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CS 497C – Introduction to UNIX Lecture 34: - Shell Programming Chin-Chih Chang

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1 CS 497C – Introduction to UNIX Lecture 34: - Shell Programming Chin-Chih Chang

2 Sample Scripts The script shown in Fig. 18.1 protects your files from accidental overwritting by the cp command. The script shown in Fig. 18.2 accepts a code and its corresponding description from the terminal, performs some rudimentary validation checks, and then adds an entry to a file desig.lst. The continue statements let you reenter the data or start a fresh cycle.

3 Sample Scripts The break statement in the inner loop quits the loop after adding the line. You should use “ $@ ” with a for loop when using multiword arguments. for file in “$@” basename is a command to strip directory and suffix from filenames $ basename hello

4 Sample Scripts You can use basename inside a for loop to change the extensions of files. for file in *.txt ; do leftname=`basename $file.txt` mv $file ${leftname}.doc done The script accepts multiple filenames and backup them without overwritting.

5 The Here Document (<<) The here document (<<) provides input to a script from the script itself. It can be used with both command substitution and variables. It is often used with commands that don’t use a filename as argument or for running interactive programs noninteractively. mail cs497c << EOF Test Date is `date`. EOF

6 let: Computation – A Second Look (ksh and bash) Some advanced features of shell programming only happen in Korn and bash shell. Korn shell is located in /usr/bin/ksh. Bash shell is located in /bin/bash. You can compute with let statement: $ let sum=256+128; echo $sum There is no space after variable. If you need space, just quote the expression:

7 let: Computation $ let sum=“3 * 6 + 4 / 2”; echo $sum $ x=22 y=28 z=5; z=$((x+y+z)); echo $z When a process is created by the shell, it makes certain features of its own environment to the child. The created process make use of the inherited parameters including: –The PID of the parent process. –The UID (owner) and GUID (group owner) of the process.

8 Sub-Shells –The current working directory. –The three standard files. –Other open files used by the parent process. –Some environment variables available in the parent process. A variable defined in the parent is visible in the child only when it is exported (export). However, when the child alters the value of the variable, the changed value is not seen by the parent.

9 Sub-Shells & Arrays The matching operators () run a group of commands in a sub-shell, but the {} don’t spawn one. Korn and bash support one-dimensional arrays where the first element has the index 0. $ prompt[2]=“Enter your name” $ echo ${prompt[2]}

10 Arrays You can use a space-delimited list enclosed with parentheses: month_arr=(0 31 28 31 30 31 30 31 31 30 31 30 31) $ echo ${month_arr[6]} In the older version of Korn, you can use the set –A statement: set –A mon_arr 0 31 28 31 30 31 30 31 31 30 31 30 31 Using the @ or * as subscript, you an display all the elements of the array as well as the number of elements.

11 Arrays & String Handling $ echo ${month_arr[@]} $ echo ${#month_arr[@]} The script in Fig. 19.2 use arrays to validate an entered date. The length of string can be found by preceding the variable name with a #. $ name=“Jay Leno”; echo ${#name} You can extract a substring: $ echo ${name:4:4}

12 Conditional Parameter Substitution Shell variables can be evaluated in a conditional manner depending on whether they are assigned a non-empty value. This is called parameter substitution with the following format: ${variable:symbol string} where symbol can be +, -, = or ?. When the + option is used, variable is evaluated to string if it has a non-null value.

13 Conditional Parameter Substitution found=`ls`; echo ${found:+“Files found."} When - option is used, variable is evaluated to string if it has a null value. The = operator additionally assigns a value to the variable. while [${x:=1} -le 10] The ? option prints an error message and exits the shell. grep $pattern ${flname:?”No file.. quitting”}

14 Shell Functions A shell function consists of a group of statements that are executed together as a bunch. Optionally, it also returns a value: function_name() { statements return value } displays some functions.

15 Shell Functions anymore() { echo $option "\n$1 ?(y/n) : \c" 1>&2 read response case "$response" in y|Y) echo 1>&2 ; return 0 ;; *) return 1 ;; esac } $ anymore “Wish to continue”

16 eval & exec eval processes a command line twice and is used to simulate arrays and execute. With eval, you can create generalized numbered prompts and variables that significantly compact code. $ prompt1=“User Name:” $ x=1; eval echo \$prompt$x exec overlays the current shell when prefixed to a command.

17 exec & trap $ exec date To debug shell scripts, use set –x at the beginning of the script so that every command line is echoed to the screen. Use trap if you want your script to respond to an interrupt in a specific way. trap ‘command_list’ signal_list It is useful in removing temporary files when a script receives a signal.

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