Copyright Sams Teach Yourself Shell Programming in 24 Hours

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$0 The name of the command being executed. For shell scripts, this is the path with which it was invoked. $n These variables correspond to the arguments with which a script was invoked. Here n is a positive decimal number corresponding to the position of an argument (the first argument is $1, the second argument is $2, and so on). $# The number of arguments supplied to a script. $* All the arguments are double quoted. If a script receives two arguments, $* is equivalent to $1 $2. $@ All the arguments are individually double quoted. If a script receives two arguments, $@ is equivalent to $1 $2. $? The exit status of the last command executed. $$ The process number of the current shell. For shell scripts, this is the process ID under which they are executing. $! The process number of the last background command. Using $0 Start by looking at $0. This variable is commonly used to determine the behavior of scripts that can be invoked with more than one name. Consider the following script: #!/bin/sh case $0 in *listtar) TARGS="-tvf $1" ;; *maketar) TARGS="-cvf $1.tar $1" ;; esac tar $TARGS You can use this script to list the contents of a tar file (t as in tape and ar as in archive, a common format for distributing files in UNIX) or to create a tar file based on the name with which the script is invoked. The tar file to read or create is specified as the first argument, $1. I called this script mytar and made two symbolic links to it called listtar and maketar as follows: $ ln -s mytar listtar $ ln -s mytar maketar If the script is invoked with the name maketar and is given a directory or filename, a tar file is created. If you had a directory called fruits with the following contents $ ls fruits apple banana mango peach pear you can invoke the script as maketar to obtain a tar file called fruit.tar containing this directory, by issuing the following command: $ ./maketar fruits
If you want to list the contents of this tar file, you can invoke the script as follows: $ ./listtar fruits.tar This gives us the following output: rwxr-xr-x 500/100 0 Nov 17 08:48 1998 fruits/ rw-r--r-- 500/100 0 Nov 17 08:48 1998 fruits/apple rw-r--r-- 500/100 0 Nov 17 08:48 1998 fruits/banana rw-r--r-- 500/100 0 Nov 17 08:48 1998 fruits/mango rw-r--r-- 500/100 0 Nov 17 08:48 1998 fruits/pear rw-r--r-- 500/100 0 Nov 17 08:48 1998 fruits/peach For this example, the output that you encounter depends on the version of tar that is installed on your machine. Some versions include more detail in the output than are shown here. Usage Statements Another common use for $0 is in the usage statement for a script, which is a short message informing the user how to invoke the script properly. All scripts used by more than one user should include such a message. In general, the usage statement is something like the following: echo "Usage: $0 [options][files]" Simplifying Script Maintenance Using $0 as I've illustrated is encountered in the install and the uninstall scripts of some software packages. Because these scripts share many of the same routines and global variables, it is desirable, for ease of maintenance, to merge them into a single script having different behavior depending on the name with which it is invoked. If you are writing scripts that need to share main routines, consider using such a scheme to simplify maintenance. If you consider the mytar script given previously, a usage statement would be a helpful addition, in case the script was called with some name other than the two names it knows about. To implement this, change the case statement as follows: case $0 in *listtar) TARGS="-tvf $1" ;; *maketar) TARGS="-cvf $1.tar $1" ;; *) echo "Usage: $0 [file|directory]" exit 0 ;;
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Sams Teach Yourself Shell Programmi
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● Using Single Quotes ● Using D
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● Questions Hour 20: Debugging
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● Part I is an introduction to UN
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Many of the examples and the answer
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$ date [ENTER] Wed Dec 9 08:49:13 P
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anga pts/0 Dec 9 08:49 $ Here, the
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The original UNIX shell was written
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There are three major versions of k
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example, when you execute the comma
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This executes the compound command
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chmod a+x ./logins Here you are usi
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Try using the man command to get mo
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Hidden Files So far you have used l
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© Copyright Macmillan Computer Pub
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Numbering Lines The cat command als
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command $ wc -w .rhosts produces th
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produces the following error: cp: w
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m: work directory The two files are
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name. Thus both of the following fi
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../docs/ch5.doc represents the file
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changes to the directory /usr/local
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If you specify several arguments in
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Sometimes when you want to create a
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$ cp -r docs /mnt/zip/backup cp: ca
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Removing Directories You can use tw
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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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is unset. Assigning a Default Value
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Page 227 and 228: Table 13.2 Formatting Sequence Type
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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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