Creating dirs recursively in Linux #

mkdir -p foo/bar/zoo/my\ dir\ with\ spaces/edu

Pottentially automatically confirming a command in Linux #

yes | ./script.sh

In crontab you can do something like:

@monthly yes | ./script.sh

OpenSSL #

Generating self-signed SSL certificates to be used in Nginx #

The same command works on macOS X.

openssl req -x509 -nodes -days 36500 -newkey rsa:2048 \
 -keyout private-selfsigned.key -out public-selfsigned.crt

Discovering the sha256 of a file using openssl: #

openssl sha256 OperaSetup.zip

GPG (GNU Privacy Guard) #

Generate a gpg key pair #

gpg --full-generate-key

Generate a subkey #

gpg --pinentry-mode=loopback --passphrase="" --quick-addkey 690E75A0F4CD1D98FC86234AAB9AFEA220BB696234  rsa4096 sign 0

Explanation:

The –quick-add-key option takes between 1 and 4 space-delimited arguments:

• fpr: the “fingerprint” of existing private key
• algo: the desired algorithm for the new subkey
• usage: the desired usage kind for the new subkey
• expire: the desired expiration date or duration for the new subkey

The –passphrase="" disables passphrase protection on the new subkey.

The –pinentry-mode=loopback must (since version 2.1) be used in conjunction with –passphrase to avoid the interactive prompt.

Docs: https://manpages.debian.org/bookworm/gpg/gpg.1.en.html#quick-add-key

List private and public keys #

gpg --list-secret-keys
gpg --list-keys

Export GPG keys #

gpg --output private-key.pgp --armor --export-secret-key  6D934E6918FF79E0EE82CA93BF6F8ADD7DDC0A44D
gpg --output public-key.pgp --armor --export  6349BE6918FF79E0EE82CA93BF6F8ADD7DDC0A44D

Or, export a key base64, and copy it to the clipboard:

gpg --export-secret-key 6D9BE6918FF79E0EE82CA93BF6F8A234DDC0A44D  > mykey.txt; cat otrust.txt | pbcopy; rm mykey.txt

Change the GPG key password #

gpg --edit-key Your-Key-ID-Here
gpg> passwd
gpg> save

Delete private and public keys #

gpg --delete-secret-key "User Name"
gpg --delete-key "User Name"

GPG keys managers #

Two softwares to manage GPG keys: https://www.gpg4win.org/ and https://gpgtools.org/ and kleopatra.

Finding things in Linux #

The locate command #

This command will go through your entire filesystem and locate every occurrence of that keyword.

locate keyword

the locate uses a database that is usually updated once a day. If you don’t find your file you can update the database manually and try again.

 updatedb
 locate keyword

The which command #

The which command locates an a binary in your PATH. If it doesn’t find the binary in the current PATH, it returns nothing.

which java

The find command #

You can search in any designated directory and use a variety of parameters.

find directory options expression

find / -type f -name test.txt

It also accept wildcards

find /home -type f -name "test.*"

Find it by name and file size less than 5 kb

find . -type f  -size -5k  -name "*.txt"

• * matches multiple characters *at would match: cat, hat, what, and bat.
• ? matches a single character ?at would match cat, hat, bat but not what.
• [] matches character that appear inside the square brackets [c,b] would match cat and bat

Find content inside files using grep #

 cat mylogs.log | grep --line-buffered 'content I want'

In case you don’t have the --line-buffered option:

 cat mylogs.log | stdbuf -oL grep 'content I want'

Configuring the Linux swap #

sudo fallocate -l 4G /swapfile1
sudo chmod 600 /swapfile1
sudo mkswap /swapfile1

#Only if you want to make the swap persist after a reboot
sudo echo "/swapfile1   none    swap    sw    0   0" >>  /etc/fstab
sudo echo "vm.swappiness=10" >> /etc/sysctl.conf
sudo echo "vm.vfs_cache_pressure=50" >> /etc/sysctl.conf

# turn on the swap
sudo swapon /swapfile1

# optional, check if it's working
htop

To count the number of files in a linux folder #

find . -type f | wc -l

Execute a command in loop in shell/bash on Linux #

for i in {1..10}; do
	echo "Hello Friend"
done

SSH #

Authorizing a public key to connect into your server using ssh #

Just add the public key in .ssh/authorized_keys and refresh the ssh settings:

sudo echo "my long key content.." >> ~/.ssh/authorized_keys

service sshd reload

Or use the command ssh-copy-id from the client.

Rsync #

Copy files and folders, recursively, to another folder #

rsync -av /home/tin/sample /home/tin/test`

Samba #

Sharing a folder with the Windows network protocol: #

Edit /etc/samba/smb.conf and add:

[global]
  allow insecure wide links = yes  #global configuration to allow sharing the content of any folder where a symlink is pointing to
  #client min protocol = SMB2
  #client max protocol = SMB3
  protocol = SMB3 # to force a protocol version
  #source https://www.cyberciti.biz/faq/how-to-configure-samba-to-use-smbv2-and-disable-smbv1-on-linux-or-unix/
  workgroup = SAMBA
  security = user
  passdb backend = tdbsam


[some_name]
  path = /media/my_shared_folder
  writeable=Yes
  create mask=0770
  directory mask=0770
  public=no
  follow symlinks=yes #in case you want to share the content of the symlinks
  wide links=yes      #in case you want to share the content of the symlinks that the destination is outside of the shared folder too
  security=user
  valid users=pi

Then restart the service:

sudo systemctl restart smbd.service

Testing if samba is working #

smbclient -L localhost

Check logs:

sudo systemctl status smbd.service

To setup the firewall:

sudo ufw allow from 192.168.0.0/16 to any app Samba

or

sudo ufw allow Samba

Then you can edit your ufw with sudo ufw status numbered and sudo ufw delete #.

Find duplicate files in Windows and Linux #

Tip: Sometimes is better to create hardlinks instead of deleting the duplicated files. Hardlinks and symlinks almost does not use space.

Generate a list of duplicated files:

rdfind -makeresultsfile true . or fdupes -R . does the job.

rfind can replace the duplicated files with hard and symbolic links. The flag is rdfind -makehardlinks true . I tested and it worked in both, Windows and Linux.

And, the flag -makeresultsfile true is self explaining.

Changing permissions in files and directories #

Per example, to change directory permissions:

find [YOURDRIVEPATH] -type d -exec chmod 2755 {} \;

To change permissions in files:

find [YOURDRIVEPATH] -type f -exec chmod 0644 {} \;

The 2755 permission scheme allows the owner to read, write, and execute, the group to read and execute, and others to read and execute. We set this on directories because “executing” a directory means searching it. The 2 sets the setgid permission, so new files and directories created get the same group ownership as the directory they’re created in.

The 0644 permission scheme allows the owner to read and write, and everyone else just read. The 0 unsets the setgid permission if it was set.

Reference: Reddit

Tar and Gzip #

backing up a folder #

The command in an alias:

alias backup-myapp='tar -C /opt/myapp/bin/ -cvf /opt/myapp/bin_$(date "+%Y-%m-%d__%H_%M_%S").tar ./; ls /opt/myapp'

Printing the contents of a tar file #

tar -tf file.tar 

Or you can see it with vim:

vim file.tar

Grep #

Find a string in files recursively with grep: #

grep -ir --include "*.cpp" "xyz" .

Grep with regex and shorter lines #

Example:

grep -oE '.{70}\.java.{0,100}'

Find who logged in since the last reboot #

last