: A company provides a common directory

that gives all employees the ability to add and

remove files in it. This directory must give user

“write” permissions. Unfortunately, the “write”

permissions also gives all employees the ability to

delete all files and subdirectories within it,

including the ones that others have added to the

directory.




151


• If the “sticky” bit were applied to this common

directory, in addition to “write” permissions, then

employees may add files to the directory.

However, they may delete only files and

directories that they have added/created.

• Example:

ls –ld /tmpdrwxrwxrwt 3 root sys 1077 Jan 25 13:30 /tmp

Sticky Bit

152

Special Permissions

• The access permission status that is displayed

using the “ls –l” command does not have a section

for special permissions

• However, since special permissions required

“execute”, they mask the execute permission when

displayed using the “ls –l” command.

r w x r w x r w x

r w s r w s r w t

SUID SGID STICKY

BIT




153

Special Permissions

• All special permissions also required the “execute” permission

to work properly.

• Usually, the SUID and SGID apply to executable files.

• The sticky bit applies to directories, which must have execute

permission for access.

• If special permissions are set on a file or a directory without

“execute” permission, the special permissions are shown in

capital letters.

r w - r w - r w -

r w S r w S r w T

SUID SGID STICKY

BIT

154

Setting Special Permissions

suid sgid stb r w x r w x r w x

4 2 1 4 2 1 4 2 1 4 2 1

7 7 7 7

Special user group others

Use the “chmod” command with octal mode:

chmod 7777 filename




155

Special Permissions: Exercise

• Give the full UNIX command to enable all users

including others to add files to directory named

“dir1”, but only to delete files that they own in

that directory.

• Answer:

____________________________________chmod 1777 dir1

156

Regular Expressions




157

Regular Expression Metacharacters

• A pattern of special characters used to match

strings in a search

• Can be controlled by special characters called

metacharacters

• Metacharacters are used by UNIX pattern

matching editors and utilities:

– Editors: ed, ex, vi

– Utilities: grep, egrep, sed, and awk

158

RE Metacharacters: use with grep

RE Metacharacter Matches…

^ A character at beginning of line

$ A character at end of line

. Any one character, except new line

* Zero or more of preceding character

\char Escape the meaning of char following it

[a-z] Any one of the enclosed characters (e.g. a-z)

[^] One character not in the set

\< Beginning of word anchor

\> End of word anchor

 Tags matched characters to be used later (max = 9)

x\{m\} Repetition of character x, m times (x,m = integer)

x\{m,\} Repetition of character x, at least m times

x\{m,n\} Repetition of character x between m and m times




159

RE Metacharacters: use with egrep

RE Metacharacter Matches…

^ A character at beginning of line

$ A character at end of line

. Any one character, except new line

* Zero or more of preceding character

\char Escape the meaning of char following it

[a-z] Any one of the enclosed characters (e.g. a-z)

[^] One character not in the set

+ One or more of the preceding characters

? Zero or one of the preceding characters

a|b Either a or b

( ) Groups characters

160

The grep Family




161

The Command: grep

• Searches the entire specified input file(s) for a pattern of characters.

• If pattern contains white space, it must be in quotes.

• All other words following RE-pattern are treated as files.

• Sends its output to stdout (default), a file, or a command pipe.

• “grep” does not change the original input file(s).

• “grep” can receive input from stdin (keyboard), a pipe, or file(s)

Syntax: grep [options] RE-pattern file(s)

162

Commonly Used Grep Options

Commonly used “grep” options:-c Print only a count of matched lines.

-i Ignore uppercase and lowercase distinctions.

-l List all files that contain the specified pattern.

-n Print matched lines and line numbers.

-s Work silently; display nothing except error messages. Useful

for checking the exit status.

-v Print lines that do not match the pattern.

-w Search for the expression as a word as if surrounded by \< \>




163

Regular Expression

An atom specifies what text is to be matched and

where it is to be found.

An operator combines regular expression atoms.

164

Atoms

An atom specifies what text is to be matched and where

it is to be found.




165

Single-Character Atom

A single character matches itself.

166

Dot Atom

A dot matches any single character except for a new

line character (\n)




167

Class Atom A class matches only single character that can be any of

the characters defined in a set, e.g. [A-C] matches either

A, B, or C.

Notes:

1) A range of characters is indicated by a dash, e.g. [A-C]

2) Can specify characters to be excluded from the set, e.g.

[^0-9] matches any character other than a number.

168

Example: Classes




169

Anchors

Anchors tell where the next character in the pattern must

be located in the text data.

170

Back References: \n

• Used to retrieve saved text in one of nine buffers

• Can refer to the text in a saved buffer by using a

back reference; e.g. \1 \2 \3 ...\9.

• More on back references later in this section.




171

Operators

no oper.

172

Sequence Operator

In a sequence operator, if a series of atoms are shown in

a regular expression, there is no operator between them.




173

Alternation Operator: |

The alternation operator ( | ) is used to defined one or more

alternatives, e.g. A | B matches A or B.

174

Repetition Operator: \{…\}

The repetition operator specifies that the atom or

expression immediately before the repetition may be

repeated.




175

Basic Repetition Forms

176

Short Form Repetition Operators: * + ?




177

Group Operator

In the group operator, when a group of characters is

enclosed in parentheses, the next operator applies to the

whole group, not only the previous characters.

178

Save Operator: $..$

• Copies a matched text string to one of nine buffers for later reference.

• Back references can then be used to retrieve saved text in one of nine buffers.

• Can refer to the text in a saved buffer by using a back reference; e.g. \1 \2 \3 ...\9.

• Example: To reverse first name and last name in the following data set; also.John DeVries

Bob Flowers

s/^$[A-Z].*$ $[A-Z].*$$/\2 \1/DeVries John

Flowers Bob




179

More Examples

• The following slides contain examples of using

regular expressions with grep, egrep and fgrep.

• Review them on your own time.

• Ask your instructor or T.A. if you have any

questions.

180

Example: Grep with RE: ^

% cat grep-datafilenorthwest NW Charles Main 300000.00

western WE Sharon Gray 53000.89

southwest SW Lewis Dalsass 290000.73

southern SO Suan Chin 54500.10

southeast SE Patricia Hemenway 400000.00

eastern EA TB Savage 440500.45

northeast NE AM Main Jr. 57800.10

north NO Ann Stephens 455000.50

central CT KRush 575500.70

Extra [A-Z]****[0-9]..$5.00

% grep '^n' grep-datafile

northwest NW Charles Main 300000.00



Print all lines beginning

with the letter n.




181

Example: Grep with RE: $

% grep '\.00$' grep-datafile



Extra [A-Z]****[0-9]..$5.00










Extra [A-Z]****[0-9]..$5.00Print all lines ending with a

period and exactly two

zero numbers.

182

Example: Grep with RE: \char










Extra [A-Z]****[0-9]..$5.00

% grep '5\..' grep-datafile

Extra [A-Z]****[0-9]..$5.00

Print all lines containing the

number 5, followed by a

literal period and any single

character. The dot

metacharacter represents a

single character, unless it is

escaped with a backslash.

When escaped, the period is

no longer a special character,

but represents itself, a literal

period.




183

Example: Grep with RE: [ ]










Extra [A-Z]****[0-9]..$5.00

% grep '^[we]' grep-datafile



Print all lines

beginning with either

a “w” or an “e”.

184

Example: Grep with RE: [^]

% cat grep-datafile










Extra [A-Z]****[0-9]..$5.00

% grep '\.[^0][^0]$' grep-datafilewestern WE Sharon Gray 53000.89



Print all lines ending

with a period and

exactly two non-zero

numbers.




185

Example: Grep with RE: x\{m\}










Extra [A-Z]****[0-9]..$5.00

% grep '[0-9]\{6\}\.' grep-datafilenorthwest NW Charles Main 300000.00






Print all lines where there

are at least six consecutive

numbers followed by a

period.

186

Example: Grep with RE: \<










Extra [A-Z]****[0-9]..$5.00

% grep '\<north' grep-datafile




Print all lines

containing a word

starting with “north”.




187

Example: Grep with RE: \< \>










Extra [A-Z]****[0-9]..$5.00

% grep '\<north\>' grep-datafile


Print the line if it

contains the word

“north”.

188

Example: grep with pipe

% ls -l | grep '^d'

drwxr-xr-x 2 krush csci 512 Feb 8 22:12 assignments

drwxr-xr-x 2 krush csci 512 Feb 5 07:43 feb3

drwxr-xr-x 2 krush csci 512 Feb 5 14:48 feb5

drwxr-xr-x 2 krush csci 512 Dec 18 14:29 grades

drwxr-xr-x 2 krush csci 512 Jan 18 13:41 jan13






% ls -l | grep -c '^d'

12

Pipe the output of the

“ls –l” command to

grep and list/select

only directory entries.

Display the number of

lines where the pattern

was found. This does

not mean the number

of occurrences of the

pattern.




189

Example: egrep with RE: a|b

% egrep 'NW|EA' grep-datafile












Extra [A-Z]****[0-9]..$5.00

Prints the line if it

contains either the

expression “NW” or the

expression “EA”.

190

Example: egrep with RE: +

% egrep '3+' grep-datafile













Extra [A-Z]****[0-9]..$5.00

Print all lines containing

one or more 3's.




191

Example: egrep with RE: ?

% egrep '2\.?[0-9]' grep-datafile











Extra [A-Z]****[0-9]..$5.00Print all lines containing

a 2, followed by zero or

one period, followed by a

number.

192

Example: egrep with RE: ( )

% egrep '(no)+' grep-datafile














one or more consecutive

occurrences of the

pattern “no”.




193

Example: egrep with RE: (a|b)

% egrep 'S(h|u)' grep-datafile













the uppercase letter “S”,

followed by either “h” or

“u”.

194

Example: fgrep

% fgrep '[A-Z]****[0-9]..$5.00' grep-datafile

Extra [A-Z]****[0-9]..$5.00










Extra [A-Z]****[0-9]..$5.00Find all lines in the file

containing the literal string

“[A-Z]****[0-9]..$5.00”.

All characters are treated as

themselves. There are no

special characters.




195

The C Shell

196

Standard UNIX Shell Overview




197

Objectives

• C Shell Environment

– Startup and logout files

– Shell variables

– History

– Command substitution

– Command sequences

– Aliases

– Directory Stacks

– Redirections and pipe

• C Shell Programming

198

Password File

ux% nismatch z036473 passwd.org_dirz036473:e6qhWRw.YeK0E:1432:202:rush kaisone:/home/mp/z036473:/bin/csh:12068::

User login id Encrypted

passworduid

gid

User

name

Home directory

Login shell

Number of days between 1/1/1970 and the last

day that the user changed their password.




199

C Shell Startup

init

getty

login

csh

Login:

Login: z036473

Passwd:

Login: z036473

Passwd:

%

200

NIS Password File

• Network Information Service password file named

“passwd.org_dir” contains user account

information.

• The command named “niscat” displays the entire

contents of the “passwd.org_dir” file.

• The command named “nismatch” displays account

information for a specified user.

Examples: Display contents of the password file.

ux% niscat passwd.org_dir




201

Login At NIU

Default shell: /bin/csh

Can be customized via startup scripts

1) /etc/.login (system shell)

2) $HOME/.cshrc (if C shell is the login shell)

3) $HOME/.login

202

Startup File: $HOME/.cshrc

• Executed once:

– at login and

– each time a C shell script is executed

(unless #!/bin/csh –f was used)

• Located in your $HOME directory

• Contains settings such as:

– Command search path

– Initial permission setting (umask)

– Terminal setting

– Mail box, etc.




203

Useful .cshrc content

• Define command aliasesalias ll “ls –al”

alias ap “man –k”

• customize common behavior:– set filec

when typing a file/directory name, if you press the esc key, it will complete the rest of the name for you.

– set ignoreeof

prevents accidental logout if you type ctrl-d at the command line.

– set noclobber

prevents overwritten of files by output direction.

204

Startup File: $HOME/.login

• Executed once only during login process


• Contains settings such as:

– Terminal setting:

special keyboard character setting

stty erase ‘^h’

• Mail directory




205

Logout File: $HOME/.logout

• Executed once when you log out


• useful content:

ps –f –u $user

displays the user’s active processes

206

C Shell Prompts

• C shell has two prompts:

– Primary prompt: %

– Secondary prompt: ?

• Primary prompt (%)

– Can be reset

– Use the command named “set prompt” to customize

your command prompt.

Example: ux% set prompt = “$USER > “

z036473 >New prompt




207

C Shell Prompts

• Secondary prompt

– Appears when writing on-line script at the command

prompt

% foreach student (z036473 z036474)

? mail $student < memo

? ^d (type ctrl-d or type end)

%

– Once the command is entered, and [Return] is pressed,

you cannot go back to previous lines.

– C shell history does not save commands typed at the

secondary prompt.

208

Variables

A shell variable is a location in memory where

values can be stored

Variables

Pre-defined

Shell Environment

User-defined

String Arithmetic

Special

Built-in Pathname




209

Pre-defined Variables

• Two types:

– Pre-defined shell variables

– Pre-defined environment variables

• Pre-defined shell variables are used to configure the shell.

– Example: history – which contains the size of the history list.

• Pre-defined environmental variables are used to configure

the shell environment.

– Example: HOME – which contains the full pathname of your

home directory.

210

Some Predefined Shell Variables

Name Contents

cwd The current working directory

history The size of the history list

ignoreeof Prevents the shell from terminating when pressing

Control-D. Use the “logout” or “exit” command.

noclobber Prevents existing files from being overridden by output

redirection (>), and non-existent files from being

appended by append (>>)

prompt The shell primary prompt

savehist The number of commands to save in the history file

“$HOME/.history”

status The exit code of the last command




211

Predefined Environment Variables

Name Meaning

HOME The full pathname of your home directory

PATH A List of directories to search for commands

MAIL The full pathname of your mailbox

USER Your user id

SHELL The full pathname of your login shell

TERM The type of your terminal

212

Command Search Path

setenv PATH /usr/local/bin:/usr/bin:/usr/ucb:$home/bin:

bin

bin

.logout

usretcdev home


z036473

.cshrc .logincsci330




213

Creating Environment Variables

• Create C shell environment variable and initialize it to a

value

• If varname does not exist, it is created; otherwise, it is

overwritten

• An Environment variable always holds exactly one value

• Example:

% setenv TERM vt100

% echo $TERM

vt100

% setenv EDITOR pico

Syntax: setenv varname value

214

User-Defined Variables

• Created by the user• Syntax: set varname=string

• Create a shell variable and initialize it to a string of characters

• varname:– 1-20 characters long

– Letters, digits, and underscore

– First character cannot be a digit

– Should not be the same as one of the pre-defined variables




215

Storing Values in Variables

Command Result Comment

set x = 123 x contains “123” x contains a number, but it is

stored as string of digits

set x = Hello x contains “Hello” Storing a character string

set name = Jane Black x contains “Jane” Only “Jane” is stored

set name = “Jane Black” x contains

“Jane Black”

When a string contains spaces

it must be quoted

set x = “Go Dons!” x contains

“Go Dons!”

When a string contains spaces

and special characters, it must

be quoted

216

Accessing the Values of a Variable

• The name of the variable must be preceded by a dollar sign

• Variable values can be used anywhere in a string

• Examples:

% set count = 7

% echo $count is greater than 6 and less than 8

7 is greater than 6 and less than 8

% echo The value of count is $count as expected

The value of count is 7 as expected

% echo My lucky number is $count

My lucky number is 7




217

Accessing the Values of a Variable

• Variable values can also be stored in another

variable

• Example:

% set count = 5

% set number = $count

% echo count contains: $count

count contains: 5

% echo number contains: $number

number contains: 5

218

Unsetting a Variables

• We can clear a variable by assigning a null value to it - examples: set z = “ “; set y =

• Another method is to use the “unset” command

• Example:% set x = 1

% echo "(x contains:" $x")"

(x contains: 1)

% unset x

% echo "(x contains:" $x")"

x: Undefined variable




219

Command Substitution

• A command surrounded by backticks (` `) is

replaced by its standard output

• Any newline in the output are replaced by spaces

Demo

220

Command Sequence (;)

• Allows you to enter a series of commands all at once

• Commands are separated by a semicolon (;)

• Useful for typing an entire sequence of commands at once

Example: To execute these commands in sequence: date, pwd, and ls

% date;pwd;ls

Mon Jul 2 11:07:10 CDT 2003

/home/ux/krush/330/Summer-03/Quizzes

message.rickert quiz1-sum03 quiz2.sum03




221

C Shell History

• C shell keeps a record of previously entered cmds

so that they can later be:

– Re-executed

– Edited

• Entered cmds are stored in the current C shell’s

history buffer ($HOME/.history)

• Commands are saved

– Per session

– Per user

222

C Shell History

• Each previously run command gets a sequential

event number

• To view the history buffer:

– Syntax: history [-rh] [count]

• If no options are supplied, list all

– Useful options:

-r displays history list in reverse order

-h inhibits the display of event numbers




223

C Shell History

• You can re-execute history events:

1) By the event number

% !5

1) By the number relative to current event

% !-3

1) By the text it contains

% !ls

224

C Shell History

How big a list ?

1) On the command line, type:

% set history = 20 (per session only)

2) Include in your $HOME/.logout file:

set savehist = 30

At logout, C shell saves the last 30 cmds you executed in

the $HOME/.history file




225

C Shell History

Two ways to display event number as part of your cmd prompt:

1) On the command line, type:% set prompt = “\! %” (per session only)

1 % echo csci 330

csci 330

2 % _

2) Include in your $HOME/.cshrc file:set prompt = “\! %” (per each login)

226

C Shell History

For more listing of history commands, see Course Notes pp. 13-7

Command Action

!! Re-execute the last command

!N Invoke event N (integer)

!str Invoke most recent event beginning with str (string)

!?str? Invoke most recent event containing str

^str1^str2 Substitute str2 for str1 in the previous cmd




227

C Shell History: Words in Event

• Can use any word from any event on the history list

• C shell numbers the words in each command starting at 0

• Example:grep ‘^[0-9]\{5\}’ | sort –t: | cut –d: -f1

Word: 0 1 2 3 4 5 6 7 8

• Can easily identify “words” since space separate them

228

C Shell History: Words in Event

• Can refer to individual words with :n ( n is the word number)

• Example:100 % ls –l assign1 assign2 assign3

101 % chmod a-w !ls:2-$

chmod a-w assign1 assign2 assign3

102 % ls –l !ls:$

ls -l assign3

-r-------- 1 krush csci 0 Feb 14 12:35 assign3

• The modifier * refers to words 1 through last (I.e. everything except the command name)103 % rm !100:2*

rm assign1 assign2 assign3

$ refers to the last

word




229

C Shell History

Demo

230

Command Substitution

• A command surrounded by backticks (` `) is

replaced by its standard output

• Any newline in the output are replaced by spaces

Demo




231

C Shell Aliases

• Important feature of C shell

• Allows you to assign a name, or alias to

command(s)

• Can use alias like any other command

• Can rename existing commands

• Can reference shell variables

• Aliases defined at the parent shell’s command line

are not inherited by subshells

232

C Shell Aliases

• C shell user-defined abbreviation for a command

• Useful if a command syntax is difficult to

remember

• Define aliases at the command line or in

“$HOME/.cshrc” file

• Alternative: create a file name “.myalias” in your

$HOME and include all alias definitions.

– Include this line in your “$HOME/.cshrc” file:

source $HOME/.myalias




233

Creating Aliases

Use the command: alias

Syntax: alias name ‘command-list’

User-defined

nickname for the

command

•The command(s) that will be

executed when the alias is invoked

•Multiple commands are separated

by a semicolon

•Metacharacters are surrounded by

single quotes

234

Passing Arguments into Aliases

• C shell assumes all arguments come at the end of

the alias definition, unless you specified otherwise

• Two common argument destinations:

!* words 1 through the last word of the current

command/event

!^ word 1 of the current command/event

• Recall:

grep ‘^[0-9]\{5\}’ demand | sort –t: | cut –d: -f1

Word: 0 1 2 3 4 5 6 7 8 9




235

Example: Creating an Alias

% alias cd ‘cd \!*; set prompt = “$cwd >”’

Note:

• \ prevents the history mechanism from evaluating the !*

• !* represents the argument from the most recent current

command in the history list (i.e. words/fields 1 through the

last word of the recent command)

• \!* allows you to pass any number of arguments to the

alias

236

Issue a Command to the C Shell

• What happens when you issue a command to the C shell?

• After history substitution, examine each word that could be a command in the following sequential order:

1. Alias Substitution

2. Built-in command (commands inside the C shell; e.g. echo)

3. Commands in your search path ($PATH)




237

In-Class Exercise: aliases

1. Write an alias named “h” to list the history of

previously executed commands.

2. Write an alias named “c” to clear the contents of the

screen.

3. Write an alias named “rm” to prompt the user for a

delete confirmation message before removing file(s).

alias h history

alias c clear

alias rm ‘rm –i \!*’

238

In-Class Exercise: aliases

4. Write an alias named “cx” to give the owner the execute permission on more or more file/directory.

5. Write an alias named “loc” to locate/find any file in your directory hierarchy.

6. Write an alias named “ldir” to display long listing of only directory names (i.e. not ordinary files).

alias cx ‘chmod u+x \!*’

alias loc ‘find $home –name “\!:1” –print’

alias ldir ‘ls –l \!* | grep “^d”’

alias fdir ‘find \!:1 –type d –print’




239

Output

• The output statement of the C shell is the “echo” command

• Syntax: echo [option] arg1 arg2 …argN

• Its arguments can be strings or variables

• Example:

% set time = “2:00pm.”

% echo "It is now $time."

It is now 2:00pm.

% echo It is now $time.

It is now 2:00pm.

% echo 'It is now $time.'

It is now $time.

The variable cannot be

embedded in single

quotes.

240

Output Redirection (>)

• Syntax: command > file

• Sends output of command to file, instead of to

terminal

• Examples:

% du > status

% (date; du) > status

Calls the disk usage command for the

current directory and redirects the

output to a file called ‘status’

( ) indicates command groups. Use it

to combine the output of multiple

commands. In this example, we place

time and date in front of the disk

usage




241

Examples: Output Redirection (>)

% date; du > usage-status

Output of ‘date’ goes to? __________________

Output of ‘du’ goes to?___________________

% date > usage-status; du > usage-status

The file “usage-status” contains ?

___________________________________

242

Input Redirection (<)

• Syntax: Command < file

• Command will read (take input) from file, instead

of from terminal

• Example:

tr “[A-Z]” “[a-z]” < report.input




243

Examples: Output / Input

• Redirecting input and output:

% tr “[A-Z]” “[a-z]” < report.input > report.output

• Output of one command becomes input of next:

% ls > temp.txt; wc < temp.txt

• Eliminate the middleman: pipe

% ls | wc

244

Appending Output

• Syntax: command >> file

• Places the stdout of command (or command group) at the end of file

• If file does not exist, C shell creates it given that the variable “noclobber” is not set

• Examples:% date >> usage-status

% ls –l >> usage-status

% du –s >> usage-status

% date > usage-status

Build the file ‘usage-status’ from

the output of the ‘date’, ‘ls’, and

‘du’ commands




245

Using “set noclobber”

% who > current-users

% date >> usage-status

% set noclobber

– Does not allow output redirection if the file already exist

– Does not allow appending if the file does not already exist

% who > current-users

current-users: File exists

% date >> usage-statis

statis: No such file or directory

Specified incorrect

filename!

246

Override the Extra Protection

• Add ! To the redirection

• ! Means “do what I mean!”

• Example:

% set noclobber

% who >! current-users

Allows redirection even if ‘current-users’ exists

% date >>! usage-statis

Creates the file named “usage-statis” even if it does not already

exist




247

Handling Standard Errors

• Sometimes a command has special output to

inform you of problems; for example:

% gcc gets.c > compile.out

“gets.c”:8: syntax error

• Syntax: command >& file

– Redirect stdout and stderr to file

• Syntax: command >>& file

– Append stdout and stderr to file

248

Examples: Redirect/Append Stderr

% gcc gets.c >& compile.out

% cat compile.out

“gets.c”:8: syntax error

% (date; gcc gets.c –o gets) >>& compile-log

Add to the file ‘compile-log’ a running record with the

date and time of the compilation of the file gets.c




249

Separating Stdout and Stderr

% cat calendar logfile >& save– Saves both stdout and stderr in file ‘save’

% cat save

July 11 Planning Meeting

July 18Budget Meeting

Cat: can’t open logfile

% (cat calendar logfile > save) >& errfile– Redirects stdout to ‘save’

– Redirects stderr to ‘errfile’

– Order is important, > goes inside ( ), and >& goes outside

stdout

stderr

250

Sending Stdout and Stderr Through a Pipe

• Syntax: command1 |& command2

Sends stdout and stderr of command1 to be stdin to

command2

• Example:

% gcc gets.c |& more

– Sends stdout and stderr to ‘more’ for convenient

viewing

% gcc gets.c |& lpr –Plpcsl

– Produces a hardcopy of the messages




251

Summary: Redirections and Pipe

Command Syntax Meaning

command < file Redirect input from file to command

command > file Redirect output from command to file

command >& file Redirect output and errors to file

command >> file Redirect output of command and appends

it to file

command1 | command2 Take/pipe output of command1 as input

to command2

command1 |& command2 Take/pipe output and errors of

command1 as input to command2

Documents

Unix introduction