Modify date of a file to be older that any other file in a certain folder

Question

I would like to modify the date of a file file in order for it to be older than the ones present in the directory dir. The value of the time difference doesn't matter, as the goal is only to make it older in order to make it older.

In order to do this, I need to touch file with a date value older than the ones present in dir. How can I recover the date value of the oldest file in dir and subtract from it a certain amount of time (e.g. 1 second)?

Answer 1

You could get the timestamps with find's -printf or with stat and sort them to get the oldest. Then subtract what you want and use it as the date specification for touch. find has the disadvantage of printing fractional seconds which have to be removed for the calculation.

oldest=$(stat -c "%Y" dir/*|sort|head -1)
touch -d "@$((oldest-1))" dir/file
# or touch -d "@$((oldest-60))" file # subtract 1 min to see the difference in normal ls -l output.

The date syntax -d @seconds-since-epoch is supported by GNU touch. It is not specified by POSIX.

The stat command is not specified by POSIX, it is part of GNU coreutils, see https://stackoverflow.com/questions/27828585/posix-analog-of-coreutils-stat-command.

So this solution should work on Linux systems, but probably not on general UNIX systems.

Answer 2

You can do this with a two-step process: copy the timestamp then adjust it to be older:

#find the eldest file in dir
eldest=$(ls -t dir | tail -1)

#duplicate the time
touch -r "dir/$eldest" myfile

#make the file one second older
touch -A -000001 myfile

Answer 3

Portably, you'd have to use perl to gather the timestamps, subtract the amount of time you'd like, then format it for touch.

t=$(perl make-oldest 1 dir/*)
if [ "$t" -gt 0 ]
then
  touch -t "$t" file
else
  echo "Sorry, unable to find a file!"
fi

... and the make-oldest perl script is:

#!/usr/bin/perl -w
use strict;
use POSIX qw(strftime);

my $subtract = shift;
my $oldest = 0;
for (@ARGV) {
  my @s = stat;
  next unless @s;
  if ($oldest) {
    $oldest = $s[9] if $s[9] < $oldest;
  } else {
    $oldest = $s[9];
  }
}

if ($oldest) {
  # convert ($oldest - $subtract) into CCCCYYMMDDhhmm.SS
  print strftime "%Y%m%d%H%M.%S\n", localtime($oldest - $subtract);
} else {
  print "0\n";
}

The intention is that your shell expands the wildcard dir/* to get the list of filenames. There are "two" arguments to the perl script: the number of seconds to subtract from the oldest file, and the list of files to gather timestamps from.

The perl script pulls off the subtraction argument then loops over the given files and keeps track of the oldest modification time. If it cannot read any files, then it will return zero (as tested by the wrapper script, above). If an oldest file was found, then we use the strftime function to convert the subtracted timestamp into the appropriate format for touch.

Answer 4

A three step solution is needed:

1. extract oldest timestamp (of only files, not dirs):

   $ oldestfile=$(find path/to/dir/ -maxdepth 1 -type f -printf '%A@ %f\n' | sort -rn | head -n 1)
   

2. extract the file name (remove timestamp):

   $ oldestfile="${oldestfile#* }"
   

3. touch (GNU) the file to the correct time:

   $ touch -r "$oldestfile" -d '-1 min' "file"
   

That will generate a file named file (if it doesn't exist) 1 minute older than the oldest file (excluding dirs) in path/to/dir/.