问：linux下这些命令到底是用什么写的？用linux这么久，有没有想过这些命令是怎么实现的。
之前我觉得linux下的命令强大得不得了，为此我还搜了一下这些命令是怎么 实现的，无果。今天下载了freeBSD源码，正好看到bin目录下有与各个命令相同的目录，就好奇进 去看了一下，这些命令远比我想象中的简单，很多我们平时常用的命令，都只有两三百行代码。
以cat为例（因为它直接编译可用，不依赖任何其他文件），加上注释才263行。考虑到cat 使用的频繁程度，我忍不住叹息：生不逢时啊！为什么cat这么简单这么常用的代码不是我写的!
我在bin/cat/目录下直接编译cat.c文件，得到a.out这个二进制文件，在当前目录下a.out的用法 和cat命令的用法一样，恩，它就是cat。
如果我们把它重命名为mycat，然后拷贝到/usr/local/bin目录下，之后mycat
这个命令就和cat命令一摸一样了，它可以完全代替cat这个命令。这就是一个终端工具的产生过程？
是的，它并不神奇，它就是一个普通的程序。
下来是cat的源码：
/*
* Copyright (c) 1989, 1993
* The Regents of the University of California. All rights reserved.
*
* This code is derived from software contributed to Berkeley by
* Kevin Fall.
*
* Redistribution and use in source and binary forms, with or without
* modification, are permitted provided that the following conditions
* are met:
* 1. Redistributions of source code must retain the above copyright
* notice, this list of conditions and the following disclaimer.
* 2. Redistributions in binary form must reproduce the above copyright
* notice, this list of conditions and the following disclaimer in the
* documentation and/or other materials provided with the distribution.
* 3. All advertising materials mentioning features or use of this software
* must display the following acknowledgement:
* This product includes software developed by the University of
* California, Berkeley and its contributors.
* 4. Neither the name of the University nor the names of its contributors
* may be used to endorse or promote products derived from this software
* without specific prior written permission.
*
* THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
* ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
* ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
* FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
* DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
* OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
* HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
* LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
* OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
* SUCH DAMAGE.
*/
#ifndef lint
static char const copyright[] =
"@(#) Copyright (c) 1989, 1993\n\
The Regents of the University of California. All rights reserved.\n";
#endif /* not lint */
#ifndef lint
#if 0
static char sccsid[] = "@(#)cat.c 8.2 (Berkeley) 4/27/95";
#endif
static const char rcsid[] =
"$FreeBSD$";
#endif /* not lint */
#include <sys/param.h>
#include <sys/stat.h>
#include <ctype.h>
#include <err.h>
#include <fcntl.h>
#include <locale.h>
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
int bflag, eflag, nflag, sflag, tflag, vflag;
int rval;
const char *filename;
void cook_args __P((char *argv[]));
void cook_buf __P((FILE *));
int main __P((int argc, char *argv[]));
void raw_args __P((char *argv[]));
void raw_cat __P((int));
int
main(argc, argv)
int argc;
char *argv[];
{
int ch;
setlocale(LC_CTYPE, "");
while ((ch = getopt(argc, argv, "benstuv")) != -1)
switch (ch) {
case 'b':
bflag = nflag = 1; /* -b implies -n */
break;
case 'e':
eflag = vflag = 1; /* -e implies -v */
break;
case 'n':
nflag = 1;
break;
case 's':
sflag = 1;
break;
case 't':
tflag = vflag = 1; /* -t implies -v */
break;
case 'u':
setbuf(stdout, NULL);
break;
case 'v':
vflag = 1;
break;
default:
(void)fprintf(stderr,
"usage: cat [-benstuv] [-] [file ...]\n");
exit(1);
}
argv += optind;
if (bflag || eflag || nflag || sflag || tflag || vflag)
cook_args(argv);
else
raw_args(argv);
if (fclose(stdout))
err(1, "stdout");
exit(rval);
}
void
cook_args(argv)
char **argv;
{
register FILE *fp;
fp = stdin;
filename = "stdin";
do {
if (*argv) {
if (!strcmp(*argv, "-"))
fp = stdin;
else if ((fp = fopen(*argv, "r")) == NULL) {
warn("%s", *argv);
rval = 1;
++argv;
continue;
}
filename = *argv++;
}
cook_buf(fp);
if (fp != stdin)
(void)fclose(fp);
} while (*argv);
}
void
cook_buf(fp)
register FILE *fp;
{
register int ch, gobble, line, prev;
line = gobble = 0;
for (prev = '\n'; (ch = getc(fp)) != EOF; prev = ch) {
if (prev == '\n') {
if (ch == '\n') {
if (sflag) {
if (!gobble && putchar(ch) == EOF)
break;
gobble = 1;
continue;
}
if (nflag && !bflag) {
(void)fprintf(stdout, "%6d\t", ++line);
if (ferror(stdout))
break;
}
} else if (nflag) {
(void)fprintf(stdout, "%6d\t", ++line);
if (ferror(stdout))
break;
}
}
gobble = 0;
if (ch == '\n') {
if (eflag)
if (putchar('$') == EOF)
break;
} else if (ch == '\t') {
if (tflag) {
if (putchar('^') == EOF || putchar('I') == EOF)
break;
continue;
}
} else if (vflag) {
if (!isascii(ch) && !isprint(ch)) {
if (putchar('M') == EOF || putchar('-') == EOF)
break;
ch = toascii(ch);
}
if (iscntrl(ch)) {
if (putchar('^') == EOF ||
putchar(ch == '\177' ? '?' :
ch | 0100) == EOF)
break;
continue;
}
}
if (putchar(ch) == EOF)
break;
}
if (ferror(fp)) {
warn("%s", filename);
rval = 1;
clearerr(fp);
}
if (ferror(stdout))
err(1, "stdout");
}
void
raw_args(argv)
char **argv;
{
register int fd;
fd = fileno(stdin);
filename = "stdin";
do {
if (*argv) {
if (!strcmp(*argv, "-"))
fd = fileno(stdin);
else if ((fd = open(*argv, O_RDONLY, 0)) < 0) {
warn("%s", *argv);
rval = 1;
++argv;
continue;
}
filename = *argv++;
}
raw_cat(fd);
if (fd != fileno(stdin))
(void)close(fd);
} while (*argv);
}
void
raw_cat(rfd)
register int rfd;
{
register int off, wfd;
ssize_t nr, nw;
static size_t bsize;
static char *buf;
struct stat sbuf;
wfd = fileno(stdout);
if (buf == NULL) {
if (fstat(wfd, &sbuf))
err(1, "%s", filename);
bsize = MAX(sbuf.st_blksize, 1024);
if ((buf = malloc(bsize)) == NULL)
err(1, "buffer");
}
while ((nr = read(rfd, buf, bsize)) > 0)
for (off = 0; nr; nr -= nw, off += nw)
if ((nw = write(wfd, buf + off, (size_t)nr)) < 0)
err(1, "stdout");
if (nr < 0) {
warn("%s", filename);
rval = 1;
}
}