executable-set-magic

[Otto Maddox]

Info node "(autotype)Executables" says that "Various interpreter modes
such as Shell script mode or AWK mode will automatically insert or
update the buffer's magic number..."

executable-insert is set to t but this doesn't happen.

C-x b foo RET
M-x sh-mode RET

No magic number is inserted, and you have to do it manually with
`C-c :'

Similarly, AWK mode doesn't automatically set it, either.
-- 
  Otto Maddox
  ottomaddox@fastmail.fm

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