command line - first EOF not working? - Ask Ubuntu

i have ubuntu 16.04 lts , when run in terminal md5sum waits input , enter hello when press ctrl+d not terminate , have press ctrl+d second time terminate that! why?

in unix, objects can read , write - ordinary files, pipes, terminals, raw disk drives - made resemble files.

a program cat reads standard input this:

n = read(0, buffer, 512); 

which asks 512 bytes. n number of bytes read, or -1 if there's error.

if did repeatedly ordinary file, you'd bunch of 512-byte reads, shorter read @ tail end of file, 0 if tried read past end of file. so, cat run until n <= 0.

reading terminal different. after type in line, terminated enter key, read returns line.

there few special characters can type. 1 ctrl-d. when type this, operating system sends of current line you've typed (but not ctrl-d itself) program doing read. , here's serendipitous thing: if ctrl-d first character on line, program sent line of length 0 - program see if got end of ordinary file. cat doesn't need differently, whether it's reading ordinary file or terminal.

another special character ctrl-z. when type it, anywhere in line, operating system discards whatever you've typed until point , sends sigtstp signal program, stops (pauses) , returns control shell.

so in example

$ cat > file.txt pa bam pshhh<ctrl+z> [2]+  stopped         cat > file.txt 

you typed characters discarded, cat stopped without having written output file.

$ cat > file.txt pa bam pshhh <ctrl+z> [2]+  stopped         cat > file.txt 

you typed in 1 line, cat read , wrote output file, , ctrl-z stopped cat.

^{source: ctrl+d ending terminal line input answer mark plotnick}