Interacting with the enviroment

Enviroment variables

It's quite simple to read and write environment variables from C:

char *envvar;

if(envvar = getenv("FOO"))
	printf( "FOO=%s\n", envvar );

putenv("FOO=bar");

When you call a child process a copy of the enviroment is passed to this process. However, if the process changes enviroment variables, these changes will be lost once the process terminates.

The parent C program:

putenv("FOO=bar");

system("sh printenv");

if(envvar = getenv("FOO"))
	printf( "FOO=%s\n", envvar );

Executes a shell script (printenv) with system()

NOTE: Enviroment variables set by a child process do not affect the same variables in the parent's enviroment.

echo "printenv: $FOO"
FOO=bar2

The resulting output is:

printenv: bar
FOO=bar

Pipes

This example shows how to remain connected to a forked child process, collecting any output it does until it finishes. We might use this kind of interface to enable the peerd program to utilise existing scripted automation or to simply call other programs with a set of parameters.

NOTE: Enviroment variables set by a child process do not affect the same variables in the parent's enviroment.

• Pipe.c
• hello.sh

The output of this program is something like:

Begin
pipe(): 3 4
fork() pid 19260
wait() returned with status 0
read(): fetched 83 bytes
hello world!
FOO=foo was set by the child
Child changing foo
FOO=child changed foo

Parent: FOO=foo was set by the parent

bash

To connect to a bash shell you can simply do:

cat > bash

This will create an asynchronous connection to the bash process.