How do I print specfic parts of a string in a file - c

Question

I'm writing a program that will read from /etc/passwd and output the username and shell.
For example, here is the first line:
root:x:0:0:root:/root:/bin/bash
I need to only output the user and the shell. In this instance it would print:
root:/bin/bash
The values are seperated by : so I just need to print the string before the first : and the string after the 6th :. How can I do that?
Here is the code I have so far:
#include <unistd.h>
#include <fcntl.h>
//printf and scanf prototype
int printf(const char *text, ...);
int scanf(const char *format, ...);
int main(void)
{
int fd;
int buff_size = 1;
char buff[512];
int size;
fd = open("/etc/passwd",O_RDONLY);
if (fd < 0)
{
printf("Error opening file \n");
return -1;
}
while ((size = read(fd,buff,1))>0)
{
buff[1] = '\0';
printf("%s",buff);
}
}
So far the code reads from /etc/passwd and prints out the whole file line by line. I need to configure the printf statement to just print the user and shell for each instance.

Answer 1

I just need to print the string before the first : and the string after the 6th :.
Filter the string, put the needed data into an output buffer, and print the output buffer.
Since you already used char buff[512]; as the original data buffer, then just create another buffer for output. i.e. char outbuff[512];
// char buff[512]; assume already defined as that in PO
// char outbuff[512]; assume already defined as that in PO
void customized_print(char* buff, char* outbuff) {
int i = 0, j = 0, cnt = 0;
while (i < 512) { // copy until first ':'
if((outbuff[j++]=buff[i++]) == ':') break;
}
// skip 5 more ':'
while (i < 512 && cnt < 5) {
if(buff[i++] == ':') cnt++;
}
// copy the remains
while (i < 512) {
if( (outbuff[j++]=buff[i++]) == '\0' ) break;
}
printf("%s\n", outbuff);
outbuff[0] = 0; // clean the outbuff
}
Another solution with a demo of full code.
If you do not need to use buff. You can use the following solution.
#include <stdio.h>
void customized_print(char* buff, char* outbuff) {
int cnt = 0;
char* p = outbuff;
while (*buff) { // copy until first ':'
if((*p++=*buff++) == ':') break;
}
// skip 5 more ':'
while (*buff && cnt < 5) {
if(*buff++ == ':') cnt++;
}
// copy the remains
while (*buff) {
if( (*p++=*buff++) == '\0' ) break;
}
printf("%s\n", outbuff);
outbuff[0]=0; // clean the outbuff
}
int main(){
char outbuff[512];
customized_print("root:x:0:0:root:/root:/bin/bash", outbuff);
customized_print("foo:x:0:0:root:/root:/bin/bash", outbuff);
return 0;
}
Compile and run:
gcc -Wall demo.c -o demo.out
./demo.out
Output:
root:/bin/bash
foo:/bin/bashh