Unix Internals - With C Programming Examples-2

Print all the environment variables using the envp argument in main() function.

/* Program to Print Environment Variables * Program Name : prenv.c * */ #include <stdio.h> int main(int argc, char *argv[], char *envp[]) { int i = 0; while (envp[i]) { printf("Environment Variable : %s\n", envp[i]); i++; } return 0; }

Print all the environment variables using extern variable environ.

/* Program to Print Environment Variables * Program Name : prenvv.c * */ #include <stdio.h> extern char **environ; int main(int argc, char *argv[]) { int i = 0; while (environ[i]) { printf("Environment Variable : %s\n", environ[i]); i++; } return 0; }

The output of both the programs are same.

Program Output:

$ ./prenv Environment Variable : _=./prenv Environment Variable : HZ=100 Environment Variable : SSH_TTY=/dev/ttyp0 Environment Variable : PATH=/bin:/usr/bin:/usr/gnu/bin:/sbin:/usr/local/bin Environment Variable : HUSHLOGIN=FALSE Environment Variable : EDITOR=emacs Environment Variable : SHELL=/bin/ksh Environment Variable : HOME=/home/reemus Environment Variable : TERM=xterm Environment Variable : PWD=/home/reemus/prog/cprog Environment Variable : TZ=EST5EDT Environment Variable : ENV=/home/reemus/.kshrc $

Unix Internals - With C Programming Examples-1

For a long time I wanted to create a series of Blog post of what I have learned as Unix System Programming and its Internals with the help of plain simple to read examples.

Program: cmdargs.c

/* Program to read and display the command-line arguments */ #include <stdio.h> int main(int argc, char *argv[]) /* argc - No. of Command-line Arguments passed to the program */ /* argv - Array of Strings - Command-line Arguments */ { int i; printf("No. of Command-line Arguments : %d\n", argc); for (i = 0; i < argc; i++) { printf("Arg[%d] = %s\n", i, argv[i]); } return 0; }

Output:

$ ./cmdargs one two three No. of Command-line Arguments : 4 Arg[0] = ./cmdargs Arg[1] = one Arg[2] = two Arg[3] = three $

As you can see the "argc" contains the number of command-line arguments passed to the program.

The very first command-line argument for the program is program name itself, followed by the other arguments passed.