Bash Special Variables Explained: $$, $?, $@, $# and More

#!/bin/bash
echo "Script name: $0"

$ ./scripts/deploy.sh
Script name: ./scripts/deploy.sh

$ bash /home/user/scripts/deploy.sh
Script name: /home/user/scripts/deploy.sh

usage() {
    echo "Usage: $0 [options] <argument>"
    echo "Example: $0 -f config.yml"
}

#!/bin/bash
echo "First argument: $1"
echo "Second argument: $2"
echo "Tenth argument: ${10}"

#!/bin/bash
echo "Number of arguments: $#"

if [ $# -lt 2 ]; then
    echo "Error: at least 2 arguments required"
    exit 1
fi

$ ./test.sh a b c
Number of arguments: 3

$ ./test.sh
Number of arguments: 0

#!/bin/bash
echo "Using \$*:"
for arg in $*; do
    echo "  - $arg"
done

echo "Using \$@:"
for arg in $@; do
    echo "  - $arg"
done

Using $*:
  - hello
  - world
  - foo
  - bar
Using $@:
  - hello
  - world
  - foo
  - bar

#!/bin/bash
echo "Using \"\$*\":"
for arg in "$*"; do
    echo "  [$arg]"
done

echo "Using \"\$@\":"
for arg in "$@"; do
    echo "  [$arg]"
done

Using "$*":
  [hello world foo bar]
Using "$@":
  [hello world]
  [foo]
  [bar]

#!/bin/bash
TEMP_FILE="/tmp/myapp_$$.tmp"
echo "Temp file: $TEMP_FILE"
echo "some data" > "$TEMP_FILE"

# Clean up on exit
trap "rm -f $TEMP_FILE" EXIT

#!/bin/bash
# Start a background task
long_running_task &
TASK_PID=$!

echo "Background task PID: $TASK_PID"

# Wait for it to finish
wait $TASK_PID
echo "Task completed"

#!/bin/bash
# Run a command with a timeout
slow_command &
PID=$!

# Check after 5 seconds
sleep 5
if kill -0 $PID 2>/dev/null; then
    echo "Command timed out, killing process"
    kill $PID
fi

#!/bin/bash
# Check whether a command succeeded
grep "error" /var/log/app.log
if [ $? -eq 0 ]; then
    echo "Errors found in log"
else
    echo "No errors"
fi

# A cleaner approach
if grep -q "error" /var/log/app.log; then
    echo "Errors found in log"
fi

#!/bin/bash
if [ ! -f "$1" ]; then
    echo "Error: file not found" >&2
    exit 1
fi

# Normal execution
process_file "$1"
exit 0

$ echo $-
himBHs

#!/bin/bash
case $- in
    *i*) echo "Interactive Shell" ;;
    *)   echo "Non-interactive Shell" ;;
esac

#!/bin/bash
# File: show_vars.sh
# Demonstrates Bash special variables

echo "===== Basic Info ====="
echo "Script name: $0"
echo "Process PID: $$"
echo "Argument count: $#"
echo "Shell options: $-"

echo ""
echo "===== Positional Parameters ====="
echo "First argument: ${1:-'(empty)'}"
echo "Second argument: ${2:-'(empty)'}"
echo "Third argument: ${3:-'(empty)'}"

echo ""
echo '===== $* vs $@ ====='
echo "Using \"\$*\":"
for arg in "$*"; do
    echo "  -> [$arg]"
done

echo "Using \"\$@\":"
for arg in "$@"; do
    echo "  -> [$arg]"
done

echo ""
echo "===== Exit Status Demo ====="
ls /nonexistent 2>/dev/null
echo "Exit code for missing directory: $?"

ls / >/dev/null
echo "Exit code for existing directory: $?"

echo ""
echo "===== Background Process ====="
sleep 1 &
echo "Background PID: $!"
wait
echo "Background process finished"

$ ./show_vars.sh "hello world" foo bar

===== Basic Info =====
Script name: ./show_vars.sh
Process PID: 28547
Argument count: 3
Shell options: hB

===== Positional Parameters =====
First argument: hello world
Second argument: foo
Third argument: bar

===== $* vs $@ =====
Using "$*":
  -> [hello world foo bar]
Using "$@":
  -> [hello world]
  -> [foo]
  -> [bar]

===== Exit Status Demo =====
Exit code for missing directory: 2
Exit code for existing directory: 0

===== Background Process =====
Background PID: 28548
Background process finished

# Recommended
for arg in "$@"; do
    process "$arg"
done

# Avoid — breaks on arguments with spaces
for arg in $*; do
    process "$arg"
done

if [ $# -eq 0 ]; then
    echo "Usage: $0 <filename>" >&2
    exit 1
fi

readonly E_SUCCESS=0
readonly E_NO_ARGS=1
readonly E_FILE_NOT_FOUND=2

if [ $# -eq 0 ]; then
    exit $E_NO_ARGS
fi

TMPFILE="/tmp/${0##*/}.$$"
trap "rm -f $TMPFILE" EXIT