Special shell parameters — process ID of the current shell In POSIX/Bourne-compatible shells, which special parameter expands to the process number (PID) of the current shell instance?

Difficulty: Easy

$!
$0
$*
None of the above

Correct Answer:

Explanation:

Introduction / Context:
Shell scripting often needs the process ID (PID) to create unique filenames, manage lock files, or send signals. POSIX shells provide built-in special parameters that expand to useful values without invoking external commands.

Given Data / Assumptions:

Shell is POSIX/Bourne-compatible (sh, bash, ksh, dash).
We need the PID of the current shell process.
We are not asking for PIDs of background jobs or parent processes.

Concept / Approach:

The special parameter expands to the PID of the current shell. Other related parameters include $! (PID of the most recently executed background pipeline), $0 (name of the shell or script), and $* (all positional parameters).

Step-by-Step Solution:

Echo the parameter: echo to see the PID.Use it in filenames:

tmpfile=/tmp/app.

toavoidcollisions.Optionallyconfirmwith

ps−p

toavoidcollisions.Optionallyconfirmwithps−p to see the matching process.Remember $! is different (last background job PID).

Verification / Alternative check:

In an interactive shell, compare echowiththePIDreportedbypsforyourshellprocess.Insideasubshell,observethatwiththePIDreportedbypsforyourshellprocess.Insideasubshell,observethat can change because it refers to that subshell's PID.

Why Other Options Are Wrong:

$!: PID of the most recent background job, not the current shell.
$0: script or shell name, not a PID.
$*: all positional parameters as a single word.
None of the above: incorrect because iscorrect.

CommonPitfalls:

Usingiscorrect.

CommonPitfalls:

Using inside subshells without realizing it changes; confusing $! and ;forgettingtoquotewhenembeddinginpathswithothervariables.

Special shell parameters — process ID of the current shell In POSIX/Bourne-compatible shells, which special parameter expands to the process number (PID) of the current shell instance?

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