Add Track Line Occurrences From Input With AWK as a Unix TIL

unix/track-line-occurrences-from-input-with-awk.md

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+# Track Line Occurrences From Input With AWK
+
+In [Deduplicate List While Preserving Original
+Order](deduplicate-list-while-preserving-original-order.md), I showed a terse
+AWK pattern that allows for sifting out all duplicate lines as they are
+encountered. This looks like `!seen[$0]++`.
+
+I thought it would be useful to look at a less dense version of this where I
+break out the conditional check, make the `print` explicit, and add lines to the
+associative array in the action block.
+
+```bash
+❯ echo "red green blue red yellow green blue red green" | tr ' ' '\n' | awk '!($0 in seen) { print; seen[$0] = 1 }'
+red
+green
+blue
+yellow
+```
+
+Let's take a look at that. The first part is the pattern that determines whether
+the action(s) runs.
+
+```bash
+!($0 in seen)
+```
+
+Here we check if the current line from the input being processed (`$0`) has
+already been added to the associative array we declared with a name of `seen`.
+If it is the first time we've seen that exact line, then it won't be in, so a
+`false` which gets negated (`!`) to `true`, so the actions are triggered.
+
+
+The second part in curly braces is a sequence of actions separated by
+semicolons.
+
+```bash
+{ print; seen[$0] = 1 }
+```
+
+The first action is `print` which will print the current line to stdout. The
+second action adds the current line to the associative array (`seen`) with a
+value of `1`. Now any time we encounter a recurring line it will be present _in_
+`seen` and the pattern will evaluate to false, preventing these actions from
+running.
+
+The whole thing then is:
+
+```bash
+awk '!($0 in seen) { print; seen[$0] = 1 }'
+```
+
+Again this is an expanded, easier-to-understand version of `awk '!seen[$0]++`
+which has the same behavior.