unixshell 48213 (temporary)

unixshell 48213 (temporary)


the main use for this tool is to create
formatted reports from raw data by reading information from
standard input and by default writing results to the standard output awk provides pattern matching language for writing short programs, with the possibility to process standard text and strings number processing, regular expression as well it is fitted with its own specific programming language making use of built-in functions this particularly interesting feature is that the awk processing language is quite similar in syntax with the C programming language a canonical form for calling the awk
utility is this: the two main components are the awk calling word followed by the program basically an awk program consists of a matching condition and its associated action now an awk executing system sequence usually consists of one of several actions where there’s more than one they appear separated by a semicolon back in our first lesson remember that the POSIX shell first processes the
contents of a command line before passing it to awk therefore the awk program
should be enclosed in quotes when invoked from the shell, so it avoids
possible conflicts with the shell interpretation that’s why we keep
bumping into these quotes, and that’s how you would recognise the beginning
and the end of an awk program now we’re coming closer to what you would see on a command line in real life the awk calling part followed by the options sequence then the program itself and in the end the name of what source file which we would use as an input data awk reads each line of input as an input record consisting of one or more fields of data when doing this awk splits each record into a set of numbered fields in which the first field is referred to as dollar one the second field is dollar two, and so far these fields are stored in proprietary variables and thus are made available for use fields are by default delimited by whitespace which is comprised of space, tab character, or combinations of the two similarly, the default separator between input records is the end of line character during this presentation we will see how we can make use of them or change them, according to our own data specific format besides the fields, we already discussed there is the dollar zero variable which actually represents
the entire input of the current record in essence awk behaves like a loop construct it will read a record from the input string
or input file and then process that record through each statement within the program when the record is fully processed awk goes to the next data row and starts it all over again in many cases these statements will make changes
in the current record before the next statement is processed it is because of this
processing that the entire awk program is usually referred to as the main processing loop in a subsequent presentation we will see several formal loop constructs available in awk the print command is the most commonly used awk command it writes data to the standard output let us consider the
following different printing scenarios if you find an awk program stating a regular expression enclosed between slash symbols and all these embedded within ticks then you should know it is the most common operation it is about matching the contents of each line of the source data against the expression and then printed to the standard output or, if the case, just ignore that line and move on to the next one so the lack of any explicit action call within the program leads to the default operation, which is
printing the content of the entire line on the contrary, a program made out of a single action the print command, with no other argument means that each line of the input source is going to be printed to the standard output entirely regardless of its content this is equivalent to this command print dollar zero which explicitly states that the whole line should be printed and in the absence of any other filtering rule it would do so unconditionally for each line of the input source finally we see another scenario, calling out the printing action this time for only print a particular field from each line of the input data note however that these are canonical examples it is not likely you will see
such programs in a real awk script but they’re meant to give you an idea about how it works okay so far assume you want to take a glance
at the content of the hosts file we’re going to bring the first field of each line of text from this file here are three commands, doing the same thing: taking on the contents of the source file passing it to the awk which will use the “print $1” program the first one sets up an anonymous pipeline from two distinct commands the first part prints the file to the standard output and second part does the printing the second command does the same thing but uses data redirection this first component is our awk filter and this component calls data directly from our targeted file using the redirection symbol the third alternative for our example brings us back to the canonical form of an awk command and again, as you can see, this is done with no filtering option please bare in mind the fact that
all these three versions of a simple awk program are correctly built and have the very same effect despite their different syntax finally, we offer an extended program for the same operation but this time with some additional text

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