get a percentage of a file

Question

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?

Answer 1

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.

Answer 2

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" | bc)) "$2"
}

---

• $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

Answer 3

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 '%s\n' 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() {
  local percent=$1 ret=0 file n
  shift
  for file do
    {
      sysseek -w end 0 &&
        sysseek -w end ' - systell(0) * percent / 100' &&
        cat
    } < $file || ret=$?
  done
  return $ret
}