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Shell scripting thrives on feedback—knowing whether each step succeeded, failed, or requires special handling. Special shell variables provide this introspection by exposing the shell environment, command-line arguments, process identifiers, and more. Leveraging these built-in indicators leads to more robust, maintainable scripts.
The image is a slide titled "Special Shell Variables," explaining that these variables allow access to information about the current shell environment and command-line arguments.

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Understanding Exit Status in Bash

Every command you run returns an exit status:
  • 0 means success
  • Any nonzero value indicates failure
The special variable $? always holds the exit status of the last executed command: 
$ ls /nonexistent
ls: cannot access '/nonexistent': No such file or directory
$ echo $?
2
$ echo "Listing current directory:"
$ ls
KodeKloud_introduction
$ echo $?
0
Use echo $? immediately after a command to capture its exit status—subsequent commands will overwrite this value.

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Exploring Common Special Shell Variables

Here’s a quick reference for the most widely used POSIX-compliant special parameters:
VariableDescriptionExample
$*All positional parameters as a single wordecho "Args: $*"
$#Number of positional parametersecho "Count: $#"
$0Name of the script or shellecho "Script: $0"
$!PID of the last background commandsleep 30 & then echo $!
$-Current shell options (flags) as a stringecho "Options: $-"
Each of these variables updates dynamically based on the context in which your script runs.
Expanding $* without quotes can lead to word splitting and unintended globbing. Use quotes or consider $@ when preserving argument boundaries is critical.

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Practical Use Cases

  1. Argument Validation 
    if [ $# -lt 2 ]; then
  echo "Usage: $0 <source> <destination>"
  exit 1
fi
  2. Logging Script Name 
    echo "[$(date)] Starting backup script: $0"
  3. Background Job Monitoring 
    long_running_task &
bg_pid=$!
wait $bg_pid
echo "Job $bg_pid finished with status $?"

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