Added notes to the knowledge base

knowledge base/xargs.md

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+# `xargs`
+
+## TL;DR
+
+```shell
+# print the command to be executed and immediately start it
+echo 1 2 3 4 | xargs -t mkdir
+
+# print the command to be executed and ask for confirmation before starting it
+echo 1 2 3 4 | xargs -p rm -rf
+
+# spawn 5 clamscan processes at a time
+# each process being given 1 argument from the list in input
+find ~ -type f -printf "'%p' " | xargs -P 5 -n 1 clamscan
+
+# replace a given string with arguments read from input
+# useful to insert the arguments in the middle of the command to execute
+find ~ -type d -name ".git" -exec dirname {} + | xargs -I // git -C "//" pull
+```