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CHAPTER 8: Automating Administrative Tasks
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In order to be properly processed by a shell, a Unix executable script must specify which interpreter the shell should use to parse and execute its contained shell code. This information is provided via a hash-bang or shbang (#!) specifier, which should always be at line 1 of the script and should precede the absolute path to the file s interpreter. For instance, in this chapter we are primarily utilizing bash scripts. To specify the bash interpreter, we use the following hash-bang specifier at the start of the script:
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NOTE: You can add an -x to the interpreter line of bash scripts to assist with debugging. This will echo the expanded variables and actual runtime code in addition to the more common output vectors like the echo command. For example, #!/bin/bash x. Using this syntax, you can also specify atypical shell interpreters, such as Perl (#!/usr/bin/perl), Python (#!/usr/bin/python), or Ruby (#!/usr/bin/ruby), the list goes on. For the most part, OS X, and most *nix variants all utilize the same directory to store interactive user shells in: the /bin/ folder. This folder is defined by BSD as housing: user utilities fundamental to both single-user and multi-user environments. This folder is very common among the *nix variants, and can usually be trusted to contain at least the Bourne shell (sh), and on most modern systems, the bash shell. However, non-shell interpreters, such as Python, Perl, or Ruby are going to vary greatly from OS to OS. Because of this, if we want our shell to be portable (which these languages provide), then providing a static path is not going to provide much utility on nonconforming systems. If portability is your goal (and certainly it s never a bad one), you may want to forgo specifying an absolute path and instead let the parent shell dynamically determine its location. To do this, utilize the following hash-bang specifier:
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The key thing to know here, is that /usr/bin/env is a very commonly supported binary, and will cause the shell to search through its $PATH to locate the Python executable. If that s found in our path, this executable will be used as the interpreter for the script. The $PATH variable is an environmental variable used by nearly all shells and specifies a number of directories that should be consulted when searching for a binary. This variable contains a colon-delimited string of directories, and will search through them in order of preference from left to right. For instance, if I run the command echo $PATH, I will see all of the directories in my path:
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Thus, if I were to run the command ifconfig, my shell would first look for the binary ifconfig in the /usr/bin folder, then in /bin, /usr/sbin, and so on until it ultimately finds the command (in this case, in the /sbin directory). If the command is not found after searching the entire path, the shell will terminate execution of the script with an error. On top of this, the PATH variable becomes a good way for a user to inject his
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CHAPTER 8: Automating Administrative Tasks
own versions of a binary in place of a system binary. For instance, Mr. Joebob Poweruser always likes to have the latest, greatest version of Perl on his system, dutifully installed at /usr/local/bin/perl. However, with a default PATH variable, when Joebob runs the command perl, he will be treated to our localization s binary stored at /usr/bin/perl. To change this, Joebob will want to modify his ~/.bash profile file, adding the line
export PATH="/usr/local/bin:$PATH"
After doing this, when Joebob starts a shell, the path /usr/local/bin will be the first folder searched in his path. Knowing all of this, it is easy to see how utilizing the /usr/bin/env in your hash-bang line can provide benefits if your script will have a wide audience. NOTE: With all the variants of Linux and Unix systems out there, it certainly can be a mental exercise to remember each one s folder hierarchy. For this purpose, many such systems provide documentation as to their particular folder eccentricities. On such systems, you can access this documentation via the hier man page by running the command man hier at your Terminal prompt. With the hash-bang out of the way, we can now start writing our script. Typically at this point in the script, we will do what is referred to as initialization. That is, we will define the variables to be utilized by the script. Initializing all of your variables at the beginning of the script provides many benefits. Primarily, it serves as a blueprint for your script. Assuming you adopt good naming conventions for your variables, the general utility and configurability of a script can often be deduced by scanning the variables, at least to an extent. To assign a variable in bash, you simply specify the variable name, followed by an equal sign, and then the value. For instance, in the following line:
USER_NAME="hunterbj"
With this line, we are assigning the global variable USER_NAME the value of hunterbj. Variables in bash can be uppercase such as PLIST_FILE and can contain underscores PLIST_FILE, and can even be camel case plistFileNumberThree, the choice is up to you, just be consistent. Notice that during assignment, we do not prepend the variable name with a $ specifier, unlike Perl. However, utilizing the global scope in bash will ultimately make your code less extensible. For instance, if you were to refactor the code into a function, you could have issues with scope conflict. To address this, you can utilize the declare statement, which will initialize the variable only in the local context:
declare USER_NAME="hunterbj" # Beau is available only the local context declare x USER_NAME="zsmith" # Zack is only available to the local and sub shells export USER_NAME="cedge" # Charles is available to the local sub shells and parent shells # (but no type assignment such as array "-a" or "-i" integer)
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