what's the difference between cat file and cat < file - unix

Question

The way I understand this is that the command
cat file
will display the contents of file and
cat < file
will take the contents of file as input. I was trying this out and created two files. One called file1.txt and another one called file2.txt. In file1.txt, I wrote file2.txt, so by typing the command
cat < file1.txt
, I am expecting the content of file2.txt to be displayed; however , both
cat file1.txt and cat < file1.txt
are displaying the content of file1.txt. I think I am misunderstanding the definition of <.

Answer 1

When cat is run with argument files, it will open and read from each of these files to standard output. So with cat file1.txt, cat opens file1.txt and print its contents to standard output:
$ cat file1.txt
file2.txt
When cat is run with no argument files, it will read from standard input (e.g., as if you typed directly to cat) and write to standard output. But < file1.txt tells the shell (e.g., bash) to open the file file1.txt, and redirect it to cat through standard input:
$ cat < file1.txt
file2.txt
As you can see, the effect is the same, that cat will read the file given; the difference is who opens the file. In the first case, cat gets told to open the file itself, while in the second case the shell opens it, and cat doesn't know anything except that there's a stream of data coming in through its standard input, that it is supposed to print.
Finally, if you want to use the contents of file1.txt as an argument to cat, you need to use the syntax
$ cat $(< file1.txt) # same as: cat file2.txt
[...contents of file2.txt]

Answer 2

If you pass one or more file names, cat will display the contents of those files. Otherwise, it will print whatever it is passed on stdin.
cat file
cat receives one file name, opens the file, and prints its contents
cat < file
The shell redirects stdin to the named file: it opens the file and feeds its contents to cat on stdin. cat receives no arguments. Without arguments, it prints whatever's on stdin—the contents of the file.
Net result: cat file and cat < file do the same thing.

Answer 3

Let see the concept of cat from its roots, say from plan9 (the evolved roots):
# \sys\src\cmd\cat.c
#include <u.h>
#include <libc.h>
void
cat(int f, char *s)
{
char buf[8192];
long n;
while((n=read(f, buf, (long)sizeof buf))>0)
if(write(1, buf, n)!=n)
sysfatal("write error copying %s: %r", s);
if(n < 0)
sysfatal("error reading %s: %r", s);
}
void
main(int argc, char *argv[])
{
int f, i;
argv0 = "cat";
if(argc == 1)
cat(0, "<stdin>");
else for(i=1; i<argc; i++){
f = open(argv[i], OREAD);
if(f < 0)
sysfatal("can't open %s: %r", argv[i]);
else{
cat(f, argv[i]);
close(f);
}
}
exits(0);
}
It's clear from the sources that if you give < file as argment to cat it reads from stdin (f=0) and writes to stdout (1):
while((n=read(f, buf, (long)sizeof buf))>0)
if(write(1, buf, n)!=n)