How to get bash not to treat [ as a test character when using command substitution? [duplicate]

Question

How can I get unmodified output from $() in bash?

I need to have bash return the results of command substitution (via $(command) or `command`), but bash treats the left square bracket as the test builtin and evaluates the text.

> echo "This shows an IP in brackets [127.0.0.1] when not in the command substitution context."
This shows an IP in brackets [127.0.0.1] when not in the command substitution context.

> echo $(echo "This should show an IP in brackets [127.0.0.1] but instead evaluates to '1'.")
This should show an IP in brackets 1 but instead evaluates to '1'.

> echo $(echo "The subtle addition of a space causes [127.0.0.1 ] to behave differently.")
The subtle addition of a space causes [127.0.0.1 ] to behave differently.

Background

Please note that the use of echo above to provide a minimal example is just that; my ultimate use will be including output from ping or tracert to show the IP of a host as seen by that host (i.e. respecting any hosts entries). In this usage, I have little control over the string being output by the command.

> grep -E "^[^#].*stackoverflow" /c/Windows/System32/drivers/etc/hosts
127.0.0.1 stackoverflow.com

> tracert -h 1 -w 1 stackoverflow.com | awk '/Tracing/'
Tracing route to stackoverflow.com [127.0.0.1]

> ping -n 1 -w 1 stackoverflow.com | awk '/Pinging/'
Pinging stackoverflow.com [127.0.0.1] with 32 bytes of data:

# Using dig or nslookup would show a real IP rather than the forced IP from the hosts entry.
> dig @ns1 +nocmd stackoverflow.com +noall +answer
stackoverflow.com.      118     IN      A       151.101.129.69
stackoverflow.com.      118     IN      A       151.101.193.69
stackoverflow.com.      118     IN      A       151.101.1.69
stackoverflow.com.      118     IN      A       151.101.65.69

> echo $(ping -n 1 -w 1 stackoverflow.com | awk '/Pinging/')
Pinging stackoverflow.com 1 with 32 bytes of data:

> echo $(ping -n 1 -w 1 stackoverflow.com | awk '/Pinging/ {print $3}')
1

> ping -n 1 -w 1 stackoverflow.com | awk '/Pinging/ {print $3}'
[127.0.0.1]

Answer 1

The problem is that the command substitution is not itself quoted. When command substitution uses the output of the echo command as the argument for the second echo command, the command results are subjected to both word splitting and file globbing. As pointed out in the comments, the characters within square brackets, [127.0.0.1] will match any files whose name is exactly one character and is one of either 1, 2, 7, or 0 (the pattern would match . only the shell ignores both the . and .. directory links). When I created a file called 1 I could replicate your result.

To avoid the results of the command substitution being subjected to globbing, you should wrap it in double quotes:

echo "$(echo "This will show an IP address in square brackets: [127.0.0.1]")"

See also the following related articles from Greg‘s Wiki (particularly the first one which covers the need for double-quoting both within and outside the command substitution):

• Command Substitution
• Quotes
• Globs