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()

echo "printenv: $FOO"
FOO=bar2

The resulting output is:

printenv: bar
FOO=bar

Pipes

• 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.