id

Overview

The id command displays user and group IDs for the current user or specified user. It provides detailed identity information including real, effective, and supplementary group memberships.

Syntax

id [options] [user]

Common Options

Option Description
-u Show only user ID
-g Show only primary group ID
-G Show all group IDs
-n Show names instead of numbers
-r Show real ID instead of effective
-z Delimit entries with NUL

Key Use Cases

  1. User identification
  2. Permission troubleshooting
  3. Security auditing
  4. Group membership verification
  5. Script access control

Examples with Explanations

Example 1: Basic Usage

id

Shows complete user and group information

Example 2: Specific User

id username

Shows information for specified user

Example 3: Numeric User ID

id -u

Returns only the numeric user ID

Example 4: Group Names

id -Gn

Shows all group names user belongs to

Understanding Output

Default output format:

uid=1000(user) gid=1000(user) groups=1000(user),4(adm),24(cdrom),27(sudo)

Components: - uid: User ID and name - gid: Primary group ID and name - groups: All group memberships

Common Usage Patterns

  1. Root check:

    [ "$(id -u)" -eq 0 ] && echo "Running as root"

  2. Group membership check:

    id -Gn | grep -q sudo && echo "User has sudo access"

  3. User validation:

    if id "$username" >/dev/null 2>&1; then
    echo "User exists"
fi

Advanced Usage

  1. Real vs effective ID:

    id -ru  # Real user ID
id -u   # Effective user ID

  2. All group information:

    id -G | tr ' ' '\n' | sort -n

  3. Formatted output:

    printf "User: %s (UID: %d)\n" "$(id -un)" "$(id -u)"

Performance Analysis

  • Very fast operation
  • No filesystem access needed
  • Minimal system resources
  • Good for frequent checks
  • Efficient in scripts

Best Practices

  1. Use numeric IDs for reliable comparisons
  2. Check both user and group permissions
  3. Handle non-existent users gracefully
  4. Use appropriate options for specific needs
  5. Consider real vs effective IDs

Security Applications

  1. Privilege escalation check:

    if [ "$(id -u)" -ne "$(id -ru)" ]; then
    echo "Running with elevated privileges"
fi

  2. Group-based access:

    if id -Gn | grep -q "admin"; then
    echo "Administrative access granted"
fi

Scripting Examples

  1. User directory creation:

    USER_ID=$(id -u)
USER_NAME=$(id -un)
mkdir -p "/data/$USER_NAME"
chown "$USER_ID" "/data/$USER_NAME"

  2. Conditional execution:

    if [ "$(id -u)" -eq 0 ]; then
    systemctl restart service
else
    echo "Root privileges required"
fi

Integration Examples

  1. Logging with user info:

    echo "$(date): User $(id -un) ($(id -u)) executed command" >> audit.log

  2. Permission validation:

    validate_user() {
    local required_group="$1"
    id -Gn | grep -q "$required_group" || {
        echo "Access denied: $required_group membership required"
        exit 1
    }
}

Troubleshooting

  1. User not found errors
  2. Permission denied issues
  3. Group membership problems
  4. Effective vs real ID confusion
  5. Numeric vs name resolution