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Piping Input to read
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his chapter deals with a gotcha that I came across while porting a script from ksh to bash. It was a gotcha only because at the time I wasn t aware of a fairly crucial difference in the behavior of the two shells. In both pdksh and bash, the last command of a pipeline is performed in a subshell. This means that a variable assigned within the subshell is not available to the parent shell. In ksh, the last command of a pipeline is executed in the original shell. This isn t an issue when using the pipe to set a variable, but if the result of a pipe is sent to a loop structure that then populates variables you will use later, that is more of a problem. Once the loop completes, the variables you were going to rely on don t exist. Included here are a few of examples of code that you might expect to work, but they actually don t. I also include some workarounds that will perform the intended tasks. The following is the part of the code that had problems when I ported it. It was used to process a file of extended output one line at a time. To perform this task in ksh, I would use the following:
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cat somefile | while read line do # Process the $line variable in some form. if [ "`echo $line | awk '{print $3}'`" = "somevalue" ] then all="$all $line" fi done
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If everything within the loop is self-contained and none of the variables in it are accessed outside the loop, this will work fine. However, the bash code parsed each line in the output of the piped command, and populated some variables based on that output. Once the loop completed, I wanted to access those values ($all in this example) for other purposes, and found that they were undefined. The following code is the first workaround that I found to overcome the problem. Unfortunately it isn t quite as elegant or intuitive as the original code because it uses a temporary file. To keep the code clean I try to avoid using temporary files, but in this case I had no choice.
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CHAPTER 10 PIPING INPUT TO READ
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while read line do # Process the $line variable in some form. if [ "`echo $line | awk '{print $3}'`" = "somevalue" ] then all="$all $line" fi done < somefile
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First the data originally piped to the while read loop is sent to a temporary file. The file is then redirected into the back end of the loop. This functions the same way as the original code, but allows the variables populated within the loop to remain usable once the loop completes. 8 offers another example of this technique. The following is a modified form of the previous example:
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THE_INPUT=`ps -ef` while read line do # Process the $line variable in some form. if [ "`echo $line | awk '{print $3}'`" = "somevalue" ] then all="$all $line" fi done <<EOF $THE_INPUT EOF
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This slight modification of the earlier example eliminates the need for a temporary file. Instead of redirecting a file into the back of the loop, we start a here-document and feed it the data we want to process through the loop. A here-document is where the shell reads input from the current source until it reaches the matching tag alone on a single line, in this case EOF. This solution works in the same way as a real file with both bash and pdksh. The following sections show four methods for reading input one line at a time. With each method, I explain what variables are available within the code for each of the four shells (bash, ksh, pdksh, and Bourne sh).
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