Difference in cat sth vs cat < sth

Question

I realized that cat sth and cat < sth both happen to give same output. Are they the same?

Answer 1

They're not completely identical. In cases where it matters, the redirection approach will generally give more annoying and obscure results (of course that might be what you want though).

$ cat < /proc/self/maps
$ cat /proc/self/maps
55c61257e000-55c61258a000 r-xp 00000000 fd:00 1180143                    /usr/bin/cat
...

Or try grep, the search program

$ grep "grep" /proc/self/exe
Binary file /proc/self/exe matches
$ grep "grep" < /proc/self/exe
$

Bit of a cheat, but you're most likely to run into this when using sudo, to run commands with root access:

$ sudo cat < /etc/shadow
bash: /etc/shadow: Permission denied
$ sudo cat /etc/shadow
root:!::0:99999:7:::
....

Answer 2

cat's job is it to concatenate everything it gets and then to print it out on stdout yet often is used to print out the contents of a single file (which of course is just concatenating the contents of all the one files given to it and then writing it to stdout). stdout (standard output) is just programs usually write output data. (There are programs (like dd) which write their output data to stdout unless stated differently by parameters and there also is the standard error channel stderr which programs write to when an error occurs.)

cat can take data given to it via stdin (standard input) or data from files passed to it as parameters.

What you're doing with cat sth is to call cat with the single parameter sth telling cat to write everything in the file sth to stdout. You could add more file names and see that the content of the files is concatenated and then written to stdout.

cat < sth changes the stdin of cat to the contents of the file sth before calling cat. cat then just reads everything from its stdin and writes it to stdout.

In both cases, cat writes the contents of the file sth to stdout. You can read more about this here and also type in man cat into a terminal to read the man page of cat describing what it does.

Note that the contents of sth are read by the user you are logged in as (who you are logged in as in the terminal is not necessarily who you are logged in as in the graphical session (if any)), whereas programs can be executed with different rights by writing sudo before them and then read with the rights of the user who executed them. This can lead to different outcomes because the user you are logged in as and the user you switch to with sudo (not necessarily root, you can specify otherwise) might have different access rights on the file you're trying to read from.

Answer 3

In the specific case of cat, printing the file sth works just as well using cat sth or cat <sth, because the cat command was designed to either print all files given as command line parameters (the cat sth case: cat will receive the string sth as a parameter, and will open the file) or, if no files were given, print standard input instead (the cat <sth case: the shell will honor the redirect by opening sth and connecting it to cat's standard input).

So the behaviour being the same in both cases is specific to cat; it may not work at all for commands designed to accept only one of standard input or command line parameters. In other cases, the behavior may be slightly different, since the command may not have access to the file name when the data is obtained through standard input; compare:

$ grep -H foo sth
sth:foo

$ grep -H foo <sth
(standard input):foo