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Top Forums Shell Programming and Scripting How to find yesterdays file - shell script Post 302428930 by jim mcnamara on Friday 11th of June 2010 09:03:36 AM
Old 06-11-2010
Unless you have GNU date there is no simple code to do this. CFA Johnson, a forum member, has a website with a shell script library to do date arithmetic. The code is good, but doing dates in shell in an involved.

Rolling your own date routines is not best practice. Finding yesterday when the date today is March 1 is a classic example. You have to know whether it is a leap year or not, then roll back to Feb 28 or 29.

The standard way to get yesterday:

calculate the current Julian day, subtract one, then convert Julian -> Gregorian date.

get current time in UNIX epoch seconds and subtract 86400, then convert epoch seconds to a Gregorian date.

For production systems always use known good libraries for date calculations.
 

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ATF-SH(1)						    BSD General Commands Manual 						 ATF-SH(1)

NAME
atf-sh [-s shell] -- interpreter for shell-based test programs SYNOPSIS
atf-sh script DESCRIPTION
atf-sh is an interpreter that runs the test program given in script after loading the atf-sh(3) library. atf-sh is not a real interpreter though: it is just a wrapper around the system-wide shell defined by ATF_SHELL. atf-sh executes the inter- preter, loads the atf-sh(3) library and then runs the script. You must consider atf-sh to be a POSIX shell by default and thus should not use any non-standard extensions. The following options are available: -s shell Specifies the shell to use instead of the value provided by ATF_SHELL. ENVIRONMENT
ATF_LIBEXECDIR Overrides the builtin directory where atf-sh is located. Should not be overridden other than for testing purposes. ATF_PKGDATADIR Overrides the builtin directory where libatf-sh.subr is located. Should not be overridden other than for testing purposes. ATF_SHELL Path to the system shell to be used in the generated scripts. Scripts must not rely on this variable being set to select a specific interpreter. EXAMPLES
Scripts using atf-sh(3) should start with: #! /usr/bin/env atf-sh Alternatively, if you want to explicitly choose a shell interpreter, you cannot rely on env(1) to find atf-sh. Instead, you have to hardcode the path to atf-sh in the script and then use the -s option afterwards as a single parameter: #! /path/to/bin/atf-sh -s/bin/bash ENVIRONMENT
ATF_SHELL Path to the system shell to be used in the generated scripts. SEE ALSO
atf-sh(3) BSD
September 27, 2014 BSD
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