Unix / Linux: Remove Bytes from End of File

Objective: Truncate or remove last n bytes from a binary (or text) file on Unix / Linux.

Let’s say that we have a binary file called foo.bin and we need to strip the last 20 bytes from the file. To do that, we can either use the dd or truncate utility. truncate is my preferred choice.

Before we strip the last 20 bytes from the foo.bin file, we first need to determine the new file size. To do that, we will need to use the stat command.

$ stat -c '%s' foo.bin
23874470

The file size is 23874470 bytes. To minus 20 from the result, we can use the following shell arithmetic expression. This step is required only if you are trying to script the whole process. The newfsize variable will be assigned with the new file size of 23874450 bytes.

$ newfsize=$(($(stat -c '%s' foo.bin) - 20))

$ echo $newfsize
23874450

If the above expression does not work, use the expr utility to evaluate the expression.

$ newfsize=$(expr $(stat -c '%s' foo.bin) - 20)

$ echo $newfsize
23874450

We can now use the truncate command to remove the last 20 bytes from the file. Note that by default, truncate will modify the input file.

$ truncate -s $newfsize foo.bin

$ stat -c '%s' foo.bin
23874450

You can also specify -20 as the size to truncate to reduce the file size by 20 bytes.

$ truncate -s -20 foo.bin

$ stat -c '%s' foo.bin
23874450

To truncate a file using dd, we can use the following command syntax. The output will be written to dd-foo.out file.

$ dd if=foo.bin of=dd-foo.out bs=1 count=$newfsize
23874450+0 records in
23874450+0 records out
23874450 bytes (24 MB) copied, 65.5627 s, 364 kB/s

We have to specify a block size of 1 byte (bs=1) and set the count parameter to the new file size. Note that this is going to be very slow as the block size of 1 byte is not optimal for disk IO.

ibrahim = { interested_in(unix, linux, android, open_source, reverse_engineering); coding(c, shell, perl, php, python, java, javascript, nodejs, angular, react); plays_on(xbox, ps4); linux_desktop_user(true); }