To write a UNIX command line interpreter (shell) in C, you can follow these general steps:

1. Read input from the user using the fgets function, which reads input from the standard input stream (stdin) until a newline character is encountered.
2. Tokenize the input into commands and arguments using the strtok function.
3. Use system calls such as fork, execve, and waitpid to create child processes and run the commands entered by the user.
4. Implement built-in shell commands such as cd, exit, and history using appropriate system calls.

Here's a simple example of how to write a command line interpreter in C:

c
 
 
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#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <sys/wait.h>

#define MAX_LINE_LENGTH 80
#define MAX_ARGS 10

int main() {
    char line[MAX_LINE_LENGTH];
    char* args[MAX_ARGS];
    int status;

    while (1) {
        printf("shell> ");
        fgets(line, MAX_LINE_LENGTH, stdin);

        // tokenize the input
        int i = 0;
        args[i] = strtok(line, " \n");
        while (args[i] != NULL) {
            i++;
            args[i] = strtok(NULL, " \n");
        }
        args[i] = NULL;

        // handle built-in commands
        if (strcmp(args[0], "cd") == 0) {
            chdir(args[1]);
            continue;
        }
        else if (strcmp(args[0], "exit") == 0) {
            exit(0);
        }

        // create a child process and execute the command
        pid_t pid = fork();
        if (pid < 0) {
            perror("fork failed");
            exit(1);
        }
        else if (pid == 0) {
            // child process
            execvp(args[0], args);
            perror("execvp failed");
            exit(1);
        }
        else {
            // parent process
            waitpid(pid, &status, 0);
        }
    }
    return 0;
}

This is just a simple example and there are many more features you can add to a shell, such as input/output redirection, piping, and background processes. For more information, you can read the following resources:

• Write a Shell in C
• Unix Shell Programming

You can use a diff tool to compare two files and highlight the differences between them. A popular diff tool is diff which is available in most Unix-based systems. Here's how to use it:

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$ diff file1.txt file2.txt

This will output a list of differences between the two files, with a > sign indicating lines present in file2 but not file1, and a < sign indicating lines present in file1 but not file2. This is a simple and effective way to identify differences between two files.

Alternatively, you can also use graphical diff tools like Beyond Compare or Meld which provide a more user-friendly interface to compare files. Here are their websites:

• Beyond Compare: https://www.scootersoftware.com/
• Meld: http://meldmerge.org/

The calc command is a Unix command that allows you to perform simple calculations from the command line. It can be used to calculate basic arithmetic operations, such as addition, subtraction, multiplication and division, as well as to compare numbers and evaluate mathematical functions. For example, calc 2 + 2 will return the result 4. For more information about calc, refer to the man page.