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Writing the Commands in Rules

   The commands of a rule consist of shell command lines to be executed
one by one.  Each command line must start with a tab, except that the
first command line may be attached to the target-and-dependencies line
with a semicolon in between.  Blank lines and lines of just comments
may appear among the command lines; they are ignored.

   Users use many different shell programs, but commands in makefiles
are always interpreted by `/bin/sh' unless the makefile specifies
otherwise.  Note: Command Execution.

   The shell that is in use determines whether comments can be written
on command lines, and what syntax they use.  When the shell is
`/bin/sh', a `#' starts a comment that extends to the end of the line.
The `#' does not have to be at the beginning of a line.  Text on a line
before a `#' is not part of the comment.

* Echoing
How to control when commands are echoed.
* Execution
How commands are executed.
* Parallel
How commands can be executed in parallel.
* Errors
What happens after a command execution error.
* Interrupts
What happens when a command is interrupted.
* Recursion
Invoking `make' from makefiles.
* Sequences
Defining canned sequences of commands.
* Empty Commands
Defining useful, do-nothing commands.

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