Output file lines from last to first in Bash

Question

I want to display the last 10 lines of my log file, starting with the last line- like a normal log reader. I thought this would be a variation of the tail command, but I can't find this anywhere.

Vivianviviana

you mean starting from the last line, then last line -1, last line -2 etc...? – Gq

Answer 1

I ended up using tail -r, which worked on my OSX (tac doesn't)

tail -r -n10

Vivianviviana

This is the best answer because it is not GNU specific and avoids a "useless use of cat". smallo.ruhr.de/award.html – Supernova

Does not work root@elk:/# tail -r -n1 /var/log/logstash-test-output.log tail: invalid option -- 'r' – Sweetscented

@KarlMorrison This won't work for non-BSD tail commands. See my answer for more help... – Gaskin

Answer 2

GNU (Linux) uses the following:

tail -n 10 <logfile> | tac

tail -n 10 <logfile> prints out the last 10 lines of the log file and tac (cat spelled backwards) reverses the order.

BSD (OS X) of tail uses the -r option:

tail -r -n 10 <logfile>

For both cases, you can try the following:

if hash tac 2>/dev/null; then tail -n 10 <logfile> | tac; else tail -n 10 -r <logfile>; fi

NOTE: The GNU manual states that the BSD -r option "can only reverse files that are at most as large as its buffer, which is typically 32 KiB" and that tac is more reliable. If buffer size is a problem and you cannot use tac, you may want to consider using @ata's answer which writes the functionality in bash.

Gaskin

I'm sure this is right, but looks like the 'tac' command doesn't exist on OSX shell... – Vivianviviana

@Vivianviviana See #742966 for some alternatives to tac, not all of which are portable. – Grethel

Sorry, I have to ding you for "useless use of cat" smallo.ruhr.de/award.html since tail has the option -r to reverse the order. So tail -r -n 10 <logfile> is the better choice. As a bonus tail -r works on non-GNU systems like OSX, Solaris, AIX, etc. – Supernova

@RichardBronosky Nice link, I've edited my answer to also include the -r option. – Gaskin

@RickSmith respectable humility, friend. I received the award in 1999 and have been diligent ever since. Also, I was GNU-only from 1995-1999 before getting plunged into Posix systems and had to relearn a lot because the admins refused to install all the sugar I was used to. – Supernova

GNU tail does not have the -r option, see gnu.org/software/coreutils/manual/html_node/… – Naima

Answer 3

tac does what you want. It's the reverse of cat.

tail -10 logfile | tac

Augustineaugustinian

Answer 4

You can do that with pure bash:

#!/bin/bash
readarray file
lines=$(( ${#file[@]} - 1 ))
for (( line=$lines, i=${1:-$lines}; (( line >= 0 && i > 0 )); line--, i-- )); do
    echo -ne "${file[$line]}"
done

./tailtac 10 < somefile

./tailtac -10 < somefile

./tailtac 100000 < somefile

./tailtac < somefile

Fenella

Answer 5

This is the perfect methods to print output in reverse order

tail -n 10 <logfile>  | tac

Zigzagger

What are the asterisks for? – Vivianviviana

Including the word "perfect" is just inviting criticism. – Supernova

Answer 6

Pure bash solution is

_tac_echo() {
  IFS=$'\n'
  echo "${BASH_ARGV[*]}"
}

_tac () {
  local -a lines
  readarray -t lines
  shopt -s extdebug
  _tac_echo "${lines[@]}"
  shopt -u extdebug
}

cat <<'EOF' | _tac
1 one line[of] smth
2 two line{of} smth
3 three line(of) smth
4 four line&of smth
EOF

which prints

4 four line&of smth
3 three line(of) smth
2 two line{of} smth
1 one line[of] smth

Prasad