Truncate Large Text File in UNIX / Linux

Posted on in Categories , last updated October 2, 2012

How do I truncate or shrink large text file under UNIX / Linux operating systems?

There are various tools and methods to truncate large text files under UNIX / Linux operating systems as follows.

Options #1: Shell Output Redirction

Your can truncate text file and make the size to zero using redirection:

> {filename}
ls -l largefile.txt
> largefile.txt
ls -l largefile.txt

Please note that largefile.txt file is created if it doesn’t exist. And largefile.txt file is overwritten if it exists.

Option #2: truncate Command

Use the truncate command to shrink or extend the size of each FILE to the specified size:

truncate -s 0 {filename.txt}
ls -lh filename.txt
truncate -s 0 filename.txt
ls -lh filename.txt

The -s option is used to set SIZE to zero. See truncate command man page for more details:
man truncate

Option #3: logrotate Utility

logrotate command is designed to ease administration of systems that generate large numbers of log files. It allows automatic rotation, compression, removal, and mailing of log files. Each log file may be handled daily, weekly, monthly, or when it grows too large. See how to use logrotate command to rotates, compresses, and mails system logs stored in /var/log and other locations under UNIX / Linux oses.

Option #4: /dev/null

The null device /dev/null act as the black hole that discards all data written to it under Unix like operating system. You can use it as follows (hat tip to Philippe Petrinko):

cp /dev/null largefile.txt

OR

cat /dev/null > largefile.txt

Posted by: Vivek Gite

The author is the creator of nixCraft and a seasoned sysadmin and a trainer for the Linux operating system/Unix shell scripting. He has worked with global clients and in various industries, including IT, education, defense and space research, and the nonprofit sector. Follow him on Twitter, Facebook, Google+.

23 comment

  1. Truncating brings up a good time to remind users about sparse files. Before you try to truncate a file, make sure the file isn’t a sparse file. With sparse files, checking the size with ls -l will NOT give you the true file size. In fact, the file may look many times larger than it really is.

    Always check the file with the command du to check the size of the file if there is any chance you may be dealing with a sparse file or if you aren’t sure.

  2. @Dave: Yes, good to know/remember.

    Nevertheless, [ls] can show blocks with [s / –size] option:


    dd if=/dev/zero of=sparse bs=1 count=1 seek=1024k # let's create a sparse-file
    ls -ls sparse # show me your blocks
    8 -rw-r--r-- 1 user1 user1 1048577 feb 24 18:38 sparse

  3. Rama Akella points out the useless use of cat . The end goal is that you don’t want to change the inode of the file you wish to truncate to zero bytes. It’s functionally equivalent to run `> file` to accomplish this.

    http://partmaps.org/era/unix/award.html#cat

    The truncate option that truncates the file in place saves me from the problem of having to dd the file to a temp file and write it back as I’m dealing with files in the hundreds of gigabytes. My outdated install of Gentoo doesn’t provide truncate in coreutils. I instead found the source from a FreeBSD machine and compiled it manually, not knowing at the time that the newer coreutils includes it.

    Locate the truncate.c file in the FreeBSD world source tree. I found it here:
    /usr/src/usr.bin/truncate/truncate.c
    Copy this to your *nix system and compile it
    gcc -o truncate truncate.c
    Test it out in case the C libraries are too divergent before you use it.

    For Gentoo I created an ebuild for those who are not able to update coreutils. I offer absolutely no warranties of my work or the files that I am using. I am redistributing the code without modification.

  4. I believe ‘truncate’ means to shorten, not (necessarily) to empty.
    I would like to shorten a very large log file to, say, 1MB.
    I wonder if Philippe P’s method demonstrates this? Thx!

    1. @jangyman

      Why would you do that?
      What part of the original file would you keep? The first 1Mbytes, or the last 1MB.

      Assuming you do not care whatever is still in your log file, (otherwise you would not consider loosing data by truncating it), your could :

      # first empty it
      cp /dev/null /somedir/mylogfile

      # enlarge your log file to 1 MB
      dd if=/dev/zero of=/somedir/mylogfile bs=1M count=1

      What do you think?

      1. You’d be better off typing
        truncate -s 1000000 /var/log/*log

        i usually do something like
        truncate -s 24000 /var/log/*log

        but my router is the linux machine, so 8 gigs is all i have of data. and some people only have 2-4 gigs.

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