Nested echo command in backticks

Question

I want to know why an extra echo will print in my shell. I'm using bash version 4.2.46(1).

echo `echo `echo $SHELL``

A interesting issue is if I replace '``' with $() it doesn't print extra echo:

echo $(echo `echo $SHELL`)

And I found that it prints extra echos in odd echo command numbers:

echo `echo `echo `echo `echo $SHELL````

Answer 1

Your two versions:

echo `echo `echo $SHELL``

and

echo $(echo `echo $SHELL`)

are not equivalent. The first backtick command substitution ends as soon as another backtick is seen:

When the old-style backquote form of substitution is used, [...] The first backquote not preceded by a backslash terminates the command substitution.

Your first version is actually equivalent to:

echo $(echo )echo $SHELL$()

which is why you get an "echo" in the output - the command you end up running (after substitutions and with extra whitespace removed) is just:

echo echo /bin/bash

so the output is "echo /bin/bash", just like if you wrote that command out directly.

---

If you must nest backticks, you can backslash the inner pairs to escape them. Your first command could be written correctly as:

echo `echo \`echo $SHELL\``

although I wouldn't recommend it — $( ... ) is made for nesting.

Answer 2

The equivalent to

echo $(echo `echo $SHELL`)

Or, even better:

echo $(echo $(echo $SHELL))

In backticks will be:

$ echo `echo \`echo $SHELL\``
/bin/sh

And that is exactly the main problem of backticks:

 Where exactly does a backtick start or end?

That is why the shell does not understand the line correctly. The shell understand the line to be:

echo echo `echo $SHELL`