How do I make a programme in C which mimic the grep command and it shoul also run in linux...only the last line in file is printing 2 times..

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write a c programme to mimic "grep" command which is in linux...and the error is if we search a pattern which is in a file...if that pattern is in last line then it is searching and printing 2 times ...exept last line if we search the pattern which is in another line it is printing exactly that line 1 time

What I have tried:

#include<stdio.h>
#include<string.h>
void main()
{
char fn[30],pat[30],temp[200];
FILE *fp;
printf("Enter file name\n");
scanf("%s",fn);
printf("Enter pattern to be searched\n");
scanf("%s",pat);
fp=fopen(fn,"r");
while(!feof(fp))
{
fgets(temp,1000,fp);
if(strstr(temp,pat)!=NULL)
printf("%s",temp);
}
fclose(fp);
}

Posted 19-May-20 1:34am

sai charan_12

Updated 19-May-20 18:33pm

v3

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Richard MacCutchan 19-May-20 8:51am

You have allocated 200 characters for temp, but then you call fgets and tell it that temp is 1000 characters long. Use #defines to set array sizes so you use the same value everywhere.

k5054 19-May-20 10:50am

Better yet, use getline() e.g.
#include <stdio.h>
#include <stdlib.h>

char *input = NULL;
size_t len = 0
ssize_t input_len;

input_len = getline(&input, &len, stdin);
/* ... process input ... */
free(input);

Richard MacCutchan 19-May-20 11:01am

And I don't think this was meant for me either. Are you having a bad day?

sai charan_12 19-May-20 11:06am

can i get a exact code for this answer please

sai charan_12 19-May-20 11:07am

i tried it a lot but i am not able to code it so please ...

Rick York 19-May-20 15:33pm

I wrote my own grep many, many years ago and I didn't bother with the fancy expression evaluation. I just accepted an argument for case sensitive match or not which is quite useful. I then added a simple wildcard matcher to accept varying file name patterns. That was really helpful too.

I recommend starting with a bit simpler implementation, something along the line of what I described.

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phil.o · Answer 1 · 2020-05-19T01:47:00

Solution 1

Start by documenting yourself about grep command, so that you can make a list of functionalities to replicate:
linux grep[^]

Then document yourself about regular expressions in C:
c regular expressions[^]

When you have accomplished former tasks, then you can start thinking about how you are going to implement it.

Posted 19-May-20 1:47am

phil.o

Comments

sai charan_12 19-May-20 7:48am

actually i am getting the output a small error can u help me

phil.o 19-May-20 7:51am

No, because you did not provide any information about what you have tried, nor what this error is.

sai charan_12 19-May-20 7:58am

sir i have given the code what i tried and i have mentioned the error in the programme now sir

OriginalGriff · Answer 2 · 2020-05-19T01:48:00

Start by reading what grep does: The Grep Command Tutorial With Examples For Beginners - OSTechNix[^] should be a start, but it's a complicated program, and once you've handled that, you will probably want to work with most of these as well: grep tutorial - Google Search[^]

Once you fully understand it, you can start to work one the "heart" of grep - the Regular Expression processor.
C doesn't have one of those built in, so you'll probably need to write your own. More reading will be required: Let’s Build a Regex Engine | Alexander Grebenyuk[^]

Do not expect this to be a "quick project" - I'd be looking at weeks to get it right myself, so a relative beginner should plane for a considerable amount more time!

Good luck!

Rick York · Answer 3 · 2020-05-19T18:34:00

Here's how I would read that file :

C++

#include<stdio.h>
#include<string.h>

#define BUFFER_LENGTH = 511;

int main( int ac, char * av[] )
{
    char lineBuffer[BUFFER_LENGTH+1];
    FILE *fp = NULL;
    int count = 0;

    if( ac < 3 )
    {
        fprintf( stderr, "A string pattern and a file name are required\n" );
        return 1;
    }

    fp = fopen( av[2], "r" );
    if( ! fp )
    {
        fprintf( stderr, "Error - unable to open %s\n", av[2] );
        return 2;
    }

    while( fgets( lineBuffer, BUFFER_LENGTH, fp ) )
    {
        if( strstr( lineBuffer, av[1] ) )
        {
            printf( "%s", lineBuffer );
            ++count;
        }
    }
    fclose( fp );
    printf( "found %d occurrences\n", count );
    return 0;
}

How do I make a programme in C which mimic the grep command and it shoul also run in linux...only the last line in file is printing 2 times..

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