zen barcode c# example For, While, and Until Statements in Objective-C

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For, While, and Until Statements
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So, at this point, we have learned how to define the flow of our program through the use of branching statements, expressions, and conditional operators. Automation, however, is rarely about performing an operation once; the benefits of automation lie in the ability to scale production as needed with minimal investment. Automation is particularly well-suited for boring, repetitive tasks that will result in hundreds, thousands, or even millions of iterations. To harness the ability of repetition and iteration, bash provides three looping statements: for, while, and until. The for loop is utilized for iterating over items.
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declare plistbuddy="/usr/libexec/PlistBuddy" declare python="/usr/bin/python" REQUIRED_COMMANDS="$plistbuddy $python" for COMMAND in $REQUIRED_COMMANDS; do
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CHAPTER 8: Automating Administrative Tasks
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if [ -x else
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$COMMAND ] ; then echo "Command: $COMMAND is installed" echo "Command: $COMMAND is missing"
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fi done
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Every element of this script is native to bash, and would output the text:
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Command: /usr/libexec/PlistBuddy is installed Command: /usr/bin/python is installed
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NOTE: To determine if a command will result in the execution of an external program, use type followed by the name of the function. If the process is external to the shell, it will specify the absolute path to the binary (as found in $PATH). For example: type echo returns echo is a shell builtin, meaning that bash will use its internal echo ability rather than the external command /bin/echo when the echo command is called in a script. The while and until statements are used for building more customized looping structures. The and operator allows us to loop while a certain criteria are met:
while [ $(ps aux | grep v "grep" | grep c "Finder" ) ge 1 ]; do echo "Finder is still running" sleep 15 done
In this example, there are a few new concepts. First and foremost, whenever we use expressions, they are primarily expecting string arguments. If we want to call an external program inside of an expression, we must designate that the text not be treated as a string, but rather as an external process. To do this, we wrap the entire command pipeline inside of $(). This wrapper tells the shell to evaluate the contents of the entire pipeline in a subshell. This same behavior applies if we want to assign the output of a command to a variable. The following syntax is used to set the value of variable $psTxt to the output of our ps command chain (this time we will use grep with pipes to accomplish the same count):
psTxt=$( ps aux | grep v "grep" | grep c "Finder" )
Examining this command chain, we see that we are utilizing the external programs ps and grep. The ps command lists running processes, and grep is a basic filtering tool. Because grep is a program, it will sometimes be found in the ps process list, so we must first filter out the grep line, using the v flag. Then we do a search for the string Finder. The c flag specifies that we will output the number of matches. If we find one or more processes, we will proceed through our loop. Next we output a simple text line stating that the program is running, then we sleep for 15 seconds. At this point, the end of our loop has been reached and we will once again test for our criteria. If the criteria match, we will proceed through our loop again, indefinitely, until our criteria fail to match. The until loop represents a different utility. In bash it does not represent true trailing logic (as it does in C), but rather serves as an inverse of the while loop. Because of this,
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CHAPTER 8: Automating Administrative Tasks
it is of rather limited use. For example, we can pretty easily replicate the logic of the above while loop, simply inversing our conditional logic:
until [ $(ps aux | grep v "grep" | grep c "Finder" ) lt 1 ]; do printf "Finder is Running\n" sleep 15 done
NOTE: Bash, like most languages provides control statements for managing individual loop iterations. For instance, the control statement continue will instruct a loop to terminate the execution for that particular instance, at which point it will return to its evaluation statement (or the next iterated item in the case of a for loop), and continue through the loop. The break statement will instruct a loop to terminate completely.
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