Shell Programming and Scripting

#!/usr/bin/env bash

# @(#) s1	Demonstrate date reformat, arithmetic, dateutils, dadd.
# "dadd" is part of dateutils, repository Debian/Jessie, MiNT, # Ubuntu, etc.
# Also: http://www.fresse.org/dateutils.

# Utility functions: print-as-echo, print-line-with-visual-space, debug.
# export PATH="/usr/local/bin:/usr/bin:/bin"
LC_ALL=C ; LANG=C ; export LC_ALL LANG
pe() { for _i;do printf "%s" "$_i";done; printf "\n"; }
pl() { pe;pe "-----" ;pe "$*"; }
db() { ( printf " db, ";for _i;do printf "%s" "$_i";done;printf "\n" ) >&2 ; }
db() { : ; }
C=$HOME/bin/context && [ -f $C ] && $C dadd

FILE=${1-data1}

pl " Input data file $FILE:"
cat $FILE

# In : MM/DD/YYYY HH:MM:SS
# Out: DD/Mon/YYYY HH:MM:SS
pl " Results:"
dadd -S -i '%m/%d/%Y' +1mo -f '%d/%b/%Y' < $FILE

exit 0

$ ./s1

Environment: LC_ALL = C, LANG = C
(Versions displayed with local utility "version")
OS, ker|rel, machine: Linux, 2.6.26-2-amd64, x86_64
Distribution        : Debian 5.0.8 (lenny, workstation) 
bash GNU bash 3.2.39
dadd 0.2.6

-----
 Input data file data1:
First line, no dates, some blank lines appear.
Today was: Mon Jul  6 08:18:06 CDT 2015

06/23/2015 20:59:12
Now is the time 06/23/2015 20:59:12 for all good men

No dates here.
When was 06/23/2015 20:59:12, another 06/23/2015 20:59:12, not 2015.06.23 20:59:12

Last line, no dates.

-----
 Results:
First line, no dates, some blank lines appear.
Today was: Mon Jul  6 08:18:06 CDT 2015

23/Jul/2015 20:59:12
Now is the time 23/Jul/2015 20:59:12 for all good men

No dates here.
When was 23/Jul/2015 20:59:12, another 23/Jul/2015 20:59:12, not 2015.06.23 20:59:12

Last line, no dates.

LC_ALL=C
export LC_ALL
fromdate="04/25/2015 12:01:02"
printf "%(%d/%b/%Y %H:%M:%S)T\n" "$fromdate"
year=$(printf "%(%Y)T\n" "$fromdate"
# ...

# current time, builtin value "now"
printf "%(%d/%b/%Y %H:%M:%S)T now" 
# epoc
epoc=$(printf "%(%s)T" now)
# date counting using epoc
((day=60*60*24))
#...