How to view live overwritten file in terminal?

Question

I have an output file which is constantly overwritten. There is a command in terminal tail -f filename which is helpful when the results are appended in the output file. However, I want to observe the first 10 or 20 lines of the constantly overwritten output file. Is there any such command?

do you want a refresh every given period or only if the file has changed — kyodev, Apr 05 '18 at 17:12
Do you want or need any guarantee that you will read the file at least once between each write? — chepner, Apr 05 '18 at 17:27

score 1 · Accepted Answer · answered Apr 05 '18 at 17:09

1

It refreshes every 1 second and prints first 100 lines of filename

watch -n 1 head -100 filename

answered Apr 05 '18 at 17:09

yausername

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nbari · Answer 2 · 2018-04-05T19:29:23.133

If using FreeBSD you would use wait_on, for example:

#!/bin/sh

while :; do
    tail file
    wait_on -w file
done

You may need to install it by using pkg install wait_on

In Linux you could use inotifywait something like:

#!/bin/sh

inotifywait --quiet --monitor --event modify file | while read; do
   tail file;
done

Based on your distro you may need to install the inotify-tools package, for example in CentOS: yum install inotify-tools

For more options check the answer to this quesion: https://superuser.com/q/181517/284722

NVRM · Answer 3 · 2018-04-05T20:19:51.513

Suggestion using php.

In the command line. This won't load the whole file in memory.

Output first 8192bytes of seed.txt.

php -r "echo fread(fopen('seed.txt','r'), 8192);"

fread() reads up to length bytes from the file pointer referenced by handle. Reading stops as soon as one of the following conditions is met http://php.net/manual/en/function.fread.php

See https://stackoverflow.com/a/15025877/2494754

How to view live overwritten file in terminal?

3 Answers3