Remove lines using vim

To remove all the lines in vim that contain certain string execute:
:g/string/d

To do the opposite i.e remove the lines that do not contain certain string execute:
:v/string/d

dd

dd is a command-line utility for Unix and Unix-like operating systems, the primary purpose of which is to convert and copy files.
dd stands for copy and convert (called dd because cc is already in use by C compiler).

[Options]
bs : BYTES read and write up to BYTES bytes at a time (default: 512)
if : FILE read from FILE instead of stdin
of : FILE write to FILE instead of stdout
status : LEVEL The LEVEL of information to print to stderr; 'none' suppresses everything but error messages, 'noxfer' suppresses the final transfer statistics, 'progress' shows periodic transfer statistics

[Examples]
1 - Wipe the disk partition
# dd if=/dev/zero out=/dev/sd<?><n>

2  - Make USB stick bootable
Step 1 : Find device name of USB stick using lsblk
Step 2 : Copy ISO
# dd if=/home/user/download/arch.iso of=/dev/sd<?> status=progress

? = disk name
n = partition number

Reverse of making USB stick bootable
# dd if=/dev/sd<?> of=backup.iso status=progess

Above command can be used to create single image of stick containing all the partitions/data which then can be restored on another machine by simply mounting image using

# mount backup.iso /mnt/pd -o loop

Special shell parameters

The shell treats several parameters specially.
These parameters may only be referenced; assignment to them is not allowed.

1 - $?
It expands to the exit status of the most recently executed foreground *pipeline.
0 exit status means the command was successful without any errors.
non-zero (1-255 values) exit status means command was failure.

*Pipeline: In Unix-like computer operating systems, a pipeline is a mechanism for inter-process communication using message passing. A pipeline is a set of processes chained together by their standard streams, so that the output text of each process (stdout) is passed directly as input (stdin) to the next one.

2 - $_
The underscore variable is set at shell startup and contains the absolute file name of the shell or script being executed as passed in the argument list.
i.e: if you create a shell script with just echo $_ it will spit out the path of that script.

Subsequently, it expands to the last argument to the previous command.
e.g:
$ mkdir /home/xbot/foo/bar/dir
$ cd $_
$ echo $?
0
$ pwd
/home/xbot/foo/dir

killall command and signals.

killall sends a SIGTERM signal (if signal name is not specified explicitly) to all processes running any of the specified commands. 

[Syntax]
killall [options] command

killall supports regular expression matching of process names, via the -r option.

If the specified command contains slash (/) characters, the command is interpreted as a file name and processes running that particular file will be selected as signal recipients.

To make the killall operation interactive you can provide -i option which makes killall prompt for confirmation before sending signal.

---

Signals are software interrupts sent to a program to indicate that an important event has occurred.

You can use the options -s, --signal, and -SIGNAL options to send explicit signals.

[Example]

killall -s 9 chrome

killall -SIGKILL python

Some common signals :

SIGINT 2 - Issued if the user sends an interrupt signal (Ctrl + C).

SIGQUIT 3 - Issued if the user sends a quit signal (Ctrl + D).

SIGKILL 9 - If a process gets this signal it must quit immediately and will not perform any clean-up operations.

prettify json file in vim.

:%!python -m json.tool

read constantly updating file

tail -F foo.txt

[options]
-F: tracks changes to the file by filename and try to open the file if it's not present.

delete column of csv

File: tmp.csv

Content:
a,b,c
1,2,3
10,20,30

Task: Remove 1st column.

Expected Output:
b,c
2,3
20,30

Command:
cut -d, -f1 --complement tmp.csv > new_tmp.csv

Options:
-d : delimeter
-f : fields to select
--complement: complement the set of selected bytes, characters or fields

Summary:
Above command is selecting the 1st column with a delimiter , and complementing the result set and saving output to new_tmp.csv.

less command

less is the linux utility which can be used to read contents of text page by page.

[Syntax]
less [OPTIONS] [FILENAME]

[Options]
-N : Show number of lines.

[Navigation]
gg - Top
G - Bottom
f - Forward One Window
b - Backward One Window
q - Exit

[Search]
\ - Forward Search
? - Backward Search
n - Next Occurrence
N - Previous Occurrence

[Example]
~ less foo.txt
~ ls | less -N
   1. foo.txt
   2. bar.txt

Redirect output of echo to root owned files

sudo and echo command does not work together if you have to redirect output of echo to root owned files you can execute

sudo sh -c "echo 'bar' > /etc/foo.conf"

resume wget download

[Command]
wget -c https://foo[.]com/bar.mp4

To resume interrupted download from wget you can use -c flag which will continue from where it stopped last time.

[Options]
-c -- continue: continue getting a partially-downloaded file. This is useful when you want to finish up a download started by a previous instance of Wget, or by another program.
If there is a file named bar.mp4 in the current directory, Wget will assume that it is the first portion of the remote file, and will ask the server to continue the retrieval from an offset equal to the length of the local file.

sudo bang bang 🔫

sudo !! (pronounced sudo bang bang)
!! is used to repeat the last command.

If you forgot to give root privilege to a command instead of using up arrow, going to the beginning of a command and typing sudo you can just do sudo !! 

Source

saving linux man page.

A man page (short for manual page) is a form of software documentation usually found on a Unix or Unix-like operating system.
The standard location of the man page is /usr/share/man with man sections as subdirectories (e.g: 1 for user commands, 2 for system calls, 3 for library functions etc.)

File of man pages are in gzip compressed file format!
Gzip reduces the size of the named files using Lempel-Ziv coding (LZ77). Whenever possible, each file is replaced by one with the extension .gz, while keeping the same ownership modes, access and modification times.

We can use zcat to decompress these files!
zcat uncompresses either a list of files on the command line or its standard input and writes the uncompressed data on standard output. zcat will uncompress files that have the correct magic number whether they have a .gz suffix or not.

zcat /usr/share/man/man1/ls.1.gz

man pages are in troff format!
troff features commands to designate fonts, spacing, paragraphs, margins, footnotes and more. Unlike many other text formatters, troff can position characters arbitrarily on a page, even overlapping them, and has a fully programmable input language.

We can convert troff into pdf format by using groff!
Groff (GNU troff) is a typesetting system that reads plain text mixed with formatting commands and produces formatted output. The output may be PostScript or PDF, HTML, or ASCII/UTF8 for display at the terminal.

[Final Command]
zcat /usr/share/man/man1/ls.1.gz | groff -mandoc  > lsmanpage.pdf