What happens if a variable is assigned with command expression in backticks

Question

Below is the code of bash:

a=`echo hello`
echo $a

output is :

hello

But I think it should be:

hello
0

Answer 1

You think wrong ;-)

Putting the command in backticks assigns the output (stdout) from the expression on the right to the variable on the left.

$? gives you the "output status" (or return code) of the command - aka the "0" you were expecting.

So:

a=`echo hello`

Runs the command "echo hello" but instead of echoing to stdout, it "echoes" to varaiable a. So a=whatever_the_command_would_have_written_to_stdout (in this case "hello") - nothing is actually written to stdout because it is "captured" by the ``s

Answer 2

You mistakenly think that a=`echo hello`:

• executes echo hello and prints its stdout output directly to the caller's stdout,
• and then assigns the exit code (return value) of the echo command to variable $a.

Neither is true; instead:

• echo hello's stdout output is captured in memory (without printing to the caller's stdout; that's how command substitutions work),
• and that captured output is assigned to $a.

A command's exit code (a return value indicating success vs. failure) is never directly returned in POSIX-like shells such as Bash.
The only way to use an exit code is either:

• explicitly, by accessing special variable $? immediately after the command ($? contains the most recent command's exit code)

• implicitly, in conditionals (a command whose exit code is 0 evaluates to true in a conditional, any other exit code implies false).

Thus, to achieve what you're really trying to do, use:

echo 'hello'  # Execute a command directly (its stdout output goes to the caller's stdout)
a=$?          # Save the previous command's exit code in var. $a
echo "$a"     # Echo the saved exit code.

Answer 3

As this [ this ] answer already mentioned, the return value for the last executed command is stored in

$? # ${?} is sometimes needed

If you wish a to contain 'hello' and the return value of echo hello in separate lines, ie

hello
0

below is one way to do it

$ a=`echo -en "hello\n" && echo -n $?` # here $? is ret val for 1st echo
$ echo -e "$a"
hello
0

Note

• -n with echo suppresses the trailing new line
• -e with echo interprets escape sequences
• Since && is the logical and operator, the second echo wouldn't have been executed had the first echo failed

• Another important point to note is that even the assignment ie

  a=b
  

  has a return value.