mjhjmtu

Is there a linux command to return the last x % of a file? I know tail can return a number of lines (-n) or number of bytes (-c), but what if I wanted to get the last 25% of a file? Is there a command to do that?

GNU split can do pretty much what you ask; given a text file in.txt, this will print the last quarter (part 4 out of 4) in terms of number of bytes (not lines), without splitting lines:

split -n l/4/4 in.txt

Here is the relevant documentation for split -n CHUNKS:

CHUNKS may be: [...] l/K/N output Kth of N to stdout without
splitting lines

In the very specific case mentioned as an example in the question,
4/4 requests the fourth quarter, or the last 25% of the input
file. For sizes that are not 1/n of the input, I do not think split
provides such a straightforward solution.

Complex bash + stat + bc + tail solution for any percentage:

get_last_chunk () 
 local p=$(bc <<<"scale=2; $1/100")
 tail -c $(printf "%.0f" $(echo "$(stat -c%s $2) * $p" 

• 
  $1 and $2 - are the function's 1st and 2nd arguments respectively


• 
  p - variable assigned with percentage value as float number (for ex. 0.14 or 0.55)


• 
  stat -c%s $2 - getting the actual size of the input file in bytes


• 
  tail -c N $2 - getting the last N bytes of the file

Or use the more simplified version:

get_last_chunk () 
 tail -c "$(($(stat -c%s - < "$2") * $1 / 100))" < "$2"))"

Signature: get_last_chunk <percent> <filename>

Sample file.txt:

apples
oranges
bananas
cherries

Test cases:

get_last_chunk 17 file.txt
ries 

get_last_chunk 77 file.txt
oranges
bananas
cherries

get_last_chunk 29 file.txt
cherries

To get the last $1% in terms of number of lines, portably (POSIXly):

last_percent() (
 percent=$1?; shift
 ret=0
 for file do
 lines=$(wc -l < "$file") &&
 tail -n "$((lines * percent / 100))" < "$file" || ret=$?
 done
 exit "$ret"
)

Example:

$ seq 12 > a; printf '%sn' aaaaaa bbbbb cccc dd > b
$ last_percent 25 a b
10
11
12
dd

For the last $1% in terms of number of bytes, replace wc -l with wc -c and tail -n with tail -c. Beware though that the first output line would likely be partial. On the same files as above, that would give:

$ last_percent 25 a b
11
12
c
dd

With ksh93, you could write it with only builtins and not a single fork as:

last_percent() (
 percent=$1; shift
 ret=0
 for file do
 command /opt/ast/bin/cat < "$file" <#((EOF*(100-percent)/100)) || ret=$?
 done
 exit "$ret"
)

Using its <#((...)) seeking operator.

The same with zsh (except that cat is not builtin there):

zmodload zsh/system zsh/stat
last_percent() ret=$?
 done
 return $ret