Reverse a hexadecimal number in bash

Process substitution results in a special file (like /dev/fd/63 in your example) that behaves like the read end of a named pipe. This file can be opened and read, but not written, not seeked.

Commands that treat their arguments as pure streams work while commands that expect to seek in files they are given (or write to them) won't work. The kind of command that will work is what is usually considered a filter: cat, grep, sed, gzip, awk, etc... An example of a command that won't work is an editor like vi or a file operation like mv.

gcc wants to be able to perform random access on its input files to detect what language they are written in. If you instead give gcc a hint about the input file's language, it's happy to stream the file:

gcc -x c <(echo 'int main(){return 0;}')

The simpler more straightforward form without process substitution also works:

echo 'int main(){return 0;}' | gcc -x c -

Note that this is not specific to bash. All shells that support process substitution behave the same way.

For the hostname -d, use hostname -f:

hostname -f | sed -e 's/^[^.]*\.//'

For IP-addresses, use ifconfig -a (look for the inet data). Your machine may have only one network device, en0, so you could do just

ifconfig en0 |awk '/inet / {print $2; }'

If you are interested in all of the network devices, keep in mind that ifconfig -l lists the devices. This lists the devices and their correspond addresses:

#!/bin/sh
for name in $(ifconfig -l)
do
    ifconfig $name |awk -v name=$name '/inet / {printf "%s: %s\n", name, $2; }'
done

Further reading:

• How do I find my IP Address from the command line?
• hostname -- set or print name of current host system