Datetime comparison in Unix

Question

I had two datetime strings like below...

2014-09-03T02:23:09Z and 2014-09-03T03:24:57Z

Now, I have to assign the two datetimes to some variables and compare them, like this:

d1=2014-09-03T02:23:09Z
d2=2014-09-03T03:24:57Z

if (d1 < d2 ) 

I browsed many websites but couldn't find a solution. I tried the approach below but it doesn't work.

#set -vx

date1=`date -d "2014-09-03T04:27:23Z" +"%Y-%m-%dT%TZ"`
date2=`date -d "2014-09-03T02:23:09Z" +"%Y-%m-%dT%TZ"`
date3=`date -d "2014-09-03T05:23:09Z" +"%Y-%m-%dT%TZ"`

#date1=`date -d "2014-09-03T04:27:23Z"`
#date2=`date -d "2014-09-03T02:23:09Z"`
#date3=`date -d "2014-09-03T05:23:09Z"`


if [ ${date3} -lt  ${date1} ] ;  then
echo "working"
else
echo "gone"
fi

if [ ${date2} -gt ${date1} ] ;  then
echo "working"
else
echo "gone"
fi

I am facing the below error:

sh compare.sh

date: invalid date `2014-09-03T04:27:23Z'
date: invalid date `2014-09-03T02:23:09Z'
date: invalid date `2014-09-03T05:23:09Z'
working
working

Can someone please guide me on how to do datetime comparison in Unix?

Answer 1

Values in the ISO 8601 date format can be compared as strings when they're all in the same time zone (Z for Zulu or UTC in the example); that's one of the primary merits of the format.

The problem with if [ "${date3}" < "${date1}" ] notation is that you're doing input redirection in the middle of your test — which is not what you intend.

If you have Bash, you can use [[ in place of [:

if [[ "${date3}" < "${date1}" ]]

Alternatively, your test (or [) command may accept:

if [ "${date3}" '<' "${date1}" ]

The quotes around the comparison prevent it being interpreted as I/O redirection. (This works on Mac OS X 10.9.4, but that's using bash again. However, /bin/[ accepts it too, provided I omit the trailing ] — it must be because it is not invoked as [ that it requires the absence of ].) POSIX test is not required to support > or < as operators — only = and != are required for string comparison.

The classic command for the task is expr:

if [ $(expr "${date3}" '<' "${date1}") == 1 ]

This is probably the most portable mechanism (7th Edition Unix had expr for this task). It is also the clumsiest notation.

---

Updated question

I'm not clear that there is anything wrong with your date expressions per se, but the fact that they give you date: invalid date '2014-09-03T04:27:23Z' errors suggests that the version of date available to you does not support the notation. It works OK on Ubuntu 14.04 with the version from date --version starting date (GNU coreutils) 8.21. Here is a script that works for me when run by bash (but not when run by sh — because on Ubuntu that is a symlink to dash rather than bash). I used the -u (UTC) option so that the input and output times agreed — beware the dreaded time zone.

Note that if you are using GNU date, you can specify +%s to get the time value in seconds since The Epoch (which is 1970-01-01 00:00:00 +00:00) as a pure number, and those can be compared numerically.

dtcmp.sh

date1=`date -u -d "2014-09-03T04:27:23Z" +"%Y-%m-%dT%TZ"`
date2=`date -u -d "2014-09-03T02:23:09Z" +"%Y-%m-%dT%TZ"`
date3=`date -u -d "2014-09-03T05:23:09Z" +"%Y-%m-%dT%TZ"`

if [ ${date3} -lt ${date1} ]
then echo "true  (claim: [ $date3 -lt $date1 ])"
else echo "false (claim: [ $date3 -lt $date1 ])"
fi

if [ ${date2} -gt ${date1} ]
then echo "true  (claim: [ $date2 -gt $date1 ])"
else echo "false (claim: [ $date2 -gt $date1 ])"
fi

if [ ${date3} '<' ${date1} ]
then echo "true  (claim: [ $date3 '<' $date1 ])"
else echo "false (claim: [ $date3 '<' $date1 ])"
fi

if [ ${date2} '>' ${date1} ]
then echo "true  (claim: [ $date2 '>' $date1 ])"
else echo "false (claim: [ $date2 '>' $date1 ])"
fi

if [[ ${date3} < ${date1} ]]
then echo "true  (claim: [[ $date3 < $date1 ]])"
else echo "false (claim: [[ $date3 < $date1 ]])"
fi

if [[ ${date2} > ${date1} ]]
then echo "true  (claim: [[ $date3 > $date1 ]])"
else echo "false (claim: [[ $date3 > $date1 ]])"
fi

if [ $(expr ${date3} '>' ${date1}) = 1 ]
then echo "true  (claim: [ \$(expr $date3 '>' $date1) = 1 ])"
else echo "false (claim: [ \$(expr $date3 '>' $date1) = 1 ])"
fi

if [ $(expr ${date2} '>' ${date1}) = 1 ]
then echo "true  (claim: [ \$(expr $date2 '>'$date1) = 1 ])"
else echo "false (claim: [ \$(expr $date2 '>' $date1) = 1 ])"
fi

Sample outputs

bash

$ bash dtcmp.sh
dtcmp.sh: line 6: [: 2014-09-03T05:23:09Z: integer expression expected
false (claim: [ 2014-09-03T05:23:09Z -lt 2014-09-03T04:27:23Z ])
dtcmp.sh: line 11: [: 2014-09-03T02:23:09Z: integer expression expected
false (claim: [ 2014-09-03T02:23:09Z -gt 2014-09-03T04:27:23Z ])
false (claim: [ 2014-09-03T05:23:09Z '<' 2014-09-03T04:27:23Z ])
false (claim: [ 2014-09-03T02:23:09Z '>' 2014-09-03T04:27:23Z ])
false (claim: [[ 2014-09-03T05:23:09Z < 2014-09-03T04:27:23Z ]])
false (claim: [[ 2014-09-03T05:23:09Z > 2014-09-03T04:27:23Z ]])
true  (claim: [ $(expr 2014-09-03T05:23:09Z '>' 2014-09-03T04:27:23Z) = 1 ])
false (claim: [ $(expr 2014-09-03T02:23:09Z '>' 2014-09-03T04:27:23Z) = 1 ])
$ 

dash

$ dash dtcmp.sh
dtcmp.sh: 6: [: Illegal number: 2014-09-03T05:23:09Z
false (claim: [ 2014-09-03T05:23:09Z -lt 2014-09-03T04:27:23Z ])
dtcmp.sh: 11: [: Illegal number: 2014-09-03T02:23:09Z
false (claim: [ 2014-09-03T02:23:09Z -gt 2014-09-03T04:27:23Z ])
false (claim: [ 2014-09-03T05:23:09Z '<' 2014-09-03T04:27:23Z ])
false (claim: [ 2014-09-03T02:23:09Z '>' 2014-09-03T04:27:23Z ])
dtcmp.sh: 26: dtcmp.sh: [[: not found
false (claim: [[ 2014-09-03T05:23:09Z < 2014-09-03T04:27:23Z ]])
dtcmp.sh: 31: dtcmp.sh: [[: not found
false (claim: [[ 2014-09-03T05:23:09Z > 2014-09-03T04:27:23Z ]])
true  (claim: [ $(expr 2014-09-03T05:23:09Z '>' 2014-09-03T04:27:23Z) = 1 ])
false (claim: [ $(expr 2014-09-03T02:23:09Z '>' 2014-09-03T04:27:23Z) = 1 ])
$

ksh

$ ksh dtcmp.sh
dtcmp.sh[6]: [: 2014-09-03T05:23:09Z: arithmetic syntax error
false (claim: [ 2014-09-03T05:23:09Z -lt 2014-09-03T04:27:23Z ])
dtcmp.sh[11]: [: 2014-09-03T02:23:09Z: arithmetic syntax error
false (claim: [ 2014-09-03T02:23:09Z -gt 2014-09-03T04:27:23Z ])
dtcmp.sh[16]: [: <: unknown operator
false (claim: [ 2014-09-03T05:23:09Z '<' 2014-09-03T04:27:23Z ])
false (claim: [ 2014-09-03T02:23:09Z '>' 2014-09-03T04:27:23Z ])
false (claim: [[ 2014-09-03T05:23:09Z < 2014-09-03T04:27:23Z ]])
false (claim: [[ 2014-09-03T05:23:09Z > 2014-09-03T04:27:23Z ]])
true  (claim: [ $(expr 2014-09-03T05:23:09Z '>' 2014-09-03T04:27:23Z) = 1 ])
false (claim: [ $(expr 2014-09-03T02:23:09Z '>' 2014-09-03T04:27:23Z) = 1 ])
$ 

zsh

$ zsh dtcmp.sh
dtcmp.sh:[:6: integer expression expected: 2014-09-03T05:23:09Z
false (claim: [ 2014-09-03T05:23:09Z -lt 2014-09-03T04:27:23Z ])
dtcmp.sh:[:11: integer expression expected: 2014-09-03T02:23:09Z
false (claim: [ 2014-09-03T02:23:09Z -gt 2014-09-03T04:27:23Z ])
dtcmp.sh:16: condition expected: <
false (claim: [ 2014-09-03T05:23:09Z '<' 2014-09-03T04:27:23Z ])
dtcmp.sh:21: condition expected: >
false (claim: [ 2014-09-03T02:23:09Z '>' 2014-09-03T04:27:23Z ])
false (claim: [[ 2014-09-03T05:23:09Z < 2014-09-03T04:27:23Z ]])
false (claim: [[ 2014-09-03T05:23:09Z > 2014-09-03T04:27:23Z ]])
true  (claim: [ $(expr 2014-09-03T05:23:09Z '>' 2014-09-03T04:27:23Z) = 1 ])
false (claim: [ $(expr 2014-09-03T02:23:09Z '>' 2014-09-03T04:27:23Z) = 1 ])
$ 

Summary

• The shells all agree that the pure numeric comparison of ISO 8601 date strings does not work.
• Only bash accepts and works correctly with the '<' and '>' operators in plain test aka [ — the other shells either do not accept it or accept it but produce the wrong answer (which is worse!).
• For the shells that support [[, the results agree and are correct.
• All the shells work correctly with the expr notation.

For maximum portability, use expr. For maximum sanity, use [[.