Linux – Extract dates from /etc/shadow

As far as I know, all unix variants have an /etc/passwd file with the traditional layout, for the sake of applications that read the file directly. Each line in the file contains colon-separated records which correspond to struct passwd fields:

1. user name (login)
2. encrypted password (if present)
3. user id (number, in decimal)
4. principal group id (number, in decimal)
5. Gecos field. This field is not used by system programs except for display purposes. It is normally a comma-separated list of fields, the first three being full name, office number and telephone number.
6. home directory
7. login shell

One thing that varies between systems is how much liberty you can take with the syntax. For example, GNU libc (i.e. Linux) ignores lines that begin with #: they are comments. GNU libc also ignores whitespace at the beginning of a line, so they can be indented. An invalid line might cause programs to stop processing the file or to skip to the next line.

Most modern systems no longer store an encrypted password in the second field. The content of that field is not a reliable indication of whether the user has a password set (and even if you found that out, this is not a reliable indication of whether the user can log in, because there are many other authentication methods such as SSH keys, one-time passwords, biometrics, smartcards, …).

When passwords aren't in /etc/passwd, where they are is system-dependent. The Rosetta Stone for Unix mentions many unix variants.

• Solaris uses /etc/shadow, and this has been copied by others including Linux. Linux and Solaris shadow files have the same format; I don't know if the other systems that have a file called /etc/shadow use the same format.
• BSD systems have /etc/master.passwd, and additionally have database files for faster access, updated by pwd_mkdb.

Remember that /etc/passwd hasn't been guaranteed to contain the full list of users for a couple of decades: users can come from other databases such as NIS (YP) or LDAP. As a system administrator, avoid edit the /etc/passwd file directly; use vipw instead, if your system provides it (and if it doesn't, consult your manuals to see what method is recommended to modify the user database).

What I wrote above goes for groups, too. The fields in /etc/group are struct group members: group name, password (largely unused), numerical group id, and a comma-separated list of user names (the users who have this group as a secondary group). Linux has a /etc/gshadow file, but this is rarely used, as group authentication is not widely practiced.

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.