2.2. Shell Commands

“Is not that nice,” you say to yourself, “but how are files actually ``re-created'', as he likes to spell it?” The re-creation is accomplished by commands you place in the makefile. These commands are passed to the Bourne shell (better known as /bin/sh) to be executed and are expected to do what is necessary to update the target file (PMake does not actually check to see if the target was created. It just assumes it is there).

Shell commands in a makefile look a lot like shell commands you would type at a terminal, with one important exception: each command in a makefile must be preceded by at least one tab.

Each target has associated with it a shell script made up of one or more of these shell commands. The creation script for a target should immediately follow the dependency line for that target. While any given target may appear on more than one dependency line, only one of these dependency lines may be followed by a creation script, unless the :: operator was used on the dependency line.

If the double-colon was used, each dependency line for the target may be followed by a shell script. That script will only be executed if the target on the associated dependency line is out-of-date with respect to the sources on that line, according to the rules I gave earlier. I'll give you a good example of this later on.

To expand on the earlier makefile, you might add commands as follows:

program         : a.o b.o c.o
        cc a.o b.o c.o -o program

a.o b.o c.o     : defs.h
a.o             : a.c
       cc -c a.c

b.o             : b.c
       cc -c b.c

c.o             : c.c
       cc -c c.c

Something you should remember when writing a makefile is, the commands will be executed if the target on the dependency line is out-of-date, not the sources. In this example, the command cc -c a.c will be executed if a.o is out-of-date. Because of the : operator, this means that should a.c or defs.h have been modified more recently than a.o, the command will be executed (a.o will be considered out-of-date).

Remember how I said the only difference between a makefile shell command and a regular shell command was the leading tab? I lied. There is another way in which makefile commands differ from regular ones. The first two characters after the initial whitespace are treated specially. If they are any combination of @ and -, they cause PMake to do different things.

In most cases, shell commands are printed before they are actually executed. This is to keep you informed of what is going on. If an @ appears, however, this echoing is suppressed. In the case of an echo command, say

echo Linking index

it would be rather silly to see

echo Linking index
Linking index

so PMake allows you to place an @ before the command to prevent the command from being printed:

@echo Linking index

The other special character is the -. In case you did not know, shell commands finish with a certain “exit status”. This status is made available by the operating system to whatever program invoked the command. Normally this status will be 0 if everything went ok and non-zero if something went wrong. For this reason, PMake will consider an error to have occurred if one of the shells it invokes returns a non-zero status. When it detects an error, PMake's usual action is to abort whatever it is doing and exit with a non-zero status itself (any other targets that were being created will continue being made, but nothing new will be started. PMake will exit after the last job finishes). This behavior can be altered, however, by placing a - at the front of a command (e.g. -mv index index.old), certain command-line arguments, or doing other things, to be detailed later. In such a case, the non-zero status is simply ignored and PMake keeps chugging along.

Because all the commands are given to a single shell to execute, such things as setting shell variables, changing directories, etc., last beyond the command in which they are found. This also allows shell compound commands (like for loops) to be entered in a natural manner. Since this could cause problems for some makefiles that depend on each command being executed by a single shell, PMake has a -B flag (it stands for backwards-compatible) that forces each command to be given to a separate shell. It also does several other things, all of which I discourage since they are now old-fashioned.

A target's shell script is fed to the shell on its (the shell's) input stream. This means that any commands, such as ci that need to get input from the terminal will not work right - they will get the shell's input, something they probably will not find to their liking. A simple way around this is to give a command like this:

ci $(SRCS) < /dev/tty

This would force the program's input to come from the terminal. If you cannot do this for some reason, your only other alternative is to use PMake in its fullest compatibility mode. See “Compatibility” in Chapter 4.