Bash – How to tail a log file by time

Try out multitail. This is an übergeneralization of tail -f. You can watch multiple files in separate windows, highlight lines based on their content, and more.

multitail -c /path/to/log

The colors are configurable. If the default color scheme doesn't work for you, write your own in the config file. For example, call multitail -cS amir_log /path/to/log with the following ~/.multitailrc:

colorscheme:amir_log
cs_re:green:INFO
cs_re:red:SEVERE

Another solution, if you're on a server where it's inconvenient to install non-standard tools, is to combine tail -f with sed or awk to add color selection control sequences. This requires tail -f to flush its standard output without delay even when its standard output is a pipe, I don't know if all implementations do this.

tail -f /path/to/log | awk '
  /INFO/ {print "\033[32m" $0 "\033[39m"}
  /SEVERE/ {print "\033[31m" $0 "\033[39m"}
'

or with sed

tail -f /path/to/log | sed --unbuffered \
    -e 's/\(.*INFO.*\)/\o033[32m\1\o033[39m/' \
    -e 's/\(.*SEVERE.*\)/\o033[31m\1\o033[39m/'

If your sed isn't GNU sed, replace \o033 by a literal escape character and remove --unbuffered.

Yet another possibility is to run tail -f in an Emacs shell buffer and use Emacs's syntax coloring abilities.

On my linux system (GNU coreutils 8.12), I was able to check (using strace) that tail -f¹ uses the lseek system call to skip over most of the file quickly:

lseek(3, 0, SEEK_CUR)                   = 0
lseek(3, 0, SEEK_END)                   = 194086
lseek(3, 188416, SEEK_SET)              = 188416

This means that the size of the tracked file should not matter in anyway.

Maybe you can check if the same applies on your system. (Obviously, it should be the case.)

—
^{1. I also tried disabling inotify support with the undocumented ---disable-inotify, just in case.}