I have this script:
#!/bin/bash
exec 4>&1
nc localhost 100 0<&4 &
echo hello
echo ....
I just want to send hello and other things to netcat but that approach doesn't work.
I know echo "hello" | nc localhost works but I don't need it, same for "nc localhost 0<file.txt echo "hello" >file.txt.
Named pipe also work, but I'd want to know if something easy like above can be achieved.
I suggest you try Bash's coprocesses:
coproc nc ( netcat localhost 100 )
The above command starts netcat in the background and links its standard input to file descriptor ${nc[1]} and its standard output to ${nc[0]}.
You can then manipulate those file descriptors in order to replace your current stdin and stdout:
#!/bin/bash
coproc nc ( netcat localhost 100 )
# Current stdin and stdout are saved in old_stdin and old_stdout,
# then they are replaced with the input and output of the coprocess
exec {old_stdin}<&0 {old_stdout}>&1 <&${nc[0]} >&${nc[1]}
echo "This is sent to netcat"
read reply
# We restore the former stdin and stdout
exec <&$old_stdin >&$old_stdout
echo "This was received from netcat: $reply"