Unexpected behaviour of watch command

Question

The following command which I want to use to monitor the number of files in a directory does not show the required value dynamically.

watch "echo \`ls -l| wc -l\`"

However following command works well.

watch "ls -l| wc -l"

What is the explanation for this behavior?

Answer 1

It's because the shell expands backticks before watch is ever executed. If you use set -x, you'll see that the command that actually ends up calling

watch "echo 42"

which explains why it's always the same.

You can escape the ` with a backslash to pass it literally:

watch "echo \`ls -l | wc -l\`"

Or more easily by using single quotes, which inhibit all expansions:

watch 'echo `ls -l | wc -l`'