KodeKloud Notes

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.

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

Note

Use echo $? immediately after a command to capture its exit status—subsequent commands will overwrite this value.

Exploring Common Special Shell Variables

Here’s a quick reference for the most widely used POSIX-compliant special parameters:

Variable	Description	Example
`$*`	All positional parameters as a single word	`echo "Args: $*"`
`$#`	Number of positional parameters	`echo "Count: $#"`
`$0`	Name of the script or shell	`echo "Script: $0"`
`$!`	PID of the last background command	`sleep 30 &` then `echo $!`
`$-`	Current shell options (flags) as a string	`echo "Options: $-"`

Each of these variables updates dynamically based on the context in which your script runs.

Warning

Expanding $* without quotes can lead to word splitting and unintended globbing. Use quotes or consider $@ when preserving argument boundaries is critical.

Practical Use Cases

Argument Validation

if [ $# -lt 2 ]; then
  echo "Usage: $0 <source> <destination>"
  exit 1
fi

Logging Script Name

echo "[$(date)] Starting backup script: $0"

Background Job Monitoring

long_running_task &
bg_pid=$!
wait $bg_pid
echo "Job $bg_pid finished with status $?"

Links and References

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