linux shell date question

shell>/bin/date -d "091029 20:18:02" +%s
1256827682

Similarly I created shell script:

#My.sh
myDate=`date +'%y%m%d_%H%M%S'`
myDat开发者_JS百科e1=`echo $myDate | sed 's/_/ /g'`
myDate2=`echo $myDate1 | sed 's/\([0-9][0-9][0-9][0-9][0-9][0-9]\) \([0-9][0-9]\)\([0-9][0-9]\)\([0-9][0-9]\)/\/bin\/date -d "\1 \2:\3:\4" +%s/'`
print $myDate2
`$myDate2`

But it doesn't execute above command. WHy?

The backticks in the last line executes the command and then executes the output of that command, which is probably not what you are trying to do -- when I run this I get the output -bash: 1256829614: command not found. Remove the backticks on the last line so that it just says

    $myDate2

In addition to the solutions above, I would like to offer assistant to get to myDate2:

$ myDate=$(date +'/bin/date -d "%y%m%d %H:%M:%S" +%%s')
$ echo $myDate
/bin/date -d "091029 09:06:29" +%s

The notation:

`$myDate2`

expands $myDate2 and executes the command (and I'll come back to why there are problems with that), and then captures the output - and tries to run the output.

What you are looking for is eval:

eval $myDate2

Handling quotes is tricky - and eval is often a part of the answer. When you build up a string with internal quotes, you need to use eval to get the shell to put the quotes back together.

One very useful tool that I have is a program called al - for argument list.

#include <stdio.h>
int main(int argc, char **argv)
{
    while (*++argv != 0)
        puts(*argv);
    return(0);
}

It prints each separate argument on a separate line. It was almost the first thing I did when looking at what you are up to.

myDate=`date +'%y%m%d_%H%M%S'`
myDate1=`echo $myDate | sed 's/_/ /g'`
myDate2=`echo $myDate1 | sed 's/\([0-9][0-9][0-9][0-9][0-9][0-9]\) \([0-9][0-9]\)\([0-9][0-9]\)\([0-9][0-9]\)/\/bin\/date -d "\1 \2:\3:\4" +%s/'`
print $myDate2
#`$myDate2`
al $myDate2
eval al $myDate2
eval $myDate2

The trace output from this was:

+ date +%y%m%d_%H%M%S
+ myDate=091029_082546
+ sed 's/_/ /g'
+ echo 091029_082546
+ myDate1='091029 082546'
+ sed 's/\([0-9][0-9][0-9][0-9][0-9][0-9]\) \([0-9][0-9]\)\([0-9][0-9]\)\([0-9][0-9]\)/\/bin\/date -d "\1 \2:\3:\4" +%s/'
+ echo 091029 082546
+ myDate2='/bin/date -d "091029 08:25:46" +%s'
+ print /bin/date -d '"091029' '08:25:46"' +%s
/bin/date -d "091029 08:25:46" +%s
+ al /bin/date -d '"091029' '08:25:46"' +%s
/bin/date
-d
"091029
08:25:46"
+%s
+ eval al /bin/date -d '"091029' '08:25:46"' +%s
+ al /bin/date -d '091029 08:25:46' +%s
/bin/date
-d
091029 08:25:46
+%s
+ eval /bin/date -d '"091029' '08:25:46"' +%s
+ /bin/date -d '091029 08:25:46' +%s
usage: date [-jnu] [-d dst] [-r seconds] [-t west] [-v[+|-]val[ymwdHMS]] ... 
            [-f fmt date | [[[mm]dd]HH]MM[[cc]yy][.ss]] [+format]

Note how when I ran 'al $myDate2' the date string was split into two arguments, but when I ran 'eval al $myDate2', the string was one argument - as required. I was testing on MacOS X, where the data command does not accept the date string format you supplied - that is a whole separate problem. But getting the string healed requires 'eval'.

I didn't even address the issue of what the shell script was trying to do.

I gather from Hai Vu's answer that we're really after the current time in seconds since the epoch; I can sort of see how that might be.

On MacOS X, that is obtained trivially:

date +'%s'

(where the single quotes really aren't needed). The MacOS X manual page also includes the example:

      date -j -f "%a %b %d %T %Z %Y" "`date`" "+%s"

This seems a bit convoluted - but would allow you to find the seconds since the epoch for any date previously given by the date command - or a date that will be given at some time in the future (by replacing the back-quoted date with the previous string).

An æon or so ago, I wrote programs 'systime' to print the current time as the number of seconds past the epoch, and also a program 'timestamp' to convert such values back into formatted dates - because none of the standard versions of the 'date' command supported such mechanisms back then (before the C standard was standard, and therefore before strftime() was widely available). I also have a program 'strptime' for converting a formatted date into a timestamp. Ah well - nice to know that the standard programs can now do it.

However, I note that the MacOS 'date' command is a superset of the POSIX standard version; I suspect that the Linux (GNU) 'date' command is a different superset of the POSIX standard, and so on for each platform.