Linux – Use touch command to set modification time of a file to the Unix epoch

datelinuxtouch

Edit: Better worded question: How do I only use the touch command to set the modification time of a file to the unix epoch?

I know that the unix epoch value can be retrieved using "data %s", but how do I use the touch command (and only that command) to set the modification time to the unix epoch?

Edit2:

So, I found that this runs without any errors:

touch -m -d ”@$(date +%s)” fileexample.txt

Is this a correct way of setting the modification time of a file to the Unix epoch?

Original Question (disregard)…:

Using the Linux manual for the “touch” command, show the command that you would 
use to set the modification time of a file to the Unix epoch.

I understand that the Unix epoch is the amount of second (or miliseconds, I forgot) that has passed since the epoch (1970, January, 01)

What does the question mean by saying: setting the time "to the Unix epoch"?

So, Is it basically asking for today's time, or 1970 01 01, or…?

I know the command for this would be:

touch -m -t time file

But what time do I set it to?

Also, am I meant to use the unix epoch format for the time in the command?

Best Answer

-t doesn't accept epoch time, -d does

   -d, --date=STRING
          parse STRING and use it instead of current time

   -t STAMP
          use [[CC]YY]MMDDhhmm[.ss] instead of current time

You need to use -d or --date instead of -t and you need to put @ before epochtime format is used, as described in date manpages:

   EXAMPLES
       Convert seconds since the epoch (1970-01-01 UTC) to a date

              $ date --date='@2147483647'

Example:

touch --date=@1442968132 test.txt

If you want to change modify time only, use -m or --time modify or --time mtime, without it both modify and access times are changed.

   -m     change only the modification time

   --time=WORD
          change the specified time: WORD is access, atime, or use: equivalent to -a WORD is modify or mtime: equivalent to -m

Examples:

$ touch --date=@1442968132 test
$ stat test
  File: test
  Size: 0           Blocks: 0          IO Block: 4096   regular empty file
Device: fd03h/64771d    Inode: 43266017    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/    user1)   Gid: ( 1000/    user1)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2015-09-23 02:28:52.000000000 +0200
Modify: 2015-09-23 02:28:52.000000000 +0200
Change: 2018-11-23 11:34:59.893888360 +0100
 Birth: -

$ touch --date=@1542968132 test
$ stat test
  File: test
  Size: 0           Blocks: 0          IO Block: 4096   regular empty file
Device: fd03h/64771d    Inode: 43266017    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/    user1)   Gid: ( 1000/    user1)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2018-11-23 11:15:32.000000000 +0100
Modify: 2018-11-23 11:15:32.000000000 +0100
Change: 2018-11-23 11:35:06.893888073 +0100
Birth: -

$ touch -m --date=@1342968132 test
$ stat test
  File: test
  Size: 0           Blocks: 0          IO Block: 4096   regular empty file
Device: fd03h/64771d    Inode: 43266017    Links: 1
Access: (0664/-rw-rw-r--)  Uid: ( 1000/    user1)   Gid: ( 1000/    user1)
Context: unconfined_u:object_r:user_home_t:s0
Access: 2018-11-23 11:15:32.000000000 +0100
Modify: 2012-07-22 16:42:12.000000000 +0200
Change: 2018-11-23 11:35:22.300887441 +0100

Related Solutions

How to get modification time for a file (in UNIX TIME!) on AIX

You can use perl:

$ perl -le 'print((stat shift)[9])' test.txt
1402577190

Or GNU date:

$ date -r test.txt +%s

You can install GNU date on AIX refer to this link.

Shell – Leap seconds and date

I didn't find a simple solution to my question, so I've written a small Bash script to solve it. You need to download the leap seconds file given in the link below and put it with the script or change the path to it. I didn't write utc2unix.sh yet, but it's very easy to adapt. Do not hesitate to comment/give suggestions...

unix2utc.sh:

#!/bin/bash

# Convert a Unix timestamp to the real number of seconds
# elapsed since the epoch.

# Note: this script only manage additional leap seconds

# Download leap-seconds.list from
# https://github.com/eggert/tz/blob/master/leap-seconds.list

# Get current timestamp if nothing is given as first param
if [ -z $1 ]; then
    posix_time=$(date --utc +%s)
else
    posix_time=$1
fi

# Get the time at which leap seconds were added
seconds_list=$(grep -v "^#" leap-seconds.list | cut -f 1 -d ' ')

# Find the last leap second (see the content of leap-seconds.list)
# 2208988800 seconds between 01-01-1900 and 01-01-1970:
leap_seconds=$(echo $seconds_list | \
               awk -v posix_time="$posix_time" \
               '{for (i=NF;i>0;i--)
                   if (($i-2208988800) < posix_time) {
                    print i-1; exit
                    }
                } END {if (($(i+1)-2208988800) == posix_time) 
                    print "Warning: POSIX time ambiguity:",
                            posix_time,
                          "matches 2 values in UTC time!",
                          "The smallest value is given." | "cat 1>&2"
                }')
# echo $leap_seconds

# Add the leap seconds to the timestamp
seconds_since_epoch=$(($posix_time + $leap_seconds))

echo $seconds_since_epoch

Just some tests:

date --utc +%s && ./unix2utc.sh -> today and at least until June 2015 the difference is 25 sec.
./unix2utc.sh 78796799 -> 78796799
./unix2utc.sh 78796801 -> 78796802
./unix2utc.sh 78796800 -> 78796800 + on stderr: Warning: POSIX time ambiguity: 78796800 matches 2 consecutive values in UTC time! Only the smallest value is given.

Best Answer

Related Solutions

How to get modification time for a file (in UNIX TIME!) on AIX

Shell – Leap seconds and date

Related Question