Part 2: Enable the Shell to read commands

This article is part <2> of the article "writing a Shell" series. In the previous article, we have completed the first step of our shell: the shell prompt. In this article, we begin to enable our Shell to start reading commands, and to do simple participle, separating commands from parameters.

Read command 0x00 use of ReadLine Library

The library we use to implement the Read command is readline that we can use the fgets() method to achieve the goal, but if we use the fgets() shell when we enter the wrong command cannot be revoked by SPACEBAR, because fgets() it is read by character.

0x01 Install ReadLine Library

sudo apt-get install libreadline-dev

0x02 using the ReadLine library

If we use the library in our program, we need to link to the code when we compile it.libreadline

Cases:

gcc -o test test.o -I /usr/include -lreadline

Processing command 0x00 Implementation ideas

For a command

cmd para1 para2 […… paraN]

We want it to be identified as:

Command: cmd
Parameter 0: cmd
Parameter 1: para1
Parameter 2: para2
......
Parameter n:paraN

In fact, the idea is very simple:

We need a command pointer to store the command, an array to store the command.

It takes two pointers to move along our entire instruction to get every command or parameter.

At the outset:

Two pointers *start , *end all pointing to the beginning of an instruction

If *start , *end pointing to a space, automatically moves backwards

Define a counter count , and the *end pointer starts to move backwards until it touches a space or line break or terminator

If count 0, indicates that the command is currently being processed, a temporary pointer is set, *p *end *start and it is moved along, assigned to *command and parameter[0] , count + 2.

If the *end line break or Terminator is stopped, it is assigned, *end *start and then *end continues to move backwards as described above until a space is encountered, which is assigned *start parameters[count-1] , count + 1.

Repeat until the *end Terminator \0 or line break is encountered\n

0X01 Implementation Code

#include "lshell.h"#include <readline/readline.h>#include <readline/history.h>intRead_command (Char**command,Char**parameters,Char*prompt) { Free(buffer); Buffer = ReadLine (prompt);if(Feof (stdin)) {printf("\ n");Exit(0); }if(buffer[0] ==' + ')    {return-1; }intCount =0;Char*start, *end;intisfinished =0; Start = end = buffer; while(isfinished = =0)    { while(*start = ="'&& *end = ="') || (*start = =' \ t '&& *end = =' \ t ')) {start++;        end++; }if(*end = =' + '|| *end = =' \ n ')        {if(Count = =0)            {return-1; } Break; } while(*end! =' + '&& *end! =' \ n '&& *end! ="') {end++; }if(Count = =0){Char*p = end; *command = start; while(P! = Start && *p! ='/') {p--; }if(*p = ='/') {p++; } parameters[0] = p; Count + =2;#ifdef DEBUG            printf("\ncommand:%s\n", *command);#endif //DEBUG }Else if(Count <= Max_args) {parameters[count-1] = start;        count++; }Else{ Break; }if(*end =' + '|| *end = =' \ n ') {*end =' + '; isfinished =1; }Else{*end =' + ';            end++;        start = end; }} parameters[count-1] = NULL;#ifdef DEBUG    printf("Input analysis:\n");printf("command:[%s]\ncommand:[%s]\nparameters:\n", *command,parameters[0]);intI for(i=0; i<count-1; i++)printf("[%s]\n], Parameters[i]);#endif    returnCount;}

Full code See:https://github.com/luoyhang003/linux_kernel_expriment/tree/master/exp2

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Part 2: Enable the Shell to read commands