CS 497C – Introduction to UNIX Lecture 33: - Shell Programming

CS 497C – Introduction to UNIXLecture 33: - Shell Programming Chin-Chih Changchang@cs.twsu.edu

The if Conditional • The if statement takes two-way decisions depending on the fulfillment of a certain condition. • There are three forms of the if statement: • if condition then commands else commands fi • if condition then commands fi • if condition then commands elif condition then commands else commands fi $ emp3.sh firewall

The if Conditional $ emp3.sh mail $ emp3a.sh firewall $ emp3a.sh mail • We saw two forms of the if conditional – if-then-fi and if-then-else-fi. There’s a third form – if-then-elif-then-else-fi. $ cronfind.sh “find */”

Test and []: Companions of if • When you utilize if to evaluate expressions, the test statement is often used as its control command. • test ueses certain operators to evaluate the condition on its right and returns either a true or false exit status, test works in three ways: • Compare two numbers. • Compare two strings or a single one for null value. • Checks a file’s attributes.

Test and []: Companions of if • Test doesn’t display any output but simply returns a value that sets the parameter $?. • The numerical comparison operators used by test are: • -eq: equal to • -ne: not equal to • -gt: greater than • -ge: greater than or equal to • -lt: less than • -le: less than or equal to

Test and []: Companions of if $ cat arg_number_check.sh #!/bin/sh if test $# -ne 3 ; then echo "You didn't enter three arguments" else echo "You entered the right number" fi • The test statement has a shorthand – a pair of rectangular brackets.

Test and []: Companions of if • The following two forms are equivalent: test $x –eq $y [$x –eq $y] • Test can be used to compare strings with the following operations: • s1 = s2: true if s1 = s2 • s1 != s2: true if s1 is not equal to s2 • stg: true if stg is assigned and not null • -n stg: true if stg is not a null string

Test and []: Companions of if • -z stg: true if stg is a null string • s1 == s2: true if s1 = s2 (korn and bash only) • The compile.sh script is used to compile the last modified c or java programs. • test can be used to test various file attributes (Page 554 Table 18.4): • -f fname: fname exists and is a regular file. • -f fname: finame exists and is readable. • -w fname: fname exists and is writable.

Test and []: Companions of if #!/bin/sh if [ $# -eq 1 ] ; then if [ $1 = "j" ] ; then file=`ls -t *.java | head -1` javac $file elif [ $1 = "c" ] ; then file=`ls -t *.c | head -1` cc $file && a.out else echo "Invalid file type" fi else echo "Usage: $0 file_type\nValid file types are c and j" fi

Test and []: Companions of if • -x fname: fname exists and is executable. • -d fname: finame exists and is a directory. • The ! negates a test, so [! –w file] negates [-w file]. • Check the file filetest.sh. $ filetest.sh emp3.lst $ filetest.sh emp.lst

The case Conditional • case is a compact string-matching construct and is closed with esac. • It uses the shell’s wild cards to match multiple patterns in egrep-style. • The * when used as the last option matches everything not matched by the previous options. The wild cards match strings and not files. • case is specially suitable for matching the filename $0.

The case Conditional • Here is its syntax: case expression in pattern1) commands1;; pattern2) commands2;; …. esac • case first matches expression with pattern1. If the match succeeds, then it executes commands1. Otherwise, then go to pattern2.

The case Conditional #!/bin/sh tput clear echo "\n 1. Find files modified in last 24 hours\n 2. The free disk space\n 3. Space consumed by this user\n 4. Exit\n\n SELECTION: \c" read choice case $choice in 1) find $HOME -mtime -1 -print ;; 2) df ;; 3) du -s $HOME ;; 4) exit ;; *) echo "Invalid option" esac

Expr: Computation and String Handling • expr is used for integer computation and string manipulation. It is used with the Bourne shell for incrementing the value of a variable. • This command combines two functions in one: • Performs arithmetic operations on integers. • Manipulates strings.

Expr: Computation and String Handling • The + (add), - (subtract), * (multiply), / (divide), and % (divide and truncate the decimal portion). $ x=3; y=5; expr $x + $y; • expr uses regular expressions to extract a substring, locate the position of a character, and evaluate the length of a string. • Korn and bash shells don’t need expr. • sleep specifies the number of seconds for which the shell will pause.

While and until: Looping • The while loop executes its body as long as the control command returns a true value. • It is used in scripts that repeatedly increment the value of a variable or provide multiple chances to a user. • You can set up an infinite loop using true as the control command. • The until loop complements while.

While and until: Looping • The syntax of the while command is: while condition is true do commands done while [! –r invoice.lst] do sleep 60 done

While and until: Looping until [–r invoice.lst] do sleep 60 done • for works with each element of a list at a time. The list can be generated by variables, wild cards, positional parameters and command substitution.

for: Looping with a List • The syntax of this construct is as follows: for variable in list; do commands done for file in chap*; do cp $file ${file}.bak echo $file copied to $file.bak done

for: Looping with a List for file in *.c; do cc -o ${file}.o $file done • All loops use the keywords do and done. • The break statement terminates a loop, while continue starts the next iteration.

CS 497C – Introduction to UNIX Lecture 33: - Shell Programming