compgen, eval, export, file: move to common (#6508)

pages/common/compgen.md

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+# compgen
+
+> A built-in command for auto-completion in Bash, which is called on pressing TAB key twice.
+> More information: <https://www.gnu.org/software/bash/manual/bash.html#index-compgen>.
+
+- List all commands that you could run:
+
+`compgen -c`
+
+- List all aliases:
+
+`compgen -a`
+
+- List all functions that you could run:
+
+`compgen -A function`
+
+- Show shell reserved keywords:
+
+`compgen -k`
+
+- See all available commands/aliases starting with 'ls':
+
+`compgen -ac {{ls}}`