Shell Scripting: Expert Recipes for Linux, Bash, and More

Libraries

The shell has no real concept of libraries in the way that Perl and C use libraries. In C, you can bring in the Math library by including its header file and linking against the library (simply called “m,” hence -lm in the following gcc call). Additional functions, including cos(), sin(), and tan(), are then available to the program.

$ cat math.c
#include <stdio.h>
#include <math.h>
int main(int argc, char *argv[])
{
  int arg=atoi(argv[1]);
  printf("cos(%d)=%0.8f\n", arg, cos(arg));
  printf("sin(%d)=%0.8f\n", arg, sin(arg));
  printf("tan(%d)=%0.8f\n", arg, tan(arg));
  return 0;
}
$ gcc -lm -o math math.c
$ ./math 30
cos(30)=0.15425145
sin(30)=-0.98803162
tan(30)=-6.40533120
$ ./math 60
cos(60)=-0.95241298
sin(60)=-0.30481062
tan(60)=0.32004039
$ ./math 90
cos(90)=-0.44807362
sin(90)=0.89399666
tan(90)=-1.99520041
$

math.c

This sample C code does no sanity tests on its input and is only here to show how linking to a library works in the C language. Also, if its results look wrong, that is because it is working in radians and not degrees.

The shell has a few ways of defining standard settings — aliases, variables, and functions, which create an environment almost indistinguishable from a set of libraries. When your shell is invoked interactively, it reads ~/.profile ...