1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
|
#!/bin/bash
# days-between.sh: Number of days between two dates.
# Usage: ./days-between.sh [M]M/[D]D/YYYY [M]M/[D]D/YYYY
#
# Note: Script modified to account for changes in Bash, v. 2.05b +,
#+ that closed the loophole permitting large negative
#+ integer return values.
ARGS=2 # Two command-line parameters expected.
E_PARAM_ERR=85 # Param error.
REFYR=1600 # Reference year.
CENTURY=100
DIY=365
ADJ_DIY=367 # Adjusted for leap year + fraction.
MIY=12
DIM=31
LEAPCYCLE=4
MAXRETVAL=255 # Largest permissible
#+ positive return value from a function.
diff= # Declare global variable for date difference.
value= # Declare global variable for absolute value.
day= # Declare globals for day, month, year.
month=
year=
Param_Error () # Command-line parameters wrong.
{
echo "Usage: `basename $0` [M]M/[D]D/YYYY [M]M/[D]D/YYYY"
echo " (date must be after 1/3/1600)"
exit $E_PARAM_ERR
}
Parse_Date () # Parse date from command-line params.
{
month=${1%%/**}
dm=${1%/**} # Day and month.
day=${dm#*/}
let "year = `basename $1`" # Not a filename, but works just the same.
}
check_date () # Checks for invalid date(s) passed.
{
[ "$day" -gt "$DIM" ] || [ "$month" -gt "$MIY" ] ||
[ "$year" -lt "$REFYR" ] && Param_Error
# Exit script on bad value(s).
# Uses or-list / and-list.
#
# Exercise: Implement more rigorous date checking.
}
strip_leading_zero () # Better to strip possible leading zero(s)
{ #+ from day and/or month
return ${1#0} #+ since otherwise Bash will interpret them
} #+ as octal values (POSIX.2, sect 2.9.2.1).
day_index () # Gauss' Formula:
{ # Days from March 1, 1600 to date passed as param.
# ^^^^^^^^^^^^^
day=$1
month=$2
year=$3
let "month = $month - 2"
if [ "$month" -le 0 ]
then
let "month += 12"
let "year -= 1"
fi
let "year -= $REFYR"
let "indexyr = $year / $CENTURY"
let "Days = $DIY*$year + $year/$LEAPCYCLE - $indexyr \
+ $indexyr/$LEAPCYCLE + $ADJ_DIY*$month/$MIY + $day - $DIM"
# For an in-depth explanation of this algorithm, see
#+ http://weblogs.asp.net/pgreborio/archive/2005/01/06/347968.aspx
echo $Days
}
calculate_difference () # Difference between two day indices.
{
let "diff = $1 - $2" # Global variable.
}
abs () # Absolute value
{ # Uses global "value" variable.
if [ "$1" -lt 0 ] # If negative
then #+ then
let "value = 0 - $1" #+ change sign,
else #+ else
let "value = $1" #+ leave it alone.
fi
}
if [ $# -ne "$ARGS" ] # Require two command-line params.
then
Param_Error
fi
Parse_Date $1
check_date $day $month $year # See if valid date.
strip_leading_zero $day # Remove any leading zeroes
day=$? #+ on day and/or month.
strip_leading_zero $month
month=$?
let "date1 = `day_index $day $month $year`"
Parse_Date $2
check_date $day $month $year
strip_leading_zero $day
day=$?
strip_leading_zero $month
month=$?
date2=$(day_index $day $month $year) # Command substitution.
calculate_difference $date1 $date2
abs $diff # Make sure it's positive.
diff=$value
echo $diff
exit 0
# Exercise:
# --------
# If given only one command-line parameter, have the script
#+ use today's date as the second.
# Compare this script with
#+ the implementation of Gauss' Formula in a C program at
#+ http://buschencrew.hypermart.net/software/datedif
|
