What I want to achieve:

I'd like to filter a system log file by date, i.e. when I do:

$ cat /var/log/syslog | grep -i "error\|warn\|kernel" 

it prints lines like these for the three last days let say:

Apr  3 06:17:38 computer_name kernel: [517239.805470] IPv6: ADDRCONF(NETDEV_CHANGE): wlp3s0: link becomes ready
Apr  4 19:34:21 computer_name kernel: [517242.523165] e1000e: enp0s25 NIC Link is Up 1000 Mbps Full Duplex, Flow Control: None
Apr  5 09:00:52 computer_name kernel: [517242.523217] IPv6: ADDRCONF(NETDEV_CHANGE): enp0s25: link becomes ready

How to grep (select, or filter):

  • by date?
  • by date+hour?

What I tried:

$ cat /var/log/syslog | grep -i "Apr  5" | grep -i "error\|warn\|kernel" 

It works as expected on the syslog file, but not on the kern.log file for example, which only returns: Binary file (standard input) matches. And when I tail this particular file I can see the same starting date format than in the syslog file.


How to achieve the same on other logs like the kern.log file?

In addition, is it possible to filter:

  • by date range?
  • by date+hour range?

Hint: if possible, with "easy-to-remember commands".


With systemd we got journalctl which easily allows fine grained filtering like this:

journalctl --since "2 days ago"   
journalctl --since "2019-03-10" --until "2019-03-11 03:00"
journalctl -b # last boot 
journalctl -k # kernel messages
journalctl -p er # by priority (emerg|alert|crit|err|warning|info|debug)
journalctl -u sshd # by unit 
journalctl _UID=1000 # by user id

Examples can be combined!

  • 5
    Ok now this is so cool! – George Udosen Apr 5 '19 at 8:44
  • 2
    Often not even sudo is required (in particular if the user is member of the adm group, which the "main" user usually is). – PerlDuck Apr 5 '19 at 9:32

In general, the kern.log is a text file. But sometimes it happens that it contains some binary data, especially when the system has crashed before and the system could not close the file properly. You may then notice lines containing text like ^@^@^@^@^@^@^@^@^@ and such.

If grep notices its input is binary, it usually stops further processing and prints ... binary file ... instead. But there's a switch to change this behaviour. From the manpage:

File and Directory Selection
   -a, --text
          Process a binary file as if it were text; 
          this is equivalent to the --binary-files=text option.

You can try the following:

$ grep -a -i "Apr  5" /var/log/kern.log  | grep -i "error\|warn\|kernel"

(But I would actually prefer the journalctl solution given in another answer.)

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