To replace empty spaces in a string with a different character or string in Bash, you can use the sed command. For example, to replace empty spaces in a string with underscores, you can use the following sed command:
$ sed 's/ /_/g' <<< "Hello world!" Hello_world!
This will replace all empty spaces in the string with underscores, and print the resulting string to standard output.
If you want to replace empty spaces in a string with a different character or string, you can simply modify the sed command by specifying the character or string you want to use in place of the empty spaces. For example, to replace empty spaces in a string with the pipe (|) character, you can use the following sed command:
$ sed 's/ /|/g' <<< "Hello world!" Hello|world!
This will replace all empty spaces in the string with the pipe character, and print the resulting string to standard output.
Overall, the sed command is a powerful tool for replacing characters or strings in a string in Bash. By using the s/old/new/g syntax, you can easily and quickly replace any character or string with a different one, allowing you to manipulate strings in a variety of ways.