环境：OS X 10.12.4

照例先看一下man page。

HEAD(1)                   BSD General Commands Manual                  HEAD(1)

NAME
     head -- display first lines of a file

SYNOPSIS
     head [-n count | -c bytes] [file ...]

DESCRIPTION
     This filter displays the first count lines or bytes of each of the specified files,or of the standard input if no files are specified.  If count is omitted
     it defaults to 10.

     If more than a single file is specified,each file is preceded by a header consisting of the string ``==> XXX <=='' where ``XXX'' is the name of the file.

EXIT STATUS
     The head utility exits 0 on success,and >0 if an error occurs.

SEE ALSO
     tail(1)

HISTORY
     The head command appeared in PWB UNIX.

BSD                              June 6,1993                              BSD

这个程序也是比较简单的，显示文件开头的n行或者n个字符，如果没有特别指出的话，默认是10行。如果没有指明文件的话，也是从标准输入读入。如果参数中指出了多个文件，每个文件的开头会有==> XXX <==的提示（XXX是文件名）。

实现中参数读取参考TCPL（The C Language Programming Language 5.10 Command-line Argument）。字符读入也使用了上回cat实现的xc_file.c缓存。

head.c

#include <stdio.h>
#include <string.h>
#include <stdlib.h>

#define COUNT 1
#define BYTES 2

#define XCERR_FORMAT 1

void xc_open(char *aFName);
void xc_close(void);
int xc_getchar(void);
void xc_readFromStdin(void);

void xc_quit(int flag);
void xc_output(int flag,int n);

/* use xc_file.c to read from file */
int main(int ac,char *av[])
{
    int ch;
    int flag = COUNT;
    int cnt = 10; /* default 10 lines */
    int moreThanOneFile = 0;
    // int readStd = 0;

    /* deal with option argument */

    while(--ac > 0 && (*++av)[0] == '-'){
        ch = *++av[0];
        switch(ch){
            case 'n':
                flag = COUNT;
                cnt = atoi(*++av); /* av[ac] == "\0" */
                --ac; /* take one argument */
                if(cnt <= 0)
                    xc_quit(XCERR_FORMAT);
                break;
            case 'c':
                flag = BYTES;
                cnt = atoi(*++av);
                --ac;
                if(cnt <= 0)
                    xc_quit(XCERR_FORMAT);
                break;
            default:
                xc_quit(XCERR_FORMAT);
        }
    }

    if(ac == 0){
        xc_readFromStdin();
        xc_output(flag,cnt);
    }else{
        if(ac > 1)
            moreThanOneFile = 1;
        while(ac--)
        {
            xc_open(*av);
            if(moreThanOneFile)
                printf("==> %s <==\n",av[0]);
            xc_output(flag,cnt);
            if(moreThanOneFile && ac > 0)
                printf("\n");
            av++;
        }
    }
    return 0;
}
void xc_quit(int flag)
{
    char str[255];
    switch(flag){
        case XCERR_FORMAT:
            strcpy(str,"Usage: head [-n count | -c bytes] [file ...]");
            break;
    }
    fprintf(stderr,"%s\n",str);
    exit(1);
}
void xc_output(int flag,int n)
{
    int ch;
    int count = 0;
    while(count < n && (ch = xc_getchar()) != EOF){
        putchar(ch);
        if(flag == COUNT){
            if(ch == '\n')
                count++;
        }else if(flag == BYTES)
            count++;
    }
    xc_close();
}

xc_file.c

#include <stdio.h>
#include <unistd.h>
#include <fcntl.h>
#include <stdlib.h>
#include <string.h>

/* BUFSIZ define in stdio.h,commonly is 1024 */
static unsigned char chBuf[BUFSIZ];
static int fd = -1;
static char fName[BUFSIZ];
static int chCur;
static int chSum;

void xc_readFromStdin(void)
{
    /* define in unistd.h */
    fd = STDIN_FILENO;
}

void xc_open(char *aFName)
{
    if((fd = open(aFName,O_RDONLY)) == -1){
        perror(aFName);
        exit(1);
    }
    strcpy(fName,aFName); /* record which file is opened */
    chCur = chSum = 0;
}

int xc_reload(void)
{
    int bytes_read;
    if((bytes_read = read(fd,chBuf,BUFSIZ)) > 0){
        chCur = 0;
        chSum = bytes_read;
        return chBuf[chCur++];
    }else if(bytes_read == -1){
        perror(fName);
        exit(1);
    }else if (bytes_read == 0)
        return EOF;
}

int xc_getchar(void)
{
    if(fd == -1)
        return EOF;
    if(chSum == chCur)
        return xc_reload();
    return chBuf[chCur++];
}

void xc_close(void)
{
    if(fd != -1)
    {
        if(close(fd) == -1){
            perror(fName);
            exit(1);
        }
        fd = -1;
    }
}

效果：