cat -b和cat -n和有什么区别-cat -s有什么区别

Question

I am currently developing a cat command in C and unix. My task is design cat -b and cat -s commands this time. However I have already have created other commands such as -t, -v, -n, -e. I got stuck at command -b as whenever I trigger the command it returns the same output what -n returns.

for example

cat -n hello.c

1  #include<studio.h>
2    int main() {
3      printf("hello");
4      return 0;
5   }

cat -b hello.c

1  #include<studio.h>
2    int main() {
3      printf("hello");
4      return 0;
5   }

My second question is how cat -s does work? as I again fired the command but the output displays a simple c file without any change.

for instance

cat -s hello.c

 #include<studio.h>
   int main() {

    printf("hello");

     return 0;
 }

Thanks in advance and for your time.

Answer 1

Let's start with a simple, six-line, file called qq.in , with two non-empty lines, two empty lines and another two non-empty:

a
b


c
d

The difference between cat -n and cat -b is that the former will number all lines while the latter will number only non-empty lines:

$ cat -n qq.in       $ cat -b qq.in
     1  a                 1  a
     2  b                 2  b
     3                    
     4                    
     5  c                 3  c
     6  d                 4  d

On the other hand, cat -s will collapse consecutive empty lines into a single empty line, as per the following (combined with -n so the effect is obvious):

$ cat -ns qq.in
     1  a
     2  b
     3
     4  c
     5  d

Note that the lines must be truly empty, these flags won't do what you expect if the lines contain a space or some other white-space. If you want to treat white-space lines as empty, you can filter them first with something like:

sed -E 's/^\s+$//' qq.in | cat -b

Answer 2

Question:

What is the difference between cat -b and can -n and how cat -s does work

From the output of man cat :

  -b, --number-nonblank number nonempty output lines, overrides -n -n, --number number all output lines -s, --squeeze-blank suppress repeated empty output lines 

When you use cat -b , the empty lines will not be numbered.

Here' what I see with a simple file:

cat -b socc.cc

1  #include 

     2  int main()
     3  {
     4     return 0;
     5  }

cat -n socc.cc

1  #include 
     2  
     3  int main()
     4  {
     5     return 0;
     6  }
     7  
     8

cat -s socc.cc

#include 

int main()
{
   return 0;
}

Answer 3

  -b, --number-nonblank number nonblank output lines -n, --number number all output lines 

ref: cat manpage

and the second question, from man page:

-s, --squeeze-blank

  never more than one single blank line 

you can implement this feature like this

1. read the file into a buffer
2. check the buffer whether it contains continuous blank lines, eg "\\n\\n\\n"
3. put them together, use only one "\\n" instead of a set of it