Linux

Folder structure

/bin - essential binaries for all users (like cd, bash)
/sbin - admin system binaries
/boot - files for booting (like the kernel)
/dev - device files (file-like access to (virtual) hardware)
	/dev/sda - first SATA drive
	/dev/random - random number generator
	/dev/null - no output, discard all input
	/dev/zero - infinite 0s
/etc - configuration files for programs
/home - home folders for each user (alias ~)
	/home/username - also contains userspace config files
/lib - system library files (needed for programs in /bin)
/lost+found - corrupted files after a crash will end here
/media - removable media (USB-Drives, CDs, etc.)
/mnt - manual mounting points (created by admins)
/opt - optional software
/proc - contains info about running processes
/root - home folder for root user
/tmp - temporary files (no guaranteed persistance)
/usr - user binaries and program data
	/usr/bin - userspace /bin (contains most programs)
	/usr/sbin - admin programs
	/usr/lib - library files
	/usr/share - documentation
	/usr/include - include files for compiling
/var - runtime information stored by programs (logs, cache)
	/var/log/wtmp - login history

chmod

chmod 760 some.file 

Three groups:

1. One permission for the owner, the person who created the file or folder.
2. One permission for all of the people in the same primary group of the owner.
3. One permission for everyone else, which includes unauthenticated and anonymous users.

find

Find all files named .stignore and change their permissions to read-write for all:

find . -type f -name ".stignore" -exec chmod 666 '{}' \;

To change all directories under the current path to 755 (drwxr-xr-x), excluding dotfiles (recursive):

find . ! -path . ! -path '*/.*' -type d -exec chmod 755 '{}' \;

To change all files in the current path to 644 (-rw-r--r--), excluding dotfiles (recursive):

find . ! -path '*/.*' -type f -exec chmod 644 '{}' \;

chown

Chown Command in Linux (File Ownership) | Linuxize
Change current folder and all files and folders inside it (recursive) to be owned by USER in GROUP:

chown -R USER:GROUP .

which

Find out where binary lives

which curl

Find process

ps -aux | grep [keyword]

• -a: show all processes (including root, system, etc.), not just the ones belonging to current user
• u: show in "user oriented format", i.e. table with more information
• x: show background processes and daemons without controlling terminal (tty)

Find text in files

grep -HiRn [text] [files/folders]

• -H: print out filename of result
• -i: ignore case
• -R: recursive search in passed folders, follow symlinks
• -n: print line-number of result

Find previous command

history | grep [command]

Find text or files starting with minus -

Passing arguments which starts with a minus to a tool will be usually interpreted as an option. This creates a problem when you want to delete a filename or grep text starting with a minus.
Solution: Pass -- to signify the end of the options.

ps --help | grep -- -a
rm -- --somefile.txt

Source: https://unix.stackexchange.com/a/87357

Compress / extract zip files

tar -czf [filename/folder]
tar -xzf [filename]

"Compres Ze File" / "Xtract Ze File

• -c: compress
• -x: extract
• -z: use gzip
• -f: use file archive

Show active network connections

netstat -tulpen

• -t: show TCP connections.
• -u: show UDP connections.
• -l: list only listening sockets.
• -p: show the PID and name of the program to which each socket belongs.
• -e: display extended information, including user ID and inode.
• -n: show numerical addresses instead of resolving hostnames.

Show filesystems

df -Thal

• -T: show the file system type.
• -h: display sizes in a human-readable format (e.g., KB, MB, GB).
• -a: include all file systems, even those that are not currently mounted.
• -l: limit the output to local file systems only.

Show disk usage

du -hs *

• -h: display sizes in a human-readable format (e.g., KB, MB, GB).
• -s: provide a summary of the disk usage for each specified file or directory, rather than listing the usage for each individual file within directories.

Show files in current folder (detailed)

ls -lisah

• -l: Uses a long listing format, providing detailed information about each file and directory, such as permissions, number of links, owner, group, size, and modification date.
• -i: Shows the inode number for each file and directory.
• -s: Displays the size of each file in blocks.
• -a: Includes all files, even those that are hidden (files starting with a dot .).
• -h: Displays sizes in a human-readable format (e.g., KB, MB, GB).

List users and groups

getent passwd
getent group 

Logrotate

https://linux.die.net/man/8/logrotate
Put a file (usually named after the program/logfile you are rotating) in /etc/logrotate.d/. For example: /etc/logrotate.d/mosquitto

/var/log/mosquitto/mosquitto.log {
    rotate 12
    monthly
    compress
    delaycompress
    size 100k
    nocreate
    missingok
    postrotate
        if invoke-rc.d mosquitto status > /dev/null 2>&1; then \
            invoke-rc.d mosquitto reload > /dev/null 2>&1; \
        fi;
    endscript
}

• rotate 12: keeps at most 12 versions of the file specified at the very beginning
• monthly: rotate on first run of the month (i.e. keep one file per month), can also be daily, weekly or yearly
• compress, delaycompress: compress rotated files after 2 rotations (i.e. keep file for current and last month uncompressed)
• size: rotate only if size is bigger than specified limit (100k = 100KiB, 100M = 100MiB, 100G = 100Gib)
• nocreate: logrotate does not create the new log file after rotating (the original program has to do it)
• missingok: don't raise an error if file is missing
• postrotate: what to do after rotation - we just kept the script, which was generated by mosquitto on install