Command ‘date +FORMAT’ — What are %a, %A, %b, etc. called

dateterminology

The date command outputs the current date and time like this: Fri Apr 12 15:04:03 UTC 2013.

To have the output date-time in a custom format we can use date +FORMAT, for example, like this: date "+%Y-%m-%dT%H:%M:%S%:z" which gives something like 2013-04-12T15:04:37+00:00.

But I want to know what the %Y, %m, %d, %H, %M, %S, etc. are collectively called (i.e. the terminology). Also, what's the T called, as it's different from the rest?

Best Answer

The term used in the POSIX specification of the date command is "conversion specifications".

The format string for the date command is closely based on the format string for C's printf function; the C standard also refers to things like %d as "conversion specifications".

T not preceded by % is just a character: "All other characters shall be copied to the output without change."

According to the change history section of the POSIX description of date:

The DESCRIPTION is updated to refer to conversion specifications, instead of field descriptors for consistency with the LC_TIME category.

So apparently an earlier version of the specification used the phrase "field descriptors", but "conversion specifications" is the current official term.

Of course that doesn't mean you have to refer to them that way.

Related Solutions

Linux – Best practice to customize date/time format system-wide

As pointed out in some of the comments, you can either copy the rest of the sections and customize LC_TIME in your custom locales file, or configure LC_TIME in /etc/locale.conf. As the locale.conf(5) man-page suggests, you could also use the kernel commandline option locale.LC_TIME= to override the setting at boot time.

Linux – How to output a date/time as “20 minutes ago” or “9 days ago”, etc

Here's an extension of the_velour_fog's answer, adapted to use second/minute/day/month/year.

#!/usr/bin/env bash

last_run="2018-06-21T21:10:18-06:00"


function rel_fmt_low_precision() {
    local SEC_PER_MINUTE=$((60))
    local   SEC_PER_HOUR=$((60*60))
    local    SEC_PER_DAY=$((60*60*24))
    local  SEC_PER_MONTH=$((60*60*24*30))
    local   SEC_PER_YEAR=$((60*60*24*365))

    local last_unix="$(date --date="$1" +%s)"    # convert date to unix timestamp
    local now_unix="$(date +'%s')"

    local delta_s=$(( now_unix - last_unix ))

    if (( delta_s <  SEC_PER_MINUTE * 2))
    then
        echo "last run "$((delta_s))" seconds ago"
        return 
    elif (( delta_s <  SEC_PER_HOUR * 2))
    then
        echo "last run "$((delta_s / SEC_PER_MINUTE))" minutes ago"
        return 
    elif (( delta_s <  SEC_PER_DAY * 2))
    then
        echo "last run "$((delta_s / SEC_PER_HOUR))" hours ago"
        return 
    elif (( delta_s <  SEC_PER_MONTH * 2))
    then
        echo "last run "$((delta_s / SEC_PER_DAY))" days ago"
        return 
    elif (( delta_s <  SEC_PER_YEAR * 2))
    then
        echo "last run "$((delta_s / SEC_PER_MONTH))" months ago"
        return 
    else
        echo "last run "$((delta_s / SEC_PER_YEAR))" years ago"
        return 
    fi
}

rel_fmt_low_precision "`date`"
rel_fmt_low_precision "2018-06-21 21:10:18"
rel_fmt_low_precision "2018-06-21 20:10:18"
rel_fmt_low_precision "2018-05-21 21:10:18"
rel_fmt_low_precision "2017-06-21 20:10:18"
rel_fmt_low_precision "2016-06-21 20:10:18"

Here's how it decides which unit to use: it uses the largest unit that gives a number of at least two.

Example: If something happened 72 hours ago, it will output in days. If something happened exactly an hour ago, it will use minutes.

Best Answer

Related Solutions

Linux – Best practice to customize date/time format system-wide

Linux – How to output a date/time as “20 minutes ago” or “9 days ago”, etc

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