Bash

ls -srSk ~/Downloads/

• s - size in blocks
• r - reverse order (largest at bottom)
• k - 1024 byte block size
• S - sort by size

ls -d ~/Downloads/*

• Shows path to all files in the directory

find . -maxdepth 0 -iname '*ansible*' -print0 | xargs -I{} --null mv {} ansible

This looks for any file name with mixed case ansible in it (in which the file may have spaces), pipes to xargs which will use mv to move the file to the directory 'ansible'.

$ date
Thu May  9 09:47:03 MDT 2019
$ TZ=Z date -d '2 hours ago' +%FT%TZ
2019-05-09T13:47:04Z

• history reuse and bang commands

An extract from something in the past:

Use

systemctl status .service

journalctl --follow

If you have bash_completion on your distro, there is tab completion of every switch and every service that is actually installed. systemctl will show all the commands systemctl --<tab> will show all the long options and systemctl start <tab> will show all installed services etc.

man systemd.index

From a mailing list:

>Does anyone have a script that sorts IPv4 addresses?  
>I've tried all sorts of tricks with "sort -n -k" etc, but can't seem to get it right.

With an older sort:
sort -t'.' -n -k1,1 -k2,2 -k3,3 -k4,4

With modern GNU sort, a "version syntax" sort is available:
sort -V 

And something else:

the canonical "sort | uniq -c | sort -nr" so beloved of admins everywhere

Source for this is from Quick hex / decimal conversion using CLI.

Since, one way or another, I have a bash console up somewhere, it would be convenient to perform decimal <-> hexadecimal conversions right there.

To convert hexadecimal to decimal, a few native ways (the third example requires the bc app installed, and as it isn't installed by default on stretch or cygwin, I don't have output):

$ echo $((0x2ac))
684

$ printf '%d\n' 0x2ac
684

$ echo 'ibase=16;obase=A;15A' | bc

To convert the other way around, the printf commands seems to be the most straight-forward:

$ printf '%x\n' 684
2ac

To use bc: a) notation for ibase, obase is decimal, b) input number needs to be in upper case, c) some playing around to make ibase/obase convert correctly.

$ echo 'ibase=10;obase=16;684' | bc

For Debian Stretch:

• Reboot computer and enter the grub menu
• 'e' to edit the entry, usually the first/default entry
• on the line starting with 'linux', at 'ro quiet', substitute 'rw init=/bin/bash', or for read only access and a different shell, use 'ro init=/bin/sh'
• press control-X or F10 to reboot
• direct access to root command line is provided (if forgot to set rw instead of ro, use 'mount -n -o rw,remount /' at the command line)
• after making changes, reboot into normal security mode

With reference to archlinunx.

Alternative is to add 'debug-shell.service' to the boot line, F10 to boot, then use ctrl+alt+f9 to reach the debug session

$ cat /proc/sys/kernel/random/uuid
2252a738-2997-4002-b6fb-2cbb272f8adc
$ cat /proc/sys/kernel/random/uuid
ba65ec83-0698-4788-97fe-822ba7cb72b0

When working with command line substitutions, one way to check for presence (or lack thereof) of a command line variable:

if [ "x$1" == "x" ];then
...
fi

If it matches, then there was no command line variable, as in 'x matches x'.

Courtesy of StackExchange, here are some methods of obtaining a list of shell variables.

Print exported environment variables:

printenv

env

To show a list of environment variables plus shell variables (unexported environment variables):

( set -o posix ; set ) | less

In KDE, to see environment variables exported/inherited to the graphic environment:

press Alt-F2
run:  xterm -e bash --noprofile --norc
run: env

Not knowing any other way to remove a large number of similarily named files from a directory (such as netflow files), and not wanting to do it manually, the following one liner creates something to start with:

ls -1 -A ft | sed "s/^/rm ft\//" > d.sh

It does a directory listing with one column, and does not include the '.' and '..' entries. It then uses sed (stream editor> to insert a rm command at the beginning of each line. The results are written out to a file for further tweaking.

chmod +x d.sh

will make the script executable.