Difficulty: Easy
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:
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.ps−p
toavoidcollisions.ps−p to see the matching process.Remember $! is different (last background job PID).Verification / Alternative check:
In an interactive shell, compare echo
withthePIDreportedbypsforyourshellprocess.Insideasubshell,observethat
withthePIDreportedbypsforyourshellprocess.Insideasubshell,observethat can change because it refers to that subshell's PID.
Why Other Options Are Wrong:
iscorrect.CommonPitfalls:
Using
iscorrect.
CommonPitfalls:
Using inside subshells without realizing it changes; confusing $! and
;forgettingtoquotewhenembeddinginpathswithothervariables.
FinalAnswer:
;forgettingtoquotewhenembeddinginpathswithothervariables.
FinalAnswer:
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