Intro To 'truncate' Command In Linux

2023-06-18 - By Robert Elder

I use the 'truncate' command to truncate or extend a file to a given size:

truncate -s 0 foo.txt

Truncate A File

Let's start by creating a file called 'README.txt' that contains a simple 'hello world' satement:

echo "Hello World!" > README.txt

I can use the 'truncate' command to truncate this readme file by providing a value of 0 to the '-s' flag:

truncate -s 0 README.txt

Using the 'ls' command, you can see that the file is now empty:

ls -l

-rw-rw-r-- 1 robert robert 0 Jun 11 09:48 README.txt

And using the 'cat' command, you can see that there is indeed nothing in the file anymore:

cat README.txt

(no output)

Specify A Target File Size

If I specify a different value to the '-s' flag, like 12, the size of the file will be explicitly changed to 12 bytes:

touch README.txt
truncate -s 12 README.txt

ls -l

-rw-rw-r-- 1 robert robert 12 Jun 11 09:54 README.txt

xxd README.txt

00000000: 0000 0000 0000 0000 0000 0000            ............

If the file was previously larger than 12 bytes, the extra data will be discarded, but the first part of the file will remain unchanged:

echo "The quick brown fox jumps over the lazy dog" > README-full.txt

truncate -s 12 README-full.txt

xxd README-full.txt

00000000: 5468 6520 7175 6963 6b20 6272            The quick br

Size Units

You can also provide size units like kilobytes or megabytes to the '-s' flag.

truncate -s 12K README.txt

ls -l README.txt

-rw-rw-r-- 1 robert robert 12288 Jun 11 10:07 README.txt

truncate -s 12M README.txt

ls -l README.txt

-rw-rw-r-- 1 robert robert 12582912 Jun 11 10:08 README.txt

Relative Size Adjustments

The 'truncate' command also supports several modifiers to the size argument:

info truncate

...
     SIZE may also be prefixed by one of the following to adjust the
     size of each FILE based on its current size:
          ‘+’  => extend by
          ‘-’  => reduce by
          ‘<’  => at most
          ‘>’  => at least
          ‘/’  => round down to multiple of
          ‘%’  => round up to multiple of
...

Here's an example of using plus to add to the current file size:

rm README.txt
touch README.txt

xxd README.txt

(no output)

truncate -s +10 README.txt

xxd README.txt

00000000: 0000 0000 0000 0000 0000                 ..........

truncate -s +10 README.txt

xxd README.txt

00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000

Here's an example of using minus to reduce the size of the same file:

truncate -s -3 README.txt

xxd README.txt

00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 00

truncate -s -3 README.txt

xxd README.txt

00000000: 0000 0000 0000 0000 0000 0000 0000       ..............

The percentage sign modifier will round the file size up to the nearest multiple:

truncate -s %13 README.txt

xxd README.txt

00000000: 0000 0000 0000 0000 0000 0000 0000 0000  ................
00000010: 0000 0000 0000 0000 0000                 ..........

ls -l  README.txt

-rw-rw-r-- 1 robert robert 26 Jun 11 10:10 README.txt

And that's why the 'truncate' command is my favourite Linux command.

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