Copyright Sams Teach Yourself Shell Programming in 24 Hours

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Sams Teach Yourself Shell Programming in 24 Hours Contents Index Hour 2: Script Basics Sections in this Chapter: Previous Chapter Next Chapter The UNIX System Summary Shell Initialization Questions Getting Help Terms Previous Section Next Section Hour 2 Script Basics In Chapter 1, "Shell Basics," I introduced the concept of a shell and commands. I showed you how the shell reads your input and executes the command you requested. In this chapter I will explain in detail what the shell is and how it works. You will learn how the shell is started during the login process and what happens when you log out. After I explain this behavior, I will show you how to group commands that are normally executed interactively into a file to create a script. Scripts are the power behind the shell because they enable you to group commands together to create new commands. The UNIX System Logging In The UNIX system consists of two components: ● Utilities ● The kernel Utilities are programs you can run or execute. The programs who and date that you saw in the previous chapter are examples of utilities. Almost every program that you know is considered a utility. Commands are slightly different than utilities. The term utility refers to the name of a program, whereas the term command refers to the program and any arguments you specify to that program to change its behavior. You might see the term command used instead of the term utility for simple commands, where only the program name to execute is given. The kernel is the heart of the UNIX system. It provides utilities with a means of accessing a machine's hardware. It also handles the scheduling and execution of commands. When a machine is turned off, both the kernel and the utilities are stored on the machine's hard disks. But when the computer is booted, the kernel is loaded from disk into memory. The kernel remains in memory until the machine is turned off. Utilities, on the other hand, are stored on disk and loaded into memory only when they are executed. For
example, when you execute the command $ who the kernel loads the who command from the machine's hard disk, places it in memory, and executes it. When the program finishes executing, it remains in the machine's memory for a short period of time before it is removed. This enables frequently used commands to execute faster. Consider what happens when you execute the date command three times: $ date Sun Dec 27 09:42:37 PST 1998 $ date Sun Dec 27 09:42:38 PST 1998 $ date Sun Dec 27 09:42:39 PST 1998 The first time the date command can be loaded from the machine's hard disk, but the second and third time the date command usually remains in the machine's memory allowing it to execute faster. The shell is a program similar to the who command. The main difference is that the shell is loaded into memory when you log in. Logging In When you first connect to a UNIX system, you usually see a prompt such as the following: login: You need to enter your username at this prompt. After you enter your username, another prompt is presented: login: ranga Password: You need to enter your password at this prompt. These two prompts are presented by a program called getty. These are its tasks: 1. Display the prompt login. 2. Wait for a user to type a username. 3. After a username has been entered, display the password prompt. 4. Wait for a user to enter a password. 5. Give the username and password entered by the user to the login command and exit.
Page 1 and 2: Sams Teach Yourself Shell Programmi
Page 3 and 4: ● Using Single Quotes ● Using D
Page 5 and 6: ● Questions Hour 20: Debugging
Page 9 and 10: ● Part I is an introduction to UN
Page 11 and 12: Many of the examples and the answer
Page 17 and 18: $ date [ENTER] Wed Dec 9 08:49:13 P
Page 19 and 20: anga pts/0 Dec 9 08:49 $ Here, the
Page 23 and 24: The original UNIX shell was written
Page 25 and 26: There are three major versions of k
Page 29: Sams Teach Yourself Shell Programmi
Page 35 and 36: This executes the compound command
Page 37 and 38: chmod a+x ./logins Here you are usi
Page 41 and 42: Try using the man command to get mo
Page 49 and 50: Hidden Files So far you have used l
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Page 53 and 54: Numbering Lines The cat command als
Page 55 and 56: command $ wc -w .rhosts produces th
Page 59 and 60: produces the following error: cp: w
Page 61 and 62: m: work directory The two files are
Page 67 and 68: name. Thus both of the following fi
Page 69 and 70: ../docs/ch5.doc represents the file
Page 71 and 72: changes to the directory /usr/local
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Page 79 and 80: $ cp -r docs /mnt/zip/backup cp: ca
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Removing Directories You can use tw
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Sams Teach Yourself Shell Programmi
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$ ls -ld /home/ranga This produces
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You can see the relative path by us
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Sections in this Chapter: File Type
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The permission block for this file
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Here, expression is a statement of
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For example, if the directory pub c
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Restrictions The super user, root,
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Running the same ls command as in t
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Tip - Background Processes That Wri
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the fg command. By default, the pro
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dhorvath 6770 3662 2 10:35:45 pts/6
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By default, a process runs with the
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Process Identifier --The process id
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Variable Names The name of a variab
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Array Variables The Bourne shell, s
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$ band[2]=mike $ band[3]=gene Tip -
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Hour 7: Variables Sections in this
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uses internally. Users can modify t
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Here, command is the name of a comm
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$ ls ch??.doc to accomplish this. M
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© Copyright Macmillan Computer Pub
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is unset. Assigning a Default Value
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© Copyright Macmillan Computer Pub
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It now gives this error result: Hel
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Sections in this Chapter: Quoting w
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To the shell, one or more spaces or
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Quoting Regular Expression Wildcard
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fi list5 Both the elif and the else
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Here you have an elif statement tha
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Consider the following if statement
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If you were to use the - n option i
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else echo "An error was encountered
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1. First the test [ -z "$DTHOME" ]
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esac Here the string contained in $
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The execution of a while loop proce
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5 4 3 2 1 0 6 5 4 3 2 1 0 7 6 5 4 3
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2. If the exit status of command is
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for i in 0 1 2 3 4 5 6 7 8 9 do ech
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3. When the user enters a value, $R
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The continue Command The continue c
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If you want to list the contents of
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In this version, the basename comma
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USAGE="Usage: ´basename $0´ [-c|-
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uuencoding a file. For readers who
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fi fi echo $MSG You could simplify
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● Formatting escape sequences Emb
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Name User Name Sriranga ranga Sriva
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Table 13.2 Formatting Sequence Type
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printf "%-32s " "$i" if [ -d "$i" ]
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anga ttyp0 Oct 27 19:42 (:0.0) amma
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Say you need to send a list of phon
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"Miscellaneous Tools," I will show
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$ exec 4>fd4.out associates the fil
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command > file 2>&1 list > file 2>&
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when the while loop exits, the valu
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Note - The shell uses the value of
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This function has three important p
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} OLDIFS="$IFS" IFS=: # print each
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} # set the directory to pop to the
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index.html RCS humor misc downloads
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unix has several additional pieces
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pqops.c:/* Function to delete a nod
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Here I specified set2 as the space
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$ tr '!?":;\[\]{}(),.\t\n' ' ' < /h
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in 69 ' 65 the 389 to 164 of 127 No
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tr '[:classname:]' 'set2' Here clas
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single letters, whereas in awk the
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able was I, ere I saw elba Because
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By adding the ^ metacharacter as fo
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\$[0-9]*\.[0-9][0-9]\/[a-zA-Z]*/ If
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Here you list the name of a fruit a
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$ rm fruit_prices.txt.$$ Performing
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Fortunately, the s command provides
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lrwxrwxrwx 1 ranga users 8 Nov 27 1
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The original awk has remained almos
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Kiwi 22 Pineapple 35 Apple 78 All t
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Pineapple $1.29 35 REORDER Apple $0
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$ ls -l produces output formatted s
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The value that you assign a variabl
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x=x+1 In awk this can be written in
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the pattern specifies. To solve you
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awk -F: '{ ... }' The major differe
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identical to expressions you looked
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The basic syntax is do { actions }
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B:amount Here amount is the balance
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The OUTPUT variable contains the >
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: ${LINES:=24} ${TERM:?"TERM not se
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Sections in this Chapter: The eval
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eports/1998 The starting directory.
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find: -type Option The -type option
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find / -name alpha -size +50 -mtime
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18 quit $ obase=16 sets the output
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The default action for the signals
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fi Init() { printf "INFO: Initializ
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4. Wait for the program to finish e
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} if [ $? -eq 0 ] ; then echo "Sub-
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A third way of enabling debugging i
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Here the debugging mode is enabled
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if [ $YN = "y" ] ; then Making this
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$ /bin/sh -nv ./buggy2.sh the outpu
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Deleting old backups, please wait..
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If you do not type a response and p
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To deactivate debugging, use either
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3 MSG="hello" 4 echo_error $MSG Her
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echo "ERROR: File $MYFILE was not f
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previous call to this function. Aft
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} Before you look at some examples
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dev/hda1 1190014 664661 463867 59%
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The number returned is in kilobytes
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getUID() { id $1 | sed -e 's/(.*$//
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63 printf "$1 (y/n)? " 64 65 if [ -
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}' 171 # Args: $1 -> the command na
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Some UNIX versions implement a work
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24 printERROR "The source $1 is not
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easily modify the script to replace
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Hour 22: Problem Solving with Shell
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To maintain your address book, you
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produces two matches: Name: Srirang
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26 # enable erasing input 27 28 stt
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2. Based on user feedback, delete t
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73 promptYESNO "Delete this entry"
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MYADDRESSBOOK="$HOME/addressbook" i
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Here, options is one or more of the
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Now that you have looked at using t
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Abstraction is a technique used to
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warning: '-' deprecated; use 'ps au
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produce the same result if the curr
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The structure of the UNIX directory
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OLDPREFIX=aaa NEWPREFIX=bbb for FIL
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history (Korn/Bash) shows the most
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Sections in this Chapter: Reserved
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^ bitwise exclusive OR operation |
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Bash array initialization USERVAR=(
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order to fix problems. default beha
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minor number UNIX uses this to asso
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important parameters. This is usual
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Sections in this Chapter: Chapter 1
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USAGE="Usage: ´basename $0´ [-v]
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Appendix C: Quiz Answers Sections i
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awk -F: ' $1 == "B" { BAL=$NF ; nex
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} if [ $DEF_TOUT -ne 0 ] ; then sle
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Sections in this Chapter: Chapter 1
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} fi if [ ! -d "$1" ] ; then printE
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numeric variables, 1st pattern-spec
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wildcards, 1st Bourne shells. See s
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(colon), 1st character, 1st, 2nd, 3
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grep, 1st, 2nd line numbers, 1st li
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stty addperson script, 1st tail, 1s
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emoving temporary files, 1st displa
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mv command, 1st, 2nd rmdir command,
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interactive mode, 1st multiple inpu
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starting directory, 1st, 2nd findin
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installation scripts disk usage, 1s
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eak command, 1st, 2nd, 3rd continue
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mvdir.sh, 1st, 2nd, 3rd, 4th, 5th,
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edirection, 1st, 2nd appending to f
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[0-9]*, 1st percent sign (%) job nu
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profiles shell specific startup wit
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hardware type, 1st, 2nd improving,
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(standard error), 1st displaying me
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prompt, 1st, 2nd, 3rd, 4th quoting
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egp command, 1st -k option sort com
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Index A B C D E F G H I J K L M N O
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