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Flow-Control Structures in Korn Shell Programming
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The Korn shell provides several flow-control structures similar to the ones found in regular programming languages, such as C or Java. These include the conditional structures that use if statements and the iterative structures that use while and for statements to loop through several steps based on certain conditions being satisfied. Besides these flow-control structures, you can use special commands to interrupt or get out of loops when necessary.
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Branching constructs let you instruct the shell program to perform alternative tasks based on whether a certain condition is true or not. For example, you can tell the program to execute a particular command if a certain file exists and to issue an error message if it doesn t. The most common form of conditional branching in all types of programming is the if-thenelse conditional structure. In UNIX and Linux programming, this conditional structure has the syntax if-then-else-fi. This conditional structure will perform one of two or more actions, depending on the results of a test.
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CHAPTER 3 ES SEN TIAL UN IX (AN D LINUX) FOR THE ORA CLE DBA
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The syntax for the if-then-else-fi structure is as follows: if condition then Action a else Action b fi Make sure that the then is on the second line. Also, notice that the control structure ends in fi (which is if spelled backwards). Here s an example of the if-then-else-fi structure: #!/usr/bin/sh LOGFILE= /tmp/dba/error.log export LOGFILE grep ORA- $LOGFILE > job.err if [ `cat job.err|wc -l` -gt 0 ] then mailx -s "Backup Job Errors" salapati@netbsa.org < job.err else mailx -s " Backup Job Completed Successfully" salapati@netbsa.org fi This script checks to see whether there are any errors in an Oracle backup job log. The script uses the mailx program, a UNIX-based mail utility, to send mail to the DBA. The -s option of the mailx utility specifies the subject line for the e-mail. The contents of the job.err file will be sent as the output of the e-mail.
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In real-world programming, you may want to execute a command several times based on some condition. UNIX provides several loop constructs to enable this, the main ones being the while-dodone loop, which executes a command while a condition is true; the for-do-done loop, which executes a command a set number of times; and the until-do-done loop, which performs the same command until some condition becomes true. The next sections examine these three loop structures in more detail.
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A while-do-done Loop
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The while-do-done loop tests a condition each time before executing the commands within the loop. If the test is successful, the commands are executed. If the test is never successful, the commands aren t executed even once. Thus, the loop ensures that the commands inside the loop get executed while a certain condition remains true. Here s the syntax for the while-do-done loop: while condition do commands done In the following example of the while-do-done loop, note that the command inside the loop executes 99 times (the lt relation ensures that as long as the value of the variable VAR1 is less than 100, the script will echo the value of the variable):
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CH APT ER 3 ES SEN TI AL UN IX (A ND LINU X) F O R TH E O RA CLE D BA
#!/usr/bin/ksh VAR1=1 while [ $VAR1 -lt 100 ] do echo "value of VAR1 is: $VAR1" ((VAR1 =VAR1+1)) done
A for-do-done Loop
You can use the for-do-done loop when you have to process a list of items. For each item in the list, the loop executes the commands within it. Processing will continue until the list elements are exhausted. The syntax of the for-do-done loop is as follows: for var in list do commands done Here s an example of a for-do-done loop (the for command uses the letter F as a variable to process the list of files in a directory): #!/usr/bin/sh ## this loop gives you a list of all files (not directories) ## in a specified directory. for F in /u01/app/oracle do if [ -f $F] then ls $F fi done
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