Linux命令每天成长快乐之cut和popen

cut是一个选取命令，以行为单位，用指定分隔符将行切割为若干字段，选取所需要的字段。

默认是tab分隔符，可通过-d指定分隔符。

-f指定选取第几段，从1开始。

示例：fdisk -l | cut -d " " -f 1 | grep dev

意思就是将fdisk -l的结果按每行进行处理，以空格分割，选取第一个字段，如果该字段里有dev字符串，则显示出来。

popen命令与system类似，可以执行shell命令。但是特殊之处在于可以将shell执行的结果（标准IO流）进行处理。

我们看下man的说明：

The  popen()  function  opens  a process by creating a pipe, forking, and invoking the shell.  Since a pipe is by definition unidirectional, the type
       argument may specify only reading or writing, not both; the resulting stream is correspondingly read-only or write-only.


       The command argument is a pointer to a null-terminated string containing a shell command line.  This command is passed to /bin/sh using the -c  flag;
       interpretation,  if  any, is performed by the shell.  The type argument is a pointer to a null-terminated string which must contain either the letter
       'r' for reading or the letter 'w' for writing. 

The return value from popen() is a normal standard I/O stream in all respects save that it must be closed with pclose() rather than fclose(3).  Writ‐
       ing  to  such  a  stream  writes  to  the standard input of the command; the command's standard output is the same as that of the process that called
       popen(), unless this is altered by the command itself.  Conversely, reading from a "popened" stream reads the command's standard output, and the com‐
       mand's standard input is the same as that of the process that called popen().

示例：

#define _LINE_LENGTH 300
int main(void) 
{
    FILE *file;
    char line[_LINE_LENGTH];
    file = popen("ls", "r");
    if (NULL != file)
    {
        while (fgets(line, _LINE_LENGTH, file) != NULL)
        {
            printf("line=%s\n", line);
        }
    }
    else
    {
        return 1;
    }
    pclose(file);
    return 0;
}