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Top Forums UNIX for Advanced & Expert Users Delete first 100 lines from a BIG File Post 302658389 by methyl on Tuesday 19th of June 2012 08:18:07 AM
Old 06-19-2012
@unoho
Sounds like your logfile is open by an application.
The usual technique when a logfile is open (like with some of the system logs) is to copy the file, then null it. This retains any file permissions and usually does not upset the application but be aware that there is a small time window where you might lose a message.

Code:
# Copy the logfile
cp -p /path/logfile /path/newname
# Null the logfile
> /path/logfile

This technique does not work for data files.
It is however much better to do logfile maintenance while the application is turned off, but in the case of some system processes this may not be possible.

If your system has the logrotate command, check it out.
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LOGSAVE(8)						      System Manager's Manual							LOGSAVE(8)

NAME
logsave - save the output of a command in a logfile SYNOPSIS
logsave [ -asv ] logfile cmd_prog [ ... ] DESCRIPTION
The logsave program will execute cmd_prog with the specified argument(s), and save a copy of its output to logfile. If the containing directory for logfile does not exist, logsave will accumulate the output in memory until it can be written out. A copy of the output will also be written to standard output. If cmd_prog is a single hyphen ('-'), then instead of executing a program, logsave will take its input from standard input and save it in logfile logsave is useful for saving the output of initial boot scripts until the /var partition is mounted, so the output can be written to /var/log. OPTIONS
-a This option will cause the output to be appended to logfile, instead of replacing its current contents. -s This option will cause logsave to skip writing to the log file text which is bracketed with a control-A (ASCII 001 or Start of Header) and control-B (ASCII 002 or Start of Text). This allows progress bar information to be visible to the user on the console, while not being written to the log file. -v This option will make logsave to be more verbose in its output to the user. AUTHOR
Theodore Ts'o (tytso@mit.edu) SEE ALSO
fsck(8) E2fsprogs version 1.44.1 March 2018 LOGSAVE(8)
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