What information is provided by the echo $$ command?

Question

Options

• AThe process ID of the current shell.
• BThe process ID for the following command.
• CThe process ID of the last command executed.
• DThe process ID of the last command which has been placed in the background.
• EThe process ID of the echo command.

• A
  86% (19)
• B
  9% (2)
• C
  5% (1)

Explanation

$$ is a special shell variable that holds the process ID (PID) of the current shell itself - so echo $$ prints that PID, making A correct.

Why the distractors are wrong:

• B is wrong because no "following command" PID exists at the time echo $$ runs - $$ never looks ahead.
• C describes $!... wait, no - $? holds the exit status of the last command, and $! holds the PID of the last backgrounded command; neither is $$.
• D describes $!, the special variable for the last background job's PID - a common confusable.
• E is wrong because $$ is evaluated by the shell before echo even runs - the echo process itself is never captured in $$.

Memory tip: Think of $$ as "self" - double-dollar, double-self. The shell telling you its own ID. Contrast with $! (the ! = "background bang") for background PIDs.

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