How to properly write a string to a named pipe and the difference between echo and printf

Question

Problem:

I need to send a command to a named pipe and need to ensure only the command is sent and not any errant or "fat-fingered" characters.

Example:

I have been using the following:

echo "command" > /path/to/namedPipe

This works fine, but I noticed something odd, that it is possible to send errant keystrokes to the pipe. For example:

echo "command" > /path/to/namedPipe straycharacters

then both <command> and <straycharacters> are written to namedPipe. I would not have guessed this would happen, but in some cases it can really mess up my program. I also noticed echo does not seem to require quotes. My command does not require quotes, just the actual <command>. What would be the best practice to write only <command> to the pipe?

Possible Solution:

printf "command" > /path/to/namedPipe straycharacters

In my terminal this only prints out <command> and not <straycharacters>, and so far it appears to be compatible with my program. However it is a little different than echo because it seems to not include a new line.

Answer 1

This command

echo "command" > /path/to/namedPipe straycharacters

is equivalent to

echo "command" straycharacters >/path/to/namedPipe

(compare this) and echo prints all its operands, so in this case straycharacters as well. Similarly your "working" solution is equivalent to

printf "command" straycharacters >/path/to/namedPipe

and it "works" only because

• command apparently doesn't include fragments that would be interpreted by the shell in a non-literal fashion (like ${PATH});
• command apparently doesn't include fragments that would be interpreted by printf in a non-literal fashion; note command is interpreted as format (see man 1 printf for details); if there was %s there, straycharacters would be substituted.

The right way to pass command (and a newline) is

printf '%s\n' 'command' >/path/to/namedPipe    # straycharacters

where

• \n is responsible for printing a newline character;
• straycharacters appear in the comment;
• all strings that should should be passed to printf literally are single-quoted (although you may want to double-quote command, e.g. if it relies on parameter substitution by design).

Having "errant or fat-fingered characters" in your code is wrong. Instead of trying to find syntax that ignores them, just remove this garbage and pay attention. This will make your code clean and nice.

Still, even without straycharacters, echo is not a right tool. See Why is printf better than echo?