When the command echo $ outputs 1, which of the following statements is true?

 The $? variable in bash is a special parameter that holds the exit status of the last command executed in the current shell. The exit status is a numerical value that indicates whether the command was successful (zero) or failed (non-zero). The echo command simply prints its arguments to the standard output. Therefore, when the command echo $? outputs 1, it means that the previous command failed with an exit status of 1. References:

[LPI Linux Essentials - Topic 103: Command Line Basics]

[Bash Special Parameters]

[Exit status - Wikipedia]