By Ron Peters
Process directors desire libraries of ideas which are inventive yet comprehensible. They don't are looking to reinvent the wheel, yet they don't are looking to reinvent filesystem administration both! specialist Shell Scripting is the final word source for all operating Linux, Unix, and OS X approach directors who want to have brief, succinct, and robust shell implementations of tough procedure scripting tasks.
* Automating small to medium approach administration tasks
* examining process facts and modifying configuration files
* Scripting Linux, Unix, and OS X functions utilizing bash, ksh, et al.
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Extra resources for Expert Shell Scripting
The option you choose will depend on your motives. Many times I have used the first type of code because scripts I’ve written need to be simple to read and support by others with varying 37 38 CHAPTER 6 ■ TESTING VARIABLES AND ASSIGNING DEFAULTS levels of shell-scripting skill. I may want to use the more terse code types if supportability is of less concern than efficiency is. The first code example checks to see if a variable (VAR) has been set. The -z (zero) test checks to see if the length of the string variable is zero.
The other conditional operators mirror this distinction. =, >, and < used for string comparisons are direct counterparts to the numeric comparison operators -eq, -ne, -gt, and -lt. There are still more operators that can be found in both the test man page and the man pages for specific shells. The square brackets in the previous examples contain the enclosed expressions to create a statement that represents the logical result (true or false) of the test described by the enclosed expressions. To be more accurate, the left square bracket ([) is both a command internal to the shell and a system command (/usr/bin/[) that can be thought of the same as the test command.
You can find an example of this in Chapter 36 in connection with password aging. Converting all temporal quantities to elapsed time also reduces the complexity of making time comparisons. Suppose, for example, that you would like to monitor time synchronization between multiple network nodes. When you convert the time on a 19 20 CHAPTER 3 ■ DATE AND TIME MANIPULATION system to seconds elapsed since the beginning of the UNIX epoch, the calculation becomes a simple subtraction. ■Caution Yes, the Network Time Protocol (NTP) keeps system clocks in sync.