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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.

Question:

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:

sudo journalctl --since "2 days ago"   

sudo journalctl --since "2019-03-10" --until "2019-03-11 03:00"

sudo journalctl -b # last boot 

sudo journalctl -k # kernel messages

sudo journalctl -p er # by priority (emerg|alert|crit|err|warning|info|debug)

sudo journalctl -u sshd # by unit 

sudo journalctl _UID=1000 # by user id

Examples can be combined together!

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.)