1 / 38

Variables in UNIX

Variables in UNIX. How to run a shell program. Shell program can be run from: terminal prompt executable files which has the shell program Any file can be executable by changing the permission chmod + file(s)

shakti
Download Presentation

Variables in UNIX

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Variables in UNIX

  2. How to run a shell program • Shell program can be run from: • terminal prompt • executable files which has the shell program • Any file can be executable by changing the permission chmod + file(s) • The executable file may have many lines, where every line has a correct shell program

  3. How to run a shell program(continue.) • If any of the lines (shell program) has error (i.e. syntax), this will not prevent other lines from being executed • Whenever the shell executes the special chracter # at the start of a word, it takes whatever characters follow the # to the end of the line as comment

  4. How to run a shell program(continue.) $ who | wc -l 11 $ cat > nu who | wc -l $ chmod +x nu $ ls -l nu -rwxr-xr-x 1 abuzneid 534 12 Oct 11 00:41 nu $ nu 11 $ nu >nufile $ cat nufile 11 $

  5. How to run a shell program(continue.) $ cat > userstate #This is userstate script echo The current date and time is: date #To print the date echo echo The number of users on the system is: who | wc -l echo echo Your current working directory is: pwd $ chmod +x userstate $ userstate The current date and time is: Thu Oct 11 00:52:46 EDT 2001 The number of users on the system is: 11 Your current working directory is: /home/abuzneid/UNIX $

  6. Variables • A shell variable begins with alphabetic or underscore "_" character, and is followed by zero or more alphabetic or underscore characters • variable=value • Must not be space before or after "=" • Example: my_bin=/usr/local/bin • Shell does not have the concept of data types

  7. Displaying the value of variables • If a valid variable name followed the $, then the shell takes this as an indication that the value stored inside that variable is to be substituted of that point • Variables can be assigned to: • numbers • alphanumeric chracters/strings • other variables • utilities or programs • null values • file names

  8. Displaying the value of variables(continue.) $ num=10 $ dir=/usr/local/bin $ echo $num 10 $ echo num num $ echo $dir /usr/local/bin $ echo dir dir $ echo $dir $num /usr/local/bin 10 $

  9. Displaying the value of variables(continue.) arguments echo 10 arguments echo /usr/local/bin 10

  10. Displaying the value of variables(continue.) $ my_UNIX_dir=/home/abuzneid/UNIX $ $my_UNIX_dir /home/abuzneid/UNIX: cannot execute $ command=who $ $command am i abuzneid pts/7 Oct 11 01:25 (216.87.102.220) $ nu='who | wc -l' $ $nu bgeorge pts/16 Oct 5 15:01 (216.87.102.204) msiles pts/3 Sep 27 17:44 (216.87.102.201) abakshi pts/7 Oct 11 01:25 (216.87.102.220) tphilip pts/11 Oct 2 14:10 (AC8C6085.ipt.aol.com) abuzneid pts/9 Oct 10 19:29 (avicenna.102.87.216.in-addr.arpa) $

  11. Displaying the value of variables(continue.) $ value1=10 $ value2=value1 $ echo $value2 value1 $ value2=$value1 $ echo $value2 10 $

  12. The null value • To set a variable to null • variable= • variable="" (double quote) • variable='‘ (single quote) • variable=" " with a space between the quotes is not a null variable (space is a character)

  13. The null value (continue.) $ echo $null_variable $ echo :$null_variable: :: $ nosuch= $ echo :$nosuch: :: $

  14. File name substitution as variables $ ls Documents foo mail personal TEST test $ x=* $ echo $x Documents TEST foo mail personal test $

  15. File name substitution as variables(continue.) $x * args ison memos phonebook . . . students arguments echo

  16. The$(variable) construct • To delimit the end of the variable name, enclose the entire name (but not the leading dollar sign) in a pair of curly braces • Example: ${filename} • Example: to rename a file from "myfile" to "myfileX"

  17. Quote Characters in UNIX

  18. Quotation

  19. The single Quote • When the shell sees the first single quote, it ignores any otherwise special characters that follow until it sees the closing quote $ cat phonebook Edward John 336-1454 Alice Stuart 334-1219 Sony Bell 332-3364 Robert Micheal 326-0569 $ grep Alice phonebook Alice Stuart 334-1219 $ grep Sony phonebook Sony Bell 332-3364 $ grep Sony Bell phonebook grep: can't open Bell phonebook:Sony Bell 332-3364 $ grep 'Sony Bell' phonebook Sony Bell 332-3364 $

  20. The single Quote(continue.) • grep Chen Tao phonebook • grep 'Chen Tao' phonebook arguments Chen Tao phonebook echo arguments Chen Tao phonebook echo

  21. The single Quote(continue.) • Single quotes preserves all white spaces $ echo one Two Three one Two Three $ echo 'one Two three' one Two three $ echo 'How are you doing, > today' How are you doing, today $

  22. The single Quote (continue.) • Quotes are needed when assigning values containing white space or special characters to shell variables $ msg='I love New York' $ echo $msg I love New York $

  23. The single Quote (continue.) • Shell does file substitution after variable name substitution even in between single quotes $ msg='* are my files' $ echo $msg Documents TEST foo list mail personal test are my files $

  24. The double quote • Three characters are not ignored inside double quotes: • Dollar sign • Back quotes • Backslashes • File names substitution (*,?) is not done inside double quotes

  25. The double quote(continue.) $ X=* $ echo $X Documents TEST foo list mail personal test $ echo '$X' $X $ echo "$X" * $

  26. The double quote(continue.) $ address="169 University Ave > Bridgeport, CT 06601" $ echo $address 169 University Ave Bridgeport, CT 06601 $ echo "$address" 169 University Ave Bridgeport, CT 06601 $

  27. The double quote (continue.) • Double quotes can be used to hide single quotes from the shell, and vice versa $ X="Hello, 'he said'" $ echo $X Hello, 'he said' $ Y='"he was here",Tom said' $ echo $Y "he was here",Tom said $

  28. The Backslash • Equivalent to placing single quotes around a single character $ echo > syntax error: `newline or ;' unexpected $ echo \> > $ X=* $ echo \$X $X $ echo \\ \ $

  29. The$(variable) construct $ myfile=foo $ mv $myfile $myfileX mv: Insufficient arguments (1) Usage: mv [-f] [-i] f1 f2 mv [-f] [-i] f1 ... fn d1 mv [-f] [-i] d1 d2 $ mv $myfile ${myfile}X $

  30. Passing Arguments

  31. Passing arguments • Whenever you execute a shell program, the shell automatically stores the first argument in the special shell variable #1, the second argument in the variable #2, and so on. [$1,$2..$9] • These special variables are known as positional parameters

  32. Passing arguments (continue.) $ cat > ison who | grep $1 $ who bgeorge pts/16 Oct 5 15:01 (216.87.102.204) abuzneid pts/17 Oct 11 01:17 (216.87.102.224) abuzneid pts/7 Oct 11 01:25 (216.87.102.220) Steve pts/9 Oct 10 19:29 (avicenna.102.87.216.in-addr.arpa) $ chmod +x ison $ ison abuzneid abuzneid pts/17 Oct 11 01:17 (216.87.102.224) abuzneid pts/7 Oct 11 01:25 (216.87.102.220) $ ison bgeorge bgeorge pts/16 Oct 5 15:01 (216.87.102.204) $

  33. The $# variable • $# is set to the number of arguments that were typed on the command line $ cat >args echo $# arguments passed echo arg1=:$1: arg2=:$2: arg3=:$3: $ chmod +x args $ args a b c 3 arguments passed arg1=:a: arg2=:b: arg3=:c: $ args a b 2 arguments passed arg1=:a: arg2=:b: arg3=:: $ args 'ls' 1 arguments passed arg1=:ls: arg2=:: arg3=:: $

  34. The $* variable • $* references all of the arguments passed to the program $ cat >args2 echo $# arguments passed echo they are :$*: $ chmod +x args2 $ args2 a b c 3 arguments passed they are :a b c: $ args2 * 10 arguments passed they are :Documents TEST args args2 fooX ison list mail personal test: $

  35. The shift command • Maximum number of variables can be referenced by the shell is 9. Shell accepts single digit following the dollar sign • The shift command allows you to effectively left shift your positional parameters • I. e. after shift is executed, whatever was previously stored inside $2 will be assigned to $1, and whatever was stored in $3 will be assigned to $2, and so on.

  36. The shift command (continue.) • The old value of $1 will be irretrievably lost • Whenever shift is executed, $# is also automatically decremented by one

  37. The shift command (continue.) • Naturally, you should first save the value of $1 before shift, if the value is needed later in the program arg1=$1 shift arg10=$9 • The shift command is very useful when processing a variable number of arguments

  38. References • UNIX SHELLS BY EXAMPLE BY ELLIE QUIGLEY • UNIX FOR PROGRAMMERS AND USERS BY G. GLASS AND K ABLES • UNIX SHELL PROGRAMMING BY S. KOCHAN AND P. WOOD

More Related