1. 一个完整的shell命令行解析器
main.c
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#include "smsh.h"
#define DFL_PROMPT "> "
/**
* 1.next_cmd 从流中读入下一条命令
* 2. splitline 将一个字符串分解为一个字符串数组 并返回
* 3. execute 来运行这个命令
*/
int main(){
char *cmdline,*prompt,**arglist;
int result;
void setup();
prompt = DFL_PROMPT;
setup(); // 关信号
while ((cmdline = next_cmd(prompt,stdin)) != NULL){
if ((arglist = splitline(cmdline)) != NULL){
result = execute(arglist);
freelist(arglist);
}
free(cmdline);
}
return 0;
}
void setup(){
signal(SIGINT,SIG_IGN);
signal(SIGQUIT,SIG_IGN);
}
void fatal(char *s1,char *s2,int n){
fprintf(stderr,"Error:%s,%s\n",s1,s2);
exit(n);
}
execute.c
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//
// Created by xm on 2022/3/1.
//
#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <signal.h>
#include <sys/wait.h>
int execute(char *argv[]){
int pid;
int child_info = -1;
if (argv[0] == NULL){
return 0;
}
if ((pid = fork()) == -1){
perror("fork");
} else if (pid == 0){
signal(SIGINT,SIG_DFL);
signal(SIGQUIT,SIG_DFL);
execvp(argv[0],argv);
perror("cannot execute command");
exit(1);
} else {
if (wait(&child_info) == -1)
perror("wait");
}
return child_info;
}
smsh.h
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//
// Created by xm on 2022/3/1.
//
#ifndef LINUX_SMSH_H
#define LINUX_SMSH_H
#define YES 1
#define NO 0
char *next_cmd();
char **splitline(char *);
void freelist(char **);
void *emalloc(size_t);
void *erealloc(void *, size_t);
int execute(char **);
void fatal(char *, char *, int );
int process();
#endif //LINUX_SMSH_H
splitline.c
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//
// Created by xm on 2022/3/1.
//
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include "smsh.h"
char * next_cmd(char *prompt, FILE *fp)
{
char *buf ; /* the buffer */
int bufspace = 0; /* total size */
int pos = 0; /* current position */
int c; /* input char */
printf("%s", prompt); /* prompt user */
while( ( c = getc(fp)) != EOF ) {
/* need space? */
if( pos+1 >= bufspace ){ /* 1 for \0 */
if ( bufspace == 0 ) /* y: 1st time */
buf = emalloc(BUFSIZ);
else /* or expand */
buf = erealloc(buf,bufspace+BUFSIZ);
bufspace += BUFSIZ; /* update size */
}
/* end of command? */
if ( c == '\n' )
break;
/* no, add to buffer */
buf[pos++] = c;
}
if ( c == EOF && pos == 0 ) /* EOF and no input */
return NULL; /* say so */
buf[pos] = '\0';
return buf;
}
/**
** splitline ( parse a line into an array of strings )
**/
#define is_delim(x) ((x)==' '||(x)=='\t')
char ** splitline(char *line)
/*
* purpose: split a line into array of white-space separated tokens
* returns: a NULL-terminated array of pointers to copies of the tokens
* or NULL if line if no tokens on the line
* action: traverse the array, locate strings, make copies
* note: strtok() could work, but we may want to add quotes later
*/
{
char *newstr();
char **args ;
int spots = 0; /* spots in table */
int bufspace = 0; /* bytes in table */
int argnum = 0; /* slots used */
char *cp = line; /* pos in string */
char *start;
int len;
if ( line == NULL ) /* handle special case */
return NULL;
args = emalloc(BUFSIZ); /* initialize array */
bufspace = BUFSIZ;
spots = BUFSIZ/sizeof(char *);
while( *cp != '\0' )
{
while ( is_delim(*cp) ) /* skip leading spaces */
cp++;
if ( *cp == '\0' ) /* quit at end-o-string */
break;
/* make sure the array has room (+1 for NULL) */
if ( argnum+1 >= spots ){
args = erealloc(args,bufspace+BUFSIZ);
bufspace += BUFSIZ;
spots += (BUFSIZ/sizeof(char *));
}
/* mark start, then find end of word */
start = cp;
len = 1;
while (*++cp != '\0' && !(is_delim(*cp)) )
len++;
args[argnum++] = newstr(start, len);
}
args[argnum] = NULL;
return args;
}
/*
* purpose: constructor for strings
* returns: a string, never NULL
*/
char *newstr(char *s, int l)
{
char *rv = emalloc(l+1);
rv[l] = '\0';
strncpy(rv, s, l);
return rv;
}
void
freelist(char **list)
/*
* purpose: free the list returned by splitline
* returns: nothing
* action: free all strings in list and then free the list
*/
{
char **cp = list;
while( *cp )
free(*cp++);
free(list);
}
void * emalloc(size_t n)
{
void *rv ;
if ( (rv = malloc(n)) == NULL )
fatal("out of memory","",1);
return rv;
}
void * erealloc(void *p, size_t n)
{
void *rv;
if ( (rv = realloc(p,n)) == NULL )
fatal("realloc() failed","",1);
return rv;
}
2. shell 编程
2.1 一个简单的例子来了解shell编程
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BOOK=/home/xm/test01/phonebook.data
echo find what name in phonebook
read NAME
if grep $NAME $BOOK > /tmp/pb.tmp
then
echo Entries for $NAME
cat /tmp/pb.tmp
else
echo No entries for $NAME
fi
rm /tmp/pb.tmp
变量 定义了 NAME 与 BOOK 两个变量。 用前缀
$来取得变量。用户输入 read命令告诉shell 从标准输入得到一个字符串
控制 if else
2.2 shell 的流程控制
一个demo：
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if date|grep Fri
then
echo time for backup. Insert tape and press enter
read x
tar cvf /dev/tape/home
fi
在date中查找字符串Fri，如果找到则命令成功，如果失败则命令失败。
程序如何表示成功？
grep 程序调用函数exit(0) 表示成功。 所有Unix程序都0退出表示成功。
diff 命令用来比较两个文本文件，相同返回0；
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ls
who
if diff filel file1.bak
then
echo no differences found,removing backup
rm file1.bak
else
echo backup differs,making it read-only
chmod -w file1.bak
fi
date
if 是如何工作的
- shell 运行if之后的命令
- shell 检查命令exit状态
- exit的状态为0，意味着成功，非0意味着失败
- 如果成功，shell执行then部分代码
- 如果失败，shell执行else部分代码
- 关键字fi标识if块的结束
3. 自己编写的shell 终端如何处理shell脚本（有点类似编译原理）
shell 脚本先略