Files along with their contents:-
~$cat a
aaa
aaa
aaa
~$cat b
bbb
bbb
bbb
~$cat c
ccc
ccc
ccc
Commands that I need explanation:-
~$cat 0< a
aaa
aaa
aaa
~$cat 0< a b
bbb
bbb
bbb
~$cat 0< a b c
bbb
bbb
bbb
ccc
ccc
ccc
I need an explanation as to why contents of file 'a' were printed when I executed ~$cat 0< a and why the contents of file 'a' were not printed when I executed ~$cat 0< a b or ~$cat 0< a b c.
cat reads stdin if run without arguments. If you give it arguments, it will read the arguments and not stdin.
If you want it to read both its argument files and stdin, make /dev/stdin one of the arguments (according to where you want it to go in the concatenation):
cat 0<a /dev/stdin b c
Notes:
- or /dev/fd/0 also works0< can simply be <
- is better since it doesn't rely on the explicit existence of those special device files. (Usually those are there, but during system startup or installation, or if /proc isn't mounted, they might not be.) So, to the OP: cat - b c <a will print out file a then b then c. You could mix it up: cat a - c < b which will print out the files in the same order as before.
/dev devices. It's cleaner. No special treatment for a magic argument of - dependent on each individual application. Getting rid of the - = stdin convention is one of the reasons why those dev devices were created.
cat -, however, is defined by the POSIX standard. Neither /dev/stdin nor /dev/fd/0 is.
0<is equivalent to just plain<, and so nobody ever says0<. (2) Nobody else seems to have explained the real answer to your question (or, at least, the entire answer), namely, your commands are equivalent tocat < a,cat b < a, andcat b c < a, because I/O redirection is not an argument, and is not visible directly to the command, and can be placed anywhere in the command line. So, in the first case,catis being run with no arguments, while in the other cases it is being run with filename argument(s).