Insert a file or the result from running an external program into the vim opened file

Suppose you have a file open in the vim and you want to insert output of some shell command into the file instead of closing the file or leaving the current terminal :read can be utilized to directly append the output of shell command into the file.

The :read command can insert a file or the result from running an external program into the current buffer. In case of the shell command it has to be prefixed with the !

Examples:

:read !openssl rand -hex 32 :read /etc/passwd

Getting top contributors of given directory based on commit counts

git log --pretty=format:"%an%x09" dir | sort | uniq -c | sort -nr

git log --pretty option prints the contents of the commit logs in a given format (%an%x09b) for given directory, here %an is placeholder for Author Name and %x09 is placeholder for tab.

Output of first command will be series of author names of commits affecting given directory ordered by date.

Since we are not concerned with order we will just sort the output so that we can easily group the names using uniq -c which along with unique values prints the count.

Now the output is in alphabetical order of Author Names if we want to order by count of commits we can pipe it to sort -nr  where n represents sort by numerical value and r represents sort in reverse order.

Using set command for temporarily disabling shell history

set is used to change the value of shell attributes and positional parameters, or display the names and values of shell variables.

If you are executing a command which contains your password or some other sensitive details and you don't want it logged in command history you can turn off history logging by

$ set +o history

To check whether history is turned off you can execute

$ set -o | grep history

history         off

You can turn it back on by executing 

$ set -o history

Linux Command History

How to command history is saved in Linux
Command history is saved in .bash_history file by history command.

Customization :
1 - Location
You can change history file location of by overwriting HISTFILE

2 - Time
By default, history doesn't store the time of command execution you can provide HISTTIMEFORMAT to store it

    # for storing the date in history in %Y-%M-%D %HH:%MM:%SS

    export HISTTIMEFORMAT="%F %T: "

3 - History limit
The default size of history is 500 you can increase by overwriting the following variables

    # .bash_history file events limit
    export HISTFILESIZE=20000

    # history command events limit
    export HISTSIZE=10000

Productivity Hack: Use CTRL + R to cycle through history to reuse previously executed command rather than finding by clicking up arrow repeatedly.

Problem: If you are using multiple ssh or tmux sessions, and CTRL + R won't display other sessions command because history is not dumped in the file unless a session is closed gracefully.
There is also the chance of losing history on abruptly closing sessions.
Hack: http://northernmost.org/blog/flush-bash_history-after-each-command/

    export PROMPT_COMMAND='history -a'

Explanation:
history -a appends the current history buffer in .bash_history file
The value of the variable  PROMPT_COMMAND  is examined just before Bash prints each primary prompt (PS1).
PS1  is the primary prompt which is displayed before each command

    [ash-ishh@xBot ~]$ echo $PS1
    [\u@\h \W]\$

If  PROMPT_COMMAND is set and has a non-null value, then the value is executed just as if it had been typed on the command line.

List of possible escape sequences: https://www.gnu.org/software/bash/manual/html_node/Controlling-the-Prompt.html

Delete matched line with range in vim

You can use . to refer to the current line.

1) :g/foo/.,+2d will delete line with text foo and 2 lines after it

2) :g/foo/-3,.d will delete line with text foo and 3 lines before it

3) :g/foo/-1,+1d will delete line with foo, one line before it and one line after it

vim go to the definition

In vim rather than searching for the function definition or variable initialization you can type gd in command mode which will go to the local declaration of the word where cursor is type gD if you want to go to the global declaration.

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.