cal

Overview

The cal command displays a calendar for a specified month and year. It’s useful for date reference, scheduling, and quick date calculations.

Syntax

cal [options] [month] [year]
cal [options] [year]

Common Options

Option	Description
-1	Display single month (default)
-3	Display previous, current, and next month
-A num	Display num months after
-B num	Display num months before
-y	Display entire year
-j	Display Julian dates (day of year)
-m	Monday as first day of week
-s	Sunday as first day of week (default)
-w	Display week numbers
--color	Colorize output

Key Use Cases

1. Quick date reference
2. Planning and scheduling
3. Date calculations
4. Historical date lookup
5. Script date validation

Examples with Explanations

Example 1: Current Month

cal

Displays calendar for current month

Example 2: Specific Month and Year

cal 12 2024

Shows December 2024 calendar

Example 3: Entire Year

cal -y 2024

Displays full year 2024 calendar

Example 4: Three Month View

cal -3

Shows previous, current, and next month

Date Range Display

1. Show months after current:

   cal -A 3  # Next 3 months

2. Show months before current:

   cal -B 2  # Previous 2 months

3. Combine before and after:

   cal -B 1 -A 1  # Previous, current, next

Julian Calendar

1. Show day of year:

   cal -j

2. Julian date for specific month:

   cal -j 3 2024  # March 2024 with day numbers

Week Display Options

1. Monday as first day:

   cal -m

2. Show week numbers:

   cal -w

3. Combine options:

   cal -mw  # Monday first + week numbers

Historical Dates

1. Historical calendar:

   cal 9 1752  # September 1752 (calendar reform)

2. Ancient dates:

   cal 1 1  # January year 1

3. Future dates:

   cal 12 2050  # December 2050

Performance Analysis

• Very fast operation
• Minimal resource usage
• No network dependencies
• Good for scripting
• Efficient date calculations

Related Commands

• date - Current date/time
• ncal - Alternative calendar
• dateutils - Date utilities
• at - Schedule commands
• crontab - Schedule recurring tasks

Best Practices

1. Use for quick date reference
2. Combine with date command
3. Consider locale settings
4. Use Julian dates for day counting
5. Helpful for scheduling scripts

Scripting Applications

1. Date validation:

   #!/bin/bash
   validate_date() {
       local month=$1 year=$2
       if cal "$month" "$year" >/dev/null 2>&1; then
           echo "Valid date"
           return 0
       else
           echo "Invalid date"
           return 1
       fi
   }

2. Business day calculation:

   count_weekdays() {
       local month=$1 year=$2
       cal "$month" "$year" | grep -E '[0-9]' | \
       tr ' ' '\n' | grep -E '^[0-9]+$' | wc -l
   }

Integration Examples

1. With date for context:

   echo "Today is $(date +%A), $(date +%B) $(date +%d)"
   cal -3

2. Planning script:

   echo "Current month schedule:"
   cal
   echo ""
   echo "Upcoming deadlines:"
   # Show project deadlines

Locale Considerations

1. Different locales affect:

   • First day of week
   • Month names
   • Date formatting

2. Set locale:

   LC_TIME=en_US.UTF-8 cal
   LC_TIME=de_DE.UTF-8 cal

Output Formatting

1. Pipe to other commands:

   cal | grep -E '[0-9]'  # Extract date lines

2. Count days in month:

   cal 2 2024 | tail -1 | awk '{print $NF}'

3. Find specific day:

   cal | grep -o '\b15\b'  # Find 15th day

Calendar Calculations

1. Days in month:

   days_in_month() {
       cal "$1" "$2" | awk 'NF {last=$NF} END {print last}'
   }

2. First day of month:

   first_day_of_week() {
       cal "$1" "$2" | awk '/^[A-Z]/ {getline; print NF}'
   }

Troubleshooting

1. Invalid month/year combinations
2. Locale-specific formatting issues
3. Terminal width limitations
4. Historical calendar accuracy
5. Leap year calculations

Advanced Usage

1. Custom formatting with ncal:

   ncal -b  # Brief format
   ncal -M  # Monday first

2. Specific day highlighting:

   cal | sed "s/$(date +%d)/[$(date +%d)]/"

Historical Context

1. Calendar reform (1752):

   cal 9 1752  # Shows 11 missing days

2. Leap year examples:

   cal 2 2000  # Leap year (divisible by 400)
   cal 2 1900  # Not leap year (divisible by 100, not 400)

Automation Examples

1. Monthly report header:

   #!/bin/bash
   echo "Monthly Report - $(date +%B %Y)"
   echo "================================"
   cal
   echo ""

2. Schedule reminder:

   # Show next month for planning
   NEXT_MONTH=$(date -d "next month" +%m)
   NEXT_YEAR=$(date -d "next month" +%Y)
   echo "Next month planning:"
   cal "$NEXT_MONTH" "$NEXT_YEAR"

Color Output

Modern cal versions support color:

cal --color=always

Environment variable:

export CAL_COLOR=always

Integration with Other Tools

1. With remind/calendar apps:

   cal && echo "" && remind ~/.reminders

2. With task managers:

   cal -3 && echo "" && task list