Reading a file from a specified line to another

Question

I currently have a shell script in C that takes a command of a file, two numbers, mth line and nth line, and supposedly should output whats in between those lines:

for example:

./output file1 2 6

would output file from line 2 to line 6

I have it implemented currently in a way of outputting the whole file, I have been trying to change it to output specifically between those lines

this is my code

#include <fcntl.h>

int main(int argc, char *argv[])
{
    int file;
    int size;
    int l[1000];

    int firstNum = atoi(argv[2]);

    int secondNum = atoi(argv[3]);


    file = open(argv[1], O_RDONLY);

    if( file == -1)
    {
        printf("\n cannot open file \n");
    }

    while ((size=read(file,l,80)) > 0)
        write(1,l,size);




}

I tried to change l and size to firstNum and secondNum, which are the numbers entered from the command line, but still did not work and outputted one single line.

What is a better way of doing so ?

Much the simplest is `char cmd[256]; snprintf(cmd, sizeof(cmd), "sed -n '%s,%sp %s", argv[2], argv[3], argv[1]); system(cmd);`. Run `sed` to print only between the two line numbers passed as arguments on the file specified. — Jonathan Leffler, Feb 04 '16 at 04:53

artm · Accepted Answer · 2016-02-04T04:55:05.777

There are several issues with your code, so just go through them sequentially:

It's better to use high level fopen rather than low level open to open a file. So it's better to write this way:
```
FILE *file = fopen(argv[1], "r");
if (file == NULL)
    exit(EXIT_FAILURE);
```

Your read is wrong as it reads exactly 80 characters instead of a line as you expect.

while ((size=read(file,l,80)) > 0)   // <-- WRONG because this reads 80 chars instead of one line

For similar reason as with open, it's better to use alternative like printf instead of low level read and write.
To read line by line, you should use library function getline.
To control what line number to print, a simple way is to have a variable tracking what line number and compare with your command line arguments.

So put them together, you would need something like this:

FILE *file = fopen(argv[1], "r");
if (file == NULL)
    exit(EXIT_FAILURE);

int line_num = 0;

char * line = NULL;
size_t len = 0;
ssize_t read = 0;
while ((read = getline(&line, &len, file)) != -1)
{
    line_num++;
    if( firstNum <= line_num && line_num <= secondNum )
    {
        printf( "line %d: %s", line_num, line );
        if( line_num == secondNum )
            break;
    }
}

you should break out of the loop when you reach `secondNum` – Barmar Feb 04 '16 at 04:51 — Barmar, Feb 04 '16 at 04:51

score 0 · Answer 2 · edited May 23 '17 at 11:59

0

Try looking at this post: C read file line by line

From there it should be a simple matter of counting lines read and discarding them until you have read firstNum lines at which point print until you've reached secondNum.

edited May 23 '17 at 11:59

Community

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answered Feb 04 '16 at 04:30

Michael Albers

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Reading a file from a specified line to another

2 Answers2