Automatically truncating long output in bash

Question

Is it possible to automatically truncate long output in bash?

Ideally I would like to define a limit, say MAXLINES, and automatically apply tail -n $MAXLINES to every command that outputs more than MAXLINES lines to the terminal (but not in other contexts, e.g. pipes, obviously).

Nahuel Fouilleul · Accepted Answer · 2019-03-21 08:29:48Z

2

May be done using PROMPT_COMMAND

MAXLINES=10
tmp_out=/tmp/$$.output
PROMPT_COMMAND='touch "$tmp_out"; tail -n "$MAXLINES" "$tmp_out" >/dev/stdin; exec >"$tmp_out"'

How it works

tmp_out=/tmp/$$.output temporary file used to store output of command
touch "$tmp_out" : create empty file if doesn't exist so that tail command doesn't fail at first call
tail -n "$MAXLINE" "$tmp" > /dev/stdin : show first maxlines of output
exec > "$tmp_out" : clear temporary file and redirects current process output (file descriptor 1) to this file, in case stderr can also be redirected to another file to be truncated (for example 2> "$tmp_err").

To run a command without redirection

exec >/dev/stdin; ... the command

or (space after { is important)

{ the command;}>/dev/stdin

To retrieve inital settings

unset PROMPT_COMMAND; exec >/dev/stdin

edited Mar 21, 2019 at 8:29

answered Mar 20, 2019 at 8:28

Nahuel Fouilleul

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4 Comments

Walter A Over a year ago

You might first want to open a new shell with bash. When you want normal output, you can exit.

Marcus Ritt Over a year ago

@Nahuel: very nice. However, when you say ls -la | less, for example, long output gets also truncated, since less doesn’t know it’s supposed to output to a terminal. Any idea how to avoid this?

Nahuel Fouilleul Over a year ago

indeed because the tail is done after the command finishes in prompt so program that work with tty can't work interactively to restore output following can be done exec >/dev/stdin ; ls -la | less

Nahuel Fouilleul Over a year ago

note also that some program like ls behaves differently when output is a tty or a file for example with escape sequence, compare touch ''$'\033''[31mhello'$'\033''[0m' and ls *hello* and ls *hello* >/dev/stdin , also ls -a in a tty will display file names in columns whereas in a file will display one file by line like ls -a1

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