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How can I get the source code for the linux utility tail?

Posted by: admin November 29, 2017 Leave a comment

Questions:

this command is really very useful but where I can get the source code to see what is going on inside .

thanks .

Answers:

The tail utility is part of the coreutils on linux.

I’ve always found FreeBSD to have far clearer source code than the gnu utilities. So here’s tail.c in the FreeBSD project:

Questions:
Answers:

Poke around the uclinux site. Since they distributed the software, they are required to make the source available one way or another.

Or, you could read man fseek and guess at how it might be done.

NB– See William’s comments below, there are cases when you can’t use seek.

Questions:
Answers:

You might find it an interesting exercise to write your own. The vast majority of the Unix command-line tools are a page or so of fairly straightforward C code.

To just look at the code, the GNU CoreUtils sources are easily found on gnu.org or your favorite Linux mirror site.

Questions:
Answers:
/`*This example implements the option n of tail command.*/`

    #define _FILE_OFFSET_BITS 64
    #include <stdio.h>
    #include <stdlib.h>
    #include <fcntl.h>
    #include <errno.h>
    #include <unistd.h>
    #include <getopt.h>

    #define BUFF_SIZE 4096

    FILE *openFile(const char *filePath)
    {
      FILE *file;
      file= fopen(filePath, "r");
      if(file == NULL)
      {
        fprintf(stderr,"Error opening file: %s\n",filePath);
        exit(errno);
      }
      return(file);
    }

    void printLine(FILE *file, off_t startline)
    {
      int fd;
      fd= fileno(file);
      int nread;
      char buffer[BUFF_SIZE];
      lseek(fd,(startline + 1),SEEK_SET);
      while((nread= read(fd,buffer,BUFF_SIZE)) > 0)
      {
        write(STDOUT_FILENO, buffer, nread);
      }
    }

    void walkFile(FILE *file, long nlines)
    {
      off_t fposition;
      fseek(file,0,SEEK_END);
      fposition= ftell(file);
      off_t index= fposition;
      off_t end= fposition;
      long countlines= 0;
      char cbyte;

      for(index; index >= 0; index --)
      {
        cbyte= fgetc(file);
        if (cbyte == '\n' && (end - index) > 1)
        {
          countlines ++;
          if(countlines == nlines)
          {
        break;
          }
         }
        fposition--;
        fseek(file,fposition,SEEK_SET);
      }
      printLine(file, fposition);
      fclose(file);
    }

    int main(int argc, char *argv[])
    {
      FILE *file;
      file= openFile(argv[2]);
      walkFile(file, atol(argv[1]));
      return 0;
    }

    /*Note: take in mind that i not wrote code to parse input options and arguments, neither code to check if the lines number argument is really a number.*/