linux_notes []

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==== tasks ====
  * [[add numbers on the command line]]
  * [[du on month end dates]]
  * [[change the default shell]]
  * [[Which shell am I using | Which shell am I using?]]
  * [[Difference between SHELL and 0 | What is the difference between \$SHELL and \$0?]]

==== what is my OS? ====
<code>
awk -F= '$1=="ID" {print $2}' /etc/os-release
</code>

Sample run:
<code>
% cat /etc/os-release
PRETTY_NAME="Debian GNU/Linux 12 (bookworm)"
NAME="Debian GNU/Linux"
VERSION_ID="12"
VERSION="12 (bookworm)"
VERSION_CODENAME=bookworm
ID=debian
HOME_URL="https://www.debian.org/"
SUPPORT_URL="https://www.debian.org/support"
BUG_REPORT_URL="https://bugs.debian.org/"
</code>

<code>
% awk -F= '$1=="ID" {print $2}' /etc/os-release     
debian
</code>

Ref:
  * I came across it in https://github.com/trimclain/.dotfiles/blob/main/Makefile

==== last reboot times ====
<code>
last reboot --time-format full
</code>

Sample run
<code>
 % last reboot --time-format full
reboot   system boot  5.10.0-17-amd64  Tue Dec  6 08:18:33 2022   still running
reboot   system boot  5.10.0-17-amd64  Tue Dec  6 08:11:34 2022 - Tue Dec  6 08:17:36 2022  (00:06)
reboot   system boot  5.10.0-17-amd64  Tue Dec  6 07:41:53 2022 - Tue Dec  6 08:11:01 2022  (00:29)
reboot   system boot  5.10.0-17-amd64  Sat Dec  3 13:18:10 2022 - Tue Dec  6 08:11:01 2022 (2+18:52)
reboot   system boot  5.10.0-17-amd64  Sat Dec  3 13:04:06 2022 - Tue Dec  6 08:11:01 2022 (2+19:06)
reboot   system boot  5.10.0-17-amd64  Wed Nov 16 11:18:41 2022 - Tue Dec  6 08:11:01 2022 (19+20:52)
reboot   system boot  5.10.0-17-amd64  Mon Sep  5 21:26:07 2022 - Wed Nov 16 11:18:01 2022 (71+14:51)
reboot   system boot  5.10.0-16-amd64  Mon Aug 15 05:27:05 2022 - Mon Sep  5 19:36:09 2022 (21+14:09)
reboot   system boot  5.10.0-16-amd64  Mon Aug  1 17:56:22 2022 - Mon Aug 15 05:26:20 2022 (13+11:29)
...
</code>

{{tag>["reboot history" "show year" "date format"]}}
==== ls and mv ====
Sample command
<code>
ls -rt *.txt | tail -n5 | tr '\n' '\0' | xargs -0 -i% mv % x1
</code>

Notes:
  * Works even if there are spaces in the filenames. Compare this with <code>
mv `ls -rt *.txt | tail -n5` x1
</code> which will not work if there are spaces in the filenames.
  * Does not work if the filenames contain newline characters.

Ref:- https://stackoverflow.com/a/937965/6305733

Related commands
<code>
ls -rt *.txt | tail -n5 | tr '\n' '\0' | xargs -0 -i% md5sum %
</code>
==== remove large directories ====

Use rsync to delete large directories.

<code>
mkdir empty_dir
rsync -a --delete empty_dir/    yourdirectory/
</code>

As per https://unix.stackexchange.com/questions/37329/efficiently-delete-large-directory-containing-thousands-of-files, it is more efficient than running "rm -rf" or some combination of find + "rm -rf".

==== stackoverflow answers I came across ====
  * prepend to a file - https://stackoverflow.com/questions/10587615/unix-command-to-prepend-text-to-a-file
  * remove leading and trailing spaces - https://unix.stackexchange.com/a/102021/198064
  * search for a string and count the number of characters per line - https://unix.stackexchange.com/questions/400650/counting-the-characters-of-each-line-with-wc

==== get file modified time in shell script ====
<code>
date +%Y%m%d_%H%M%S -r $input_file
</code>

Used it in | https://github.com/KamarajuKusumanchi/rutils/blob/master/python3/black_on_selected.sh

Found it in | https://stackoverflow.com/questions/16391208/print-a-files-last-modified-date-in-bash

==== tput: unknown terminal "xterm-256color" ====
When I moved my miniconda3 installation from /home/rajulocal/miniconda3 to /opt/rajulocal/miniconda3, I started getting
<code>
tput: unknown terminal "xterm-256color"
</code>

To fix it, I did
<code>
conda install --force-reinstall ncurses
</code>

It turns out that the --fore-reinstall option was important since simply doing
<code>
conda install ncurses
</code>
was not installing ncurses as it was already uptodate.

Ref:- https://stackoverflow.com/questions/32798940/tput-unknown-terminal-xterm-256color