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Special Variables: Shell Provides These

Shell provides special variables. They're always available. Use them.

Here's the thing: Special variables are useful. Learn them. Use them.

Script Information​

$0: Script Name​

echo "Script: $0"

My take: $0 is the script name. Use it for logging.

$#: Argument Count​

echo "Arguments: $#"

My take: $# is argument count. Check it before using arguments.

Arguments​

$1, $2, $3...: Positional Arguments​

echo "First: $1"
echo "Second: $2"

My take: $1 is first argument. $2 is second. Simple.

$@: All Arguments​

echo "All: $@"

My take: $@ is all arguments. Use it to pass them along.

$*: All Arguments (Different)​

echo "All: $*"

My take: $* is similar but different. Use $@ usually.

Process Information​

$$: Process ID​

echo "PID: $$"

My take: $$ is the process ID. Use it for temp files.

$?: Exit Code​

command
if [ $? -eq 0 ]; then
    echo "Success"
fi

My take: $? is exit code. Check it after commands.

Common Patterns​

Check Arguments​

if [ $# -eq 0 ]; then
    echo "Usage: $0 <arg>"
    exit 1
fi

Use All Arguments​

for arg in "$@"; do
    echo "$arg"
done

What's Next?​

Now that you understand special variables, let's talk about Environment Variables.

Personal note: Special variables seemed confusing at first. Then I learned them. Now I use them constantly. They're useful. Learn them.