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Top Forums UNIX for Dummies Questions & Answers Command expansion and variables Post 302821517 by BrandonD on Friday 14th of June 2013 03:53:40 PM
Old 06-14-2013
Command expansion and variables

Command expansions are pretty useful in some situations, however, I am not understanding why expansions do not update files, folders, directories, or even the current time, when there are placed in variables. For example:

Code:
#create variable
d=$(date)


result

Code:
echo $d
Fri Jun 14 15:48:04 EDT 2013
echo $d

Fri Jun 14 15:48:04 EDT 2013
echo $d

Fri Jun 14 15:48:04 EDT 2013
echo $d

Fri Jun 14 15:48:04 EDT 2013
echo $d

I would assume that it would up the the current time.
 

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getdate(3C)															       getdate(3C)

NAME
getdate() - convert user format date and time SYNOPSIS
Obsolescent Interface DESCRIPTION
The function converts user definable date and/or time specifications pointed to by string into a The structure declaration is in the header file (see ctime(3C)). User-supplied templates are used to parse and interpret the input string. The templates are text files created by the user and identified via the environment variable should be set to indicate the full path name of the template file. The first line in the template that matches the input specification is used for interpretation and conversion into the internal time format. Upon successful completion, returns a pointer to a otherwise, it returns NULL and the symbol is set to indicate the error. The following field descriptors are supported: same as abbreviated weekday name full weekday name abbreviated month name full month name locale's appropriate date and time representation century number (00 through 99; leading zeros are permitted but not required) day of the month (01 through 31; the leading 0 is optional) same as date as abbreviated month name hour (00 through 23) hour (01 through 12) month number (01 through 12) minute (00 through 59) same as locale's equivalent of either AM or PM time as time as seconds (00 through 61) insert a tab time as weekday number (Sunday = 0 through Saturday = 6) locale's appropriate date representation locale's appropriate time representation year without century (00 through 99). For inputs 69-99, the 20th century(1900s) is assumed, and for inputs 00-68, the 21st century(2000s) is assumed. year as ccyy (e.g., 1986) time zone name or no characters if no time zone exists. If the time zone supplied by is not the same as the time zone expects, an invalid specification error is returned. calcu- lates the expected time zone from the environment variable. Month and weekday names may consist of any combination of uppercase and lowercase letters. The user can request that the input date or time specification be in a specific language by setting the category (see setlocale(3C)). For descriptors that allow leading zeros, leading zeros are optional. However, the number of digits used for those descriptors must not exceed two, including leading zeros. Extra whitespace in either the template file or in string is ignored. The field descriptors and are not supported if they include unsupported field descriptors. The following example shows the possible contents of a template: The following are examples of valid input specifications for the above template: If the category is set to a German locale that includes as a weekday name and as a month name, the following would be valid: This example shows how local date and time specification can be defined in the template: +---------------------------+------------------+ | Invocation | Line in Template | +---------------------------+------------------+ |getdate("11/27/86") | %m/%d/%y | |getdate("27.11.86") | %d.%m.%y | |getdate("86-11-27") | %y-%m-%d | |getdate("Friday 12:00:00") | %A %H:%M:%S | +---------------------------+------------------+ The following rules apply when converting the input specification into the internal format: o If only the weekday is given, today is assumed if the given day is equal to the current day, and next week if it is less. o If only the month is given, the current month is assumed if the given month is equal to the current month, and next year if it is less and no year is given (the first day of the month is assumed if no day is given). o If no hour, minute and second are given, the current hour, minute and second are assumed. o If no date is given, today is assumed if the given hour is greater than the current hour and tomorrow is assumed if it is less. The following examples help to illustrate the above rules assuming that the current date is and the category is set to the default locale. +-------------+----------+------------------------------+ | | Line in | | | Input | Template | Date | +-------------+----------+------------------------------+ |Mon | %a | Mon Sep 22 12:19:47 EDT 1986 | |Sun | %a | Sun Sep 28 12:19:47 EDT 1986 | |Fri | %a | Fri Sep 26 12:19:47 EDT 1986 | |September | %B | Mon Sep 1 12:19:47 EDT 1986 | |January | %B | Thu Jan 1 12:19:47 EST 1987 | |December | %B | Mon Dec 1 12:19:47 EST 1986 | |Sep Mon | %b %a | Mon Sep 1 12:19:47 EDT 1986 | |Jan Fri | %b %a | Fri Jan 2 12:19:47 EST 1987 | |Dec Mon | %b %a | Mon Dec 1 12:19:47 EST 1986 | |Jan Wed 1989 | %b %a %Y | Wed Jan 4 12:19:47 EST 1989 | |Fri 9 | %a %H | Fri Sep 26 09:00:00 EDT 1986 | |Feb 10:30 | %b %H:%S | Sun Feb 1 10:30:00 EST 1987 | |10:30 | %H:%M | Tue Sep 23 10:30:00 EDT 1986 | |13:30 | %H:%M | Mon Sep 22 13:30:00 EDT 1986 | +-------------+----------+------------------------------+ Obsolescent Interface converts user format date and time. Also see the section. ERRORS
Upon failure, returns NULL and the symbol is set to indicate the error. The following is a complete list of the settings and their interpretation: the environment variable is null or undefined, the template file cannot be opened for reading, failed to get file status information, the template file is not a regular file, an error is encountered while reading the template file, memory allocation failed (not enough memory available), there is no line in the template that matches the input, invalid input specification. For example, February 31; or the time specified cannot be represented in the data type in 32-bit HP-UX (which represents Tuesday January 19 03:14:07 UTC, 2038) or exceeds the maximum date supported in 64-bit HP-UX (which is Friday December 31 23:59:59 UTC, 9999). WARNINGS
The return value for points to data whose content is overwritten by each call by the same thread. is an obsolescent interface supported only for compatibility with existing DCE applications. New multi-threaded applications should use SEE ALSO
ctime(3C), ctype(3C), setlocale(3C), strftime(3C), thread_safety(5). getdate(3C)
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