Using backticks and pipes in Ruby

Question

This bug appears when using shell commands in a ruby script. The commands are not executed as what would happen in the bash terminal.

The first two commands are reasonable, the third is not.

`echo foo` # => "foo\n"
`echo -n foo` # => "foo"
`echo -n foo | cat` # => "-n foo\n"

In bash, we would have the following:

$ echo foo # => "foo\n"
$ echo -n foo # => "foo"
$ echo -n foo | cat # => "foo"

Is there a way I am messing up the parameter passing to the calls to echo in the Ruby commands? If you're unfamiliar, the command echo returns the string it is given. Without the -n flag, it appends a newline character, when you add the -n, it does not. cat repeats whatever it is given.

Answer 1

First, Ruby uses /bin/sh, not bash, for the back tick operator. It is likely that on your system, /bin/sh is a link to /bin/bash, but bash behaves differently when invoked as sh, for better POSIX compliance. However, it's not perfect, as your example shows. The POSIX specification states that echo "shall not support any options", but leaves handling of an initial argument -n to be implementation-specific.

In a quick test with /bin/sh as a link to /bin/bash, bash -c "echo -n foo" and bash -c "echo -n foo | cat" produced identical, newline-free results, but sh -c "echo -n foo" and sh -c "echo -n foo | cat" showed the results you report. (I am not sure how other shells, such as ksh, dash, or zsh, behave when invoked as sh, but by default they all treat -n as a newline suppressor.)

For predictable results, use printf instead, which never prints a newline unless a \n is included in the format string.

printf 'foo\n'
printf 'foo'
printf 'foo' | cat

UPDATE: This appears to be a bug in bash 3.2 that was fixed at some point in the 4.x series. With 4.1 or later, sh -c "echo -n foo | cat and sh -c "echo -n foo" produce the same output.