UNIX环境高级编程1-5从标准输入读命令并执行

从标准输入读命令并执行

    char buf[MAXLINE]; /* from apue.h */

    printf("%% "); /* print prompt (printf requires %% to print %) */

    // We use the standard I/O function fgets to read one line at a time from the

    // standard input. When we type the end-of-file character (which is often

    // Control-D) as the first character of a line, fgets returns a null pointer, the loop

    // stops, and the process terminates. In Chapter 18, we describe all the special

    // terminal characters--end of file, backspace one character, erase entire line, and

    // so on -- and how to change them.

    // Because each line returned by fgets is terminated with a newline character,

    // followed by a null byte, we use the standard C function strlen to calculate the

    // length of the string, and then replace the newline with a null byte. We do this

    // because the execlp function wants a null-terminated argument, not a

    // We call fork to create a new process, which is a copy of the caller. We say that

    // the caller is the parent and that the newly created process is the child. Then

    // fork returns the non-negative process ID of the new child process to the parent,

    // and returns 0 to the child. Because fork creates a new process, we say that it is

    // called once--by the parent--but returns twice--in the parent and in the child.

    // In the child, we call execlp to execute the command that was read from the

    // standard input. This replaces the child process with the new program file. The

    // combination of fork followed by exec is called spawning a new process on

    // some operating systems. In the UNIX System, the two parts are separated into

    // individual functions. We'll say a lot more about these functions in Chapter 8.

    // Because the child calls execlp to execute the new program file, the parent

    // wants to wait for the child to terminate. This is done by calling waitpid,

    // specifying which process to wait for: the pid argument, which is the process ID

    // of the child. The waitpid function also returns the termination status of the

    // child--the status variable--but in this simple program, we don't do anything

    // with this value. We could examine it to determine how the child terminated

    // The most fundamental limitation of this program is that we can't pass

    // arguments to the command we execute. We can't, for example, specify the name

    // of a directory to list. We can execute ls only on the working directory. To allow

    // arguments would require that we parse the input line, separating the arguments

    // by some convention, probably spaces or tabs, and then pass each argument as a

    // separate parameter to the execlp function. Nevertheless, this program is still a

    // useful demonstration of the UNIX System's process control functions.

while (fgets(buf, MAXLINE, stdin) != NULL)

        if (buf[strlen(buf) - 1] == '
')

            buf[strlen(buf) - 1] = 0; /* replace newline with null */

            // if the execlp execute successfully, execlp will not return

            // if it execute failed, return -1, and the error number store in errno variable

            err_ret("couldn't execute: %s", buf);

if ((pid = waitpid(pid, &status, 0)) < 0)

all: shell1
shell1: shell1.c
	g++ -g -Wall shell1.c ../lib/libapue.a -I ../include -o shell1
clean:
	rm shell1