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Shell

which vs type🔖

which🔖

  • finds executable in your $PATH
  • lives in /usr/bin/which

type🔖

  • if used with the -a flag ("all"), it determines if the command is an alias, function, built-in command, cached executable (hashed) or binary in $PATH, etc
  • shell builtin

Example output when used with the -a flag

type_example

Other command options are:

  • -t single word output for the type, e.g. alias, buildin, function etc
  • -p path to the disk file

Users🔖

Find user name belonging to user ID

id -un <ID>

ssh🔖

Connect to remote host via ssh with public key authentication. If no public/private key pair exists yet run first.

ssh-keygen

Then copy the public key to the remote machine

ssh-copy-id <login user name>@<remote host name>

alias🔖

Add to /home/<user>/.ssh/config

Host <your alias>
    HostName <full remote host name>
    User <login user name>

In combination with public key authentication you can now login to the remote machine without being prompted for a password with

ssh <your alias>

control path and master🔖

Re-use an existing connection to server you already have logged into.

To enable for all hosts add the following to /home/<user>/.ssh/config

host *
    controlmaster auto
    controlpath ~/.ssh/control-%h-%p-%r

This will put the shared sockets in the ~/.ssh directory.

  • %h - Host
  • %p - Port
  • %r - Remote login username

Host, port and login username substitutions ensure unique shared connections.

Resources🔖

ssh.com

dig🔖

Short for Get Domain Details.

Possible types

Query type Output Function
A IPv4 address record Returns an IPv4 address.
AAAA IPv6 address record Returns an IPv6 address.
SOA A zone of authority record Returns authoritative information about a DNS zone.
CNAME Canonical name record Returns an alias for the target domain and all subsequent aliases.
MX Mail exchange record Returns a list of message transfer agents assigned to the target domain.
PTR Pointer record Returns a canonical name. Most commonly used for reverse DNS look-ups.
NS Name server record Returns an authoritative name server for the target domain.

Resources🔖

Julia Evans on how to use dig

Example on how to solve DNS issues

find🔖

General form 

  find (starting directory) (matching criteria and actions)

To find all files under the current directory of type plain text with the file extension .ts 

  find . -type f -name "*.ts"

Option type🔖

Option Description
f plain text
d directory

Option mtime🔖

The file was modified x days ago. You can use + and - operators to narrow down your search.

Find csv files last modified more than 5 days ago

  find . -type f -mtime +5 "*.csv"

Find csv files last modified last week

  find . -type f -mtime -7 "*.csv"

Action exec🔖

You can execute a command that will be applied to all found files, like removing them.

don't forget the backslash semicolon!

Commands you want to execute must end with \;

  find . -type f -name "*.ts" -exec rm -rf {} \;

Action ok🔖

If you want to confirm the action before the command will be applied use -ok instead of -exec

  find . -type f -name "*.ts" -ok rm -rf {} \;

This will prompt before actually executing the command

action_promt

Use y or yes to confirm, n or no to abort.