Tip 72: Export one-off variables

23 February 12. [link] PDF version

level: intermediate POSIX
purpose: faster control of your programs

Part of a series of tips on POSIX and C. Start from the tip intro page, or get 21st Century C, the book based on this series.

First, I hope you are by now familiar with the difference between a shell variable and an exported shell variable.

The shell variable is just some text that gets substituted in later use on the shell. Like any old variable, it saves typing and redundancy.

ls $workingdir
cd $workingdir

Users of shells in the C shell family need to use set VAR value and setenv VAR value. That said, I'm going to use the POSIX-standard forms for the rest of this.

When a variable is exported, then it rises in importance, and is now a means of communicating to child programs. When a program spawns a child (in C, when you call fork(); on the shell, when you type in a command and hit <enter>), then the environment variables as they stand are sent to the child program. Appendix A of Modeling with Data covers how obnoxiously easy it is to read environment variables from C.

Great--so we can call export VAR=value and our child programs will have VAR on hand. But we can do it even faster.

Try this:

PANTS=kakhi env

You may know env as a program that does nothing but list the environment variables. When you run this, you should see PANTS there at the end of the list. Or run it through grep (env|grep PANTS) to make sure it's there.

So by setting a variable on the same line as the command, the command receives the variable as an export. You can do this with several variables if need be:

PANTS=kakhi PLANTS="ficus fern" env

Notice how we handled the space in the PLANTS variable.

Now that you've tried this trick on your command line, try running env again without preface. You will find that PANTS isn't there. The variable really did get set and exported for the duration of a single command.

What can we do with this?

We can write our C programs around this. As per Appendix A, which gives a complete example on p 384, we can call int varvalue = atoi(genenv("VAR")); in our C programs to get the value of an environment variable and convert from string to integer. So you could do something like

RUNS=32 ./do_simulation

for the variables that you would be tweaking all the time. People are more used to flags like -r 32, and they're about as easy to parse, but if you like this form, run with it.

make cares deeply about environment variables, and expects to be making use of them. Also, the configure script that you use to set up programs packaged with Autotools (think ./configure; make;sudo make install) also cares about environment variables.

Now things are getting useful. Make has a default rule for compiling C code, and (among others) it depends on two key variables: CFLAGS and LDLIBS. You might as well set up your CFLAGS once and for all via

export CFLAGS="-g -Wall -O3 -std=gnu99"

(or whatever works for you) in your .bashrc, .zshrc or whatever is appropriate.

If your program depends on any libraries, then they are specified via LDLIBS. Now for the magic: if you have those two variables set, then for a simple program, you don't need a makefile at all. We set CFLAGS for good above, so let's say we have a program that depends on glib-2.0 and is named pantspress.c. Then you may be able to compile it via

LDLIBS=-lglib-2.0 make pantspress

You will be running Make many times on a program, so you might want to at some point generate a makefile. It may consist of only a single line defining LDLIBS=-lglib-2.0 and still work.

But you only run configure once or twice on a package, and it may require odd flags to get working. In this case, the form here is great:

#turn off optimization, generate all warnings, link to glib:
CFLAGS="-g -Wall -O0" LDADD="-lglib-2.0" ./configure

Did you notice that I used LDADD this time? GNU Configure likes that better than GNU Make's LDLIBS. Neither is POSIX-standard.

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