Bash Specialty: Arithmetic, Dates & Time

Comprehensive Reference Guide — Detailed examples with explanations for system administrators

Part 1: Bash Arithmetic Operations

Arithmetic Syntax Methods

Method	Syntax	Use Case	Notes
$(( ))	`result=$((5 + 3))`	Most common, recommended	POSIX compliant, returns value
let	`let result=5+3`	Variable assignment	No spaces around operators
expr	`result=$(expr 5 + 3)`	Legacy scripts	Requires spaces, slower
bc	`echo "5.5 + 3.2" \| bc`	Floating-point math	External command, very powerful

Arithmetic Operators

Operator	Description	Example	Result
`+`	Addition	`$((5 + 3))`	8
`-`	Subtraction	`$((10 - 4))`	6
`*`	Multiplication	`$((6 * 7))`	42
`/`	Division (integer)	`$((17 / 5))`	3
`%`	Modulo (remainder)	`$((17 % 5))`	2
`**`	Exponentiation	`$((2 ** 10))`	1024
`++`	Increment	`$((i++))`	i + 1
`--`	Decrement	`$((i--))`	i - 1
`<<`	Bit shift left	`$((1 << 4))`	16
`>>`	Bit shift right	`$((16 >> 2))`	4
`&`	Bitwise AND	`$((12 & 10))`	8
`\|`	Bitwise OR	`$((12 \| 10))`	14

Arithmetic Examples

Basic Arithmetic with Variables

Fundamental arithmetic operations commonly used in system scripts.

#!/bin/bash
# Basic arithmetic operations with variables

a=15
b=4

sum=$((a + b))
diff=$((a - b))
product=$((a * b))
quotient=$((a / b))
remainder=$((a % b))

echo "Variables: a=$a, b=$b"
echo "Sum: $sum"
echo "Difference: $diff"
echo "Product: $product"
echo "Quotient: $quotient (integer division)"
echo "Remainder: $remainder"

# Exponentiation
power=$((2 ** 8))
echo "2^8 = $power"

# Compound operations
compound=$(( (a + b) * 2 - a / b ))
echo "Compound: $compound"

Output: Variables: a=15, b=4 Sum: 19 Difference: 11 Product: 60 Quotient: 3 (integer division) Remainder: 3 2^8 = 256 Compound: 35

⚠️ Integer Division

Bash arithmetic only handles integers. 15 / 4 = 3, not 3.75. For floating-point, use bc.

Increment/Decrement and Assignment Operators

#!/bin/bash
# Increment, decrement, and compound assignment

count=10
echo "Initial: $count"

# Post-increment (use then increment)
echo "Post-increment: $((count++))"
echo "After: $count"

# Pre-increment (increment then use)
echo "Pre-increment: $((++count))"

# Compound assignments
value=100
((value += 25))
echo "After += 25: $value"

((value *= 2))
echo "After *= 2: $value"

((value /= 5))
echo "After /= 5: $value"

Output: Initial: 10 Post-increment: 10 After: 11 Pre-increment: 12 After += 25: 125 After *= 2: 250 After /= 5: 50

Floating-Point Arithmetic with bc

#!/bin/bash
# Floating-point arithmetic with bc

# Basic division
result=$(echo "scale=2; 15 / 4" | bc)
echo "15 / 4 = $result"

# Percentage calculation
total=847
used=523
percent=$(echo "scale=2; ($used / $total) * 100" | bc)
echo "Usage: ${percent}%"

# Square root
sqrt=$(echo "scale=4; sqrt(2)" | bc)
echo "sqrt(2) = $sqrt"

# Pi calculation
pi=$(echo "scale=10; 4*a(1)" | bc -l)
echo "Pi = $pi"

# Disk usage example
disk_total=500000000000
disk_used=327000000000
disk_percent=$(echo "scale=1; ($disk_used / $disk_total) * 100" | bc)
echo "Disk: ${disk_percent}%"

Output: 15 / 4 = 3.75 Usage: 61.74% sqrt(2) = 1.4142 Pi = 3.1415926532 Disk: 65.4%

Bitwise Operations for System Administration

#!/bin/bash
# Bitwise operations for permissions and flags

# Permission bits
read_bit=4
write_bit=2
exec_bit=1

rwx=$((read_bit | write_bit | exec_bit))
rw=$((read_bit | write_bit))
echo "rwx = $rwx, rw = $rw"

# Check permission
perms=5  # r-x
if (( perms & write_bit )); then
    echo "Write IS set"
else
    echo "Write NOT set"
fi

# Powers of 2 with shift
for i in {0..7}; do
    echo "2^$i = $((1 << i))"
done

# Network mask from CIDR
cidr=24
mask=$(( 0xFFFFFFFF << (32 - cidr) ))
echo "/$cidr mask: $(( (mask >> 24) & 0xFF )).$(( (mask >> 16) & 0xFF )).$(( (mask >> 8) & 0xFF )).$(( mask & 0xFF ))"

Output: rwx = 7, rw = 6 Write NOT set 2^0 = 1 2^1 = 2 2^2 = 4 ... /24 mask: 255.255.255.0

Arithmetic in Loops and Arrays

#!/bin/bash
# Arithmetic in loops and arrays

# C-style for loop - sum 1 to 10
sum=0
for ((i=1; i<=10; i++)); do
    ((sum += i))
done
echo "Sum 1-10: $sum"

# Array statistics
data=(23 45 67 89 12 34 56 78 90 11)
count=${#data[@]}
sum=0
min=${data[0]}
max=${data[0]}

for val in "${data[@]}"; do
    ((sum += val))
    ((val < min)) && min=$val
    ((val > max)) && max=$val
done

avg=$(echo "scale=2; $sum / $count" | bc)
echo "Count: $count, Sum: $sum"
echo "Min: $min, Max: $max, Avg: $avg"

# Fibonacci
fib[0]=0; fib[1]=1
for ((i=2; i<10; i++)); do
    ((fib[i] = fib[i-1] + fib[i-2]))
done
echo "Fibonacci: ${fib[*]}"

Output: Sum 1-10: 55 Count: 10, Sum: 505 Min: 11, Max: 90, Avg: 50.50 Fibonacci: 0 1 1 2 3 5 8 13 21 34

Arithmetic Comparisons and Conditionals

#!/bin/bash
# Arithmetic comparisons

value=42

# Using (( )) for tests
if ((value > 40)); then
    echo "$value > 40 is TRUE"
fi

# Range check
if ((value >= 40 && value <= 50)); then
    echo "$value is in range 40-50"
fi

# Practical: disk warning
disk=78
if ((disk >= 90)); then
    echo "CRITICAL: ${disk}%"
elif ((disk >= 70)); then
    echo "WARNING: ${disk}%"
else
    echo "OK: ${disk}%"
fi

# Divisibility (FizzBuzz style)
for n in {1..15}; do
    if ((n % 3 == 0 && n % 5 == 0)); then
        echo "$n: FizzBuzz"
    elif ((n % 3 == 0)); then
        echo "$n: Fizz"
    elif ((n % 5 == 0)); then
        echo "$n: Buzz"
    fi
done

Output: 42 > 40 is TRUE 42 is in range 40-50 WARNING: 78% 3: Fizz 5: Buzz 6: Fizz ... 15: FizzBuzz

Number Base Conversions

#!/bin/bash
# Number base conversions

# Different base inputs
echo "Decimal 255: 255"
echo "Hex 0xFF: $((0xFF))"
echo "Octal 0377: $((0377))"
echo "Binary 2#11111111: $((2#11111111))"

# Decimal to other bases
value=200
printf "Decimal %d = Hex %X = Octal %o\n" $value $value $value

# IP to integer
ip="192.168.1.100"
IFS='.' read -r a b c d <<< "$ip"
int=$(( (a << 24) + (b << 16) + (c << 8) + d ))
printf "IP %s = %d\n" "$ip" "$int"

# Chmod octal to symbolic
for perm in 755 644 700; do
    mode=$((8#$perm))
    sym=""
    for i in 0 1 2; do
        d=$(( (mode >> (2-i)*3) & 7 ))
        ((d & 4)) && sym+="r" || sym+="-"
        ((d & 2)) && sym+="w" || sym+="-"
        ((d & 1)) && sym+="x" || sym+="-"
    done
    echo "$perm = $sym"
done

Output: Decimal 255: 255 Hex 0xFF: 255 Octal 0377: 255 Binary 2#11111111: 255 Decimal 200 = Hex C8 = Octal 310 IP 192.168.1.100 = 3232235876 755 = rwxr-xr-x 644 = rw-r--r-- 700 = rwx------

Random Numbers and Practical Calculations

#!/bin/bash
# Random numbers and conversions

# Random in range
echo "Random 1-100: $(( RANDOM % 100 + 1 ))"
echo "Random die: $(( RANDOM % 6 + 1 ))"

# Bytes to human readable
bytes_to_human() {
    local b=$1
    local units=("B" "KB" "MB" "GB" "TB")
    local u=0
    while ((b >= 1024 && u < 4)); do
        b=$(echo "scale=2; $b / 1024" | bc)
        ((u++))
    done
    echo "$b ${units[$u]}"
}

echo "1048576 bytes = $(bytes_to_human 1048576)"
echo "5368709120 bytes = $(bytes_to_human 5368709120)"

# Seconds to human
secs=90061
days=$((secs / 86400))
hrs=$(( (secs % 86400) / 3600 ))
mins=$(( (secs % 3600) / 60 ))
echo "$secs seconds = ${days}d ${hrs}h ${mins}m"

# Percentage change
old=150; new=180
change=$((new - old))
pct=$(echo "scale=1; ($change * 100) / $old" | bc)
echo "Change: $old → $new = ${pct}%"

Output: Random 1-100: 67 Random die: 4 1048576 bytes = 1.00 MB 5368709120 bytes = 5.00 GB 90061 seconds = 1d 1h 1m Change: 150 → 180 = 20.0%

Using awk for Advanced Arithmetic

#!/bin/bash
# Advanced arithmetic with awk

# Basic
echo | awk '{print "15/4 =", 15/4}'
echo | awk '{print "2^10 =", 2^10}'

# Math functions
awk 'BEGIN {
    print "sin(1) =", sin(1)
    print "sqrt(2) =", sqrt(2)
    print "exp(1) =", exp(1)
}'

# Statistics from data
echo "23 45 67 89 12" | tr ' ' '\n' | awk '
{
    sum += $1
    count++
    if (NR==1 || $1 < min) min=$1
    if (NR==1 || $1 > max) max=$1
}
END {
    print "Count:", count
    print "Sum:", sum
    print "Mean:", sum/count
    print "Min:", min, "Max:", max
}'

Output: 15/4 = 3.75 2^10 = 1024 sin(1) = 0.841471 sqrt(2) = 1.41421 exp(1) = 2.71828 Count: 5 Sum: 236 Mean: 47.2 Min: 12 Max: 89

System Resource Calculator

#!/bin/bash
# System resource calculations

echo "=== Memory Analysis ==="
total_kb=16384000
used_kb=12582912
free_kb=$((total_kb - used_kb))

total_gb=$(echo "scale=2; $total_kb / 1048576" | bc)
used_pct=$(echo "scale=1; ($used_kb * 100) / $total_kb" | bc)

echo "Total: ${total_gb} GB"
echo "Used: ${used_pct}%"

if (( $(echo "$used_pct > 90" | bc -l) )); then
    echo "Status: CRITICAL"
elif (( $(echo "$used_pct > 75" | bc -l) )); then
    echo "Status: WARNING"
else
    echo "Status: OK"
fi

echo ""
echo "=== CPU Load ==="
cores=8
load=4.25
util=$(echo "scale=0; ($load * 100) / $cores" | bc)
echo "Load: $load on $cores cores = ${util}%"

echo ""
echo "=== Network Rate ==="
rx_start=10485760000
rx_end=10737418240
interval=60
rx_rate=$(( (rx_end - rx_start) / interval ))
rx_mbps=$(echo "scale=2; ($rx_rate * 8) / 1000000" | bc)
echo "RX: $rx_mbps Mbps"

Output: === Memory Analysis === Total: 15.62 GB Used: 76.8% Status: WARNING === CPU Load === Load: 4.25 on 8 cores = 53% === Network Rate === RX: 33.55 Mbps

Part 2: Dates & Time Operations

Quick Reference

date — Display current date/time
date +FORMAT — Custom format
date -d "STRING" — Parse date string
date -d "DATE +/- OFFSET" — Date arithmetic

Common Format Specifiers

Specifier	Description	Example
`%Y`	4-digit year	2025
`%m`	Month (01-12)	11
`%d`	Day (01-31)	18
`%H`	Hour (00-23)	14
`%M`	Minute (00-59)	30
`%S`	Second (00-59)	45
`%A`	Full weekday	Tuesday
`%B`	Full month	November
`%s`	Unix epoch	1731958245
`%Z`	Timezone	EST
`%z`	TZ offset	-0500

Date & Time Examples

Basic Date Formatting

#!/bin/bash
# Basic date formatting

echo "Default: $(date)"
echo "UTC: $(date -u)"

# Common formats
echo "ISO 8601: $(date +%Y-%m-%dT%H:%M:%S%z)"
echo "ISO Date: $(date +%Y-%m-%d)"
echo "US Date: $(date +%m/%d/%Y)"
echo "EU Date: $(date +%d/%m/%Y)"

# For filenames
echo "Filename: $(date +%Y%m%d_%H%M%S)"
echo "Daily: $(date +%Y-%m-%d)"

# Time formats
echo "24-hour: $(date +%H:%M:%S)"
echo "12-hour: $(date '+%I:%M:%S %p')"

# Special
echo "Epoch: $(date +%s)"
echo "Week: $(date +%W)"
echo "Day of year: $(date +%j)"

# Practical
echo "[$(date '+%Y-%m-%d %H:%M:%S')] Log message"
echo "backup-$(date +%Y%m%d-%H%M%S).tar.gz"

Output: Default: Tue Nov 18 14:30:45 EST 2025 UTC: Tue Nov 18 19:30:45 UTC 2025 ISO 8601: 2025-11-18T14:30:45-0500 ISO Date: 2025-11-18 US Date: 11/18/2025 EU Date: 18/11/2025 Filename: 20251118_143045 24-hour: 14:30:45 12-hour: 02:30:45 PM Epoch: 1731958245 [2025-11-18 14:30:45] Log message backup-20251118-143045.tar.gz

Date Arithmetic

#!/bin/bash
# Date arithmetic with -d

echo "Today: $(date '+%Y-%m-%d')"

# Adding time
echo "Tomorrow: $(date -d 'tomorrow' '+%Y-%m-%d')"
echo "+3 days: $(date -d '+3 days' '+%Y-%m-%d')"
echo "+1 week: $(date -d '+1 week' '+%Y-%m-%d')"
echo "+1 month: $(date -d '+1 month' '+%Y-%m-%d')"
echo "+1 year: $(date -d '+1 year' '+%Y-%m-%d')"

# Subtracting
echo "Yesterday: $(date -d 'yesterday' '+%Y-%m-%d')"
echo "-7 days: $(date -d '-7 days' '+%Y-%m-%d')"
echo "-30 days: $(date -d '-30 days' '+%Y-%m-%d')"
echo "-3 months: $(date -d '-3 months' '+%Y-%m-%d')"

# Time arithmetic
echo "+2 hours: $(date -d '+2 hours' '+%H:%M:%S')"
echo "-30 min: $(date -d '-30 minutes' '+%H:%M:%S')"

# Retention dates
echo "Delete before:"
echo "  7 days: $(date -d '-7 days' '+%Y-%m-%d')"
echo "  30 days: $(date -d '-30 days' '+%Y-%m-%d')"
echo "  90 days: $(date -d '-90 days' '+%Y-%m-%d')"

Output: Today: 2025-11-18 Tomorrow: 2025-11-19 +3 days: 2025-11-21 +1 week: 2025-11-25 +1 month: 2025-12-18 Yesterday: 2025-11-17 -7 days: 2025-11-11 -30 days: 2025-10-19 +2 hours: 16:30:45 Delete before: 7 days: 2025-11-11 30 days: 2025-10-19 90 days: 2025-08-20

Parsing and Converting Dates

#!/bin/bash
# Date parsing and conversion

# Various input formats
echo "From 'Nov 18, 2025': $(date -d 'Nov 18, 2025' '+%Y-%m-%d')"
echo "From '11/18/2025': $(date -d '11/18/2025' '+%Y-%m-%d')"
echo "From '2025-11-18': $(date -d '2025-11-18' '+%Y-%m-%d')"

# Format conversion
us_date="11/18/2025"
iso=$(date -d "$us_date" '+%Y-%m-%d')
echo "US → ISO: $us_date → $iso"

# Epoch conversions
epoch=$(date +%s)
echo "Now: $epoch"
echo "From epoch: $(date -d @1731958245 '+%Y-%m-%d %H:%M:%S')"

human="2025-11-18 14:30:45"
to_epoch=$(date -d "$human" '+%s')
echo "'$human' → $to_epoch"

# Parse syslog format
syslog="Nov 18 14:30:45"
parsed=$(date -d "$syslog" '+%Y-%m-%d %H:%M:%S')
echo "Syslog: $syslog → $parsed"

Output: From 'Nov 18, 2025': 2025-11-18 From '11/18/2025': 2025-11-18 From '2025-11-18': 2025-11-18 US → ISO: 11/18/2025 → 2025-11-18 Now: 1731958245 From epoch: 2025-11-18 14:30:45 '2025-11-18 14:30:45' → 1731958245 Syslog: Nov 18 14:30:45 → 2025-11-18 14:30:45

💡 Epoch Parsing

Use @ prefix for epoch: date -d @1731958245

Calculating Time Differences

#!/bin/bash
# Time difference calculations

date1="2025-01-01"
date2="2025-11-18"

epoch1=$(date -d "$date1" '+%s')
epoch2=$(date -d "$date2" '+%s')
diff=$((epoch2 - epoch1))

echo "From $date1 to $date2:"
echo "  $((diff / 86400)) days"
echo "  $((diff / 3600)) hours"

# Age calculation
birth="1982-06-25"
today=$(date '+%Y-%m-%d')
age_days=$(( ($(date -d "$today" '+%s') - $(date -d "$birth" '+%s')) / 86400 ))
echo "Age: $((age_days / 365)) years"

# Days until event
event="2025-12-25"
now=$(date '+%s')
event_epoch=$(date -d "$event" '+%s')
days=$(( (event_epoch - now) / 86400 ))
echo "Days until $event: $days"

# Script timing
start=$(date '+%s')
sleep 1
end=$(date '+%s')
echo "Elapsed: $((end - start)) seconds"

Output: From 2025-01-01 to 2025-11-18: 321 days 7704 hours Age: 43 years Days until 2025-12-25: 37 Elapsed: 1 seconds

Working with Timezones

#!/bin/bash
# Timezone operations

echo "Local: $(date '+%H:%M %Z')"
echo "UTC: $(TZ=UTC date '+%H:%M %Z')"

echo ""
echo "US Timezones:"
echo "  Eastern: $(TZ=America/New_York date '+%H:%M %Z')"
echo "  Central: $(TZ=America/Chicago date '+%H:%M %Z')"
echo "  Mountain: $(TZ=America/Denver date '+%H:%M %Z')"
echo "  Pacific: $(TZ=America/Los_Angeles date '+%H:%M %Z')"

echo ""
echo "International:"
echo "  London: $(TZ=Europe/London date '+%H:%M %Z')"
echo "  Tokyo: $(TZ=Asia/Tokyo date '+%H:%M %Z')"

# Convert specific time
eastern="2025-11-18 14:00:00"
epoch=$(TZ=America/New_York date -d "$eastern" '+%s')
echo ""
echo "Meeting: $eastern Eastern"
echo "  Pacific: $(TZ=America/Los_Angeles date -d @$epoch '+%H:%M %Z')"
echo "  UTC: $(TZ=UTC date -d @$epoch '+%H:%M %Z')"
echo "  Tokyo: $(TZ=Asia/Tokyo date -d @$epoch '+%H:%M %Z (%Y-%m-%d)')"

Output: Local: 14:30 EST UTC: 19:30 UTC US Timezones: Eastern: 14:30 EST Central: 13:30 CST Mountain: 12:30 MST Pacific: 11:30 PST International: London: 19:30 GMT Tokyo: 04:30 JST Meeting: 2025-11-18 14:00:00 Eastern Pacific: 11:00 PST UTC: 19:00 UTC Tokyo: 04:00 JST (2025-11-19)

Weekday Operations

#!/bin/bash
# Weekday operations

echo "Today: $(date '+%A') ($(date '+%u'))"

# Find weekdays
echo "Next Monday: $(date -d 'next monday' '+%Y-%m-%d')"
echo "Last Friday: $(date -d 'last friday' '+%Y-%m-%d')"

# Check weekend
dow=$(date '+%u')
if ((dow >= 6)); then
    echo "It's the weekend!"
else
    echo "It's a weekday"
fi

# First/last of month
echo "First of month: $(date -d "$(date '+%Y-%m-01')" '+%Y-%m-%d (%A)')"
echo "Last of month: $(date -d "$(date '+%Y-%m-01') +1 month -1 day" '+%Y-%m-%d (%A)')"

# Business days check
is_weekend() {
    (($(date -d "$1" '+%u') >= 6))
}

# Count business days
start="2025-11-01"
end="2025-11-18"
bdays=0
curr="$start"
while [[ "$curr" < "$end" ]]; do
    is_weekend "$curr" || ((bdays++))
    curr=$(date -d "$curr +1 day" '+%Y-%m-%d')
done
echo "Business days $start to $end: $bdays"

Output: Today: Tuesday (2) Next Monday: 2025-11-24 Last Friday: 2025-11-14 It's a weekday First of month: 2025-11-01 (Saturday) Last of month: 2025-11-30 (Sunday) Business days 2025-11-01 to 2025-11-18: 11

Log Rotation and File Dating

#!/bin/bash
# Log rotation filenames

echo "=== Filename Formats ==="
echo "Daily: app-$(date '+%Y-%m-%d').log"
echo "Hourly: app-$(date '+%Y-%m-%d_%H').log"
echo "Full: app-$(date '+%Y%m%d_%H%M%S').log"
echo "Weekly: app-$(date '+%Y-W%W').log"
echo "Monthly: app-$(date '+%Y-%m').log"

echo ""
echo "=== Rotation Names ==="
base="application.log"
for i in {1..5}; do
    echo "${base}.$(date -d "-$i days" '+%Y-%m-%d')"
done

echo ""
echo "=== Backup Naming ==="
echo "Daily: backup-$(date '+%Y-%m-%d').tar.gz"
echo "Weekly: backup-$(date -d 'last sunday' '+%Y-%m-%d').tar.gz"
echo "Monthly: backup-$(date '+%Y-%m').tar.gz"

echo ""
echo "=== Cleanup Schedule ==="
echo "Delete daily older than: $(date -d '-7 days' '+%Y-%m-%d')"
echo "Delete weekly older than: $(date -d '-4 weeks' '+%Y-%m-%d')"
echo "Delete monthly older than: $(date -d '-12 months' '+%Y-%m-%d')"

Output: === Filename Formats === Daily: app-2025-11-18.log Hourly: app-2025-11-18_14.log Full: app-20251118_143045.log === Rotation Names === application.log.2025-11-17 application.log.2025-11-16 application.log.2025-11-15 ... === Cleanup Schedule === Delete daily older than: 2025-11-11 Delete weekly older than: 2025-10-21 Delete monthly older than: 2024-11-18

Relative Date Expressions

#!/bin/bash
# Natural language dates

echo "=== Basic ==="
echo "now: $(date -d 'now' '+%Y-%m-%d %H:%M')"
echo "today: $(date -d 'today' '+%Y-%m-%d')"
echo "yesterday: $(date -d 'yesterday' '+%Y-%m-%d')"
echo "tomorrow: $(date -d 'tomorrow' '+%Y-%m-%d')"

echo ""
echo "=== Time Terms ==="
echo "noon: $(date -d 'noon' '+%H:%M')"
echo "midnight: $(date -d 'midnight' '+%H:%M')"

echo ""
echo "=== Complex ==="
echo "3 days ago: $(date -d '3 days ago' '+%Y-%m-%d')"
echo "2 weeks hence: $(date -d '2 weeks hence' '+%Y-%m-%d')"
echo "next year: $(date -d 'next year' '+%Y-%m-%d')"

echo ""
echo "=== Report Periods ==="
echo "Today: $(date '+%Y-%m-%d')"
echo "This week: $(date -d 'last monday' '+%Y-%m-%d') to $(date -d 'next sunday' '+%Y-%m-%d')"
echo "This month: $(date '+%Y-%m-01') to $(date -d "$(date '+%Y-%m-01') +1 month -1 day" '+%Y-%m-%d')"

echo ""
echo "=== Quarter ==="
month=$(date '+%-m')
quarter=$(( (month - 1) / 3 + 1 ))
echo "Current: Q$quarter"

Output: === Basic === now: 2025-11-18 14:30 today: 2025-11-18 yesterday: 2025-11-17 tomorrow: 2025-11-19 === Time Terms === noon: 12:00 midnight: 00:00 === Report Periods === Today: 2025-11-18 This week: 2025-11-17 to 2025-11-23 This month: 2025-11-01 to 2025-11-30 === Quarter === Current: Q4

High-Precision Timing

#!/bin/bash
# Nanosecond timing

echo "Epoch: $(date '+%s')"
echo "With ns: $(date '+%s%N')"
echo "Milliseconds: $(date '+%s%3N')"

# Benchmark function
benchmark() {
    local start=$(date '+%s%N')
    "$@"
    local end=$(date '+%s%N')
    local ms=$(( (end - start) / 1000000 ))
    echo "Elapsed: ${ms}ms"
}

benchmark sleep 0.1

# Code timing
start=$(date '+%s%N')
for i in {1..1000}; do echo "$i" > /dev/null; done
end=$(date '+%s%N')
echo "Loop: $(( (end - start) / 1000 )) µs"

# Unique IDs
echo "ID: $(date '+%s%N')-$$-$RANDOM"

# Using SECONDS
SECONDS=0
sleep 1
echo "SECONDS: $SECONDS"

Output: Epoch: 1731958245 With ns: 1731958245123456789 Milliseconds: 1731958245123 Elapsed: 100ms Loop: 2100 µs ID: 1731958245123456789-12345-28463 SECONDS: 1

💡 SECONDS Variable

Bash's $SECONDS counts since shell start. Reset with SECONDS=0.

System Maintenance Scheduler

#!/bin/bash
# Maintenance scheduler

echo "=== Current Time ==="
echo "Local: $(date '+%Y-%m-%d %H:%M:%S %Z')"
echo "UTC: $(date -u '+%Y-%m-%d %H:%M:%S %Z')"
echo "Day: $(date '+%j')/365, Week: $(date '+%W')/52"

echo ""
echo "=== Maintenance Window ==="
hour=$(date '+%H')
dow=$(date '+%u')

if ((dow >= 6)); then
    echo "✓ Weekend - extended window"
elif ((hour >= 2 && hour < 4)); then
    echo "✓ In window (2-4 AM)"
else
    echo "⚠ Outside window"
fi

echo ""
echo "=== Retention Policy ==="
printf "%-20s %s\n" "Daily backups:" "$(date -d '-7 days' '+%Y-%m-%d')"
printf "%-20s %s\n" "Weekly backups:" "$(date -d '-4 weeks' '+%Y-%m-%d')"
printf "%-20s %s\n" "Monthly backups:" "$(date -d '-12 months' '+%Y-%m-%d')"

echo ""
echo "=== Upcoming ==="
today=$(date '+%s')
for event in "2025-11-27:Thanksgiving" "2025-12-25:Christmas" "2026-01-01:New Year"; do
    IFS=':' read -r dt name <<< "$event"
    days=$(( ($(date -d "$dt" '+%s') - today) / 86400 ))
    echo "  $name: $days days"
done

echo ""
echo "=== Global Team ==="
for loc in "HQ:America/New_York" "West:America/Los_Angeles" "EU:Europe/London"; do
    IFS=':' read -r n tz <<< "$loc"
    printf "  %-6s %s\n" "$n" "$(TZ=$tz date '+%H:%M %Z')"
done

Output: === Current Time === Local: 2025-11-18 14:30:45 EST UTC: 2025-11-18 19:30:45 UTC Day: 322/365, Week: 46/52 === Maintenance Window === ⚠ Outside window === Retention Policy === Daily backups: 2025-11-11 Weekly backups: 2025-10-21 Monthly backups: 2024-11-18 === Upcoming === Thanksgiving: 9 days Christmas: 37 days New Year: 44 days === Global Team === HQ 14:30 EST West 11:30 PST EU 19:30 GMT

🔢 Bash Specialty: Arithmetic, Dates & Time

📋 Table of Contents

Overview

Key Concepts