如何在自己的shell中实现自己的cd命令

我在一个迷你项目中工作:"我自己的命令解释器(壳)",像Bash shell或Sh shell.直到现在它可以执行每个预定义的命令,如ls,ps,pwd,date.除此之外,我已经实现了一些其他操作,如输入重定向(<),输出重定向(>),PIPE(|)功能.除此之外,我已经实现了自己的用户命令,如pid,ppid,quit,hist.

vvdnlt208@vvdnlt208-Vostro-3446:~/project/Vector/sourabh/unix/proj2$ ./my_sh  
<1 SOURABH> ls
 a     cd.c       execute.o  inredir.c  main.o   multiexec.o  my_sh   sh.c                sigign.c  test.c
addhist.c  dir        header.h   inredir.o  Makefile     mypipe.c     outredir.c  showhist.c  sigign.o  tetest.c
addhist.o  execute.c  inredir    main.c     multiexec.c  mypipe.o     outredir.o    showhist.o  test2.c
<2 SOURABH> ps
PID TTY          TIME CMD
6120 pts/10   00:00:01 bash
6510 pts/10   00:00:00 grep
6512 pts/10   00:00:00 grep
6514 pts/10   00:00:00 grep
6516 pts/10   00:00:00 grep
7772 pts/10   00:00:00 my_sh
7774 pts/10   00:00:00 ps
<3 SOURABH> pwd
/home/vvdnlt208/project/Vector/sourabh/unix/proj2
<4 SOURABH> date
Fri Nov 14 16:44:24 IST 2014
<5 SOURABH> 
<6 SOURABH> 
<7 SOURABH> 
<8 SOURABH> hist
1 ls
2 ps
3 pwd
4 date
8 hist
<9 SOURABH> ls >a.txt
<10 SOURABH> pid
7772
<11 SOURABH> ppid

但我在执行cd命令时面临一些困难......

<12 SOURABH> cd
cd: No such file or directory
<13 SOURABH> cd dir
cd: No such file or directory
<14 SOURABH> 

有人告诉我自己实施它...请帮我这样做.提前致谢.