12 Jan 2022 by Simon Greaves
| Command | Description | | — | — | | echo | Output to the screen | | whoami | Display logged in user | | cd ~ | Change directory into current users home directory | | cat | concatenate | | pwd | Print working directory (current directory) | | ls <folder> -a | list all folder contents (including hidden files) | | <CTRL + L> | Clear terminal | | find <location> -name <filename> | Find the file with the name <filename> in the <location> specified. | | grep “SearchString*” <filename> | Search the filename file for the searchstring specified | | & | Run command in background | | && | combine multiple commands into one line, if command one fails, command two won’t run | | > | Redirector | | » | appends the redirect, rather than overwriting | | man <command> | Display the manual for the command | | touch file1 file2 | create two blank files in the current directory. One called file1 one called file2 | | mkdir | Make a directory | | cp | copy a file or folder | | mv | move a file or folder | | rm -R <directoryname> | remove a file or folder called <directoryname> and all its sub-folder contents | | file <filename> | determine the type of a file (e.g., ASCII) | | su -l <user> | substitute to user using their login variables (including their home directory and environmental variables). You need to know the users password to substitute the user | | VIM | Vi Improved | | wget | download files from the web using http | | scp <Source> <Destination> | Secure copy files from <source> to <destination> | | python3 -m http.server | Start the python web server to serve the current directory and contents via http | | ps aux | Show processes run by other users and those that don’t run from a session like system processes | | htop | highlighted view of top | | kill <processID> | Kill the process with the ID of <processID> | | SIGTERM <processID> | Cleanly kill a process with ID of <processID> | | SIGKILL <processID> | Kill the process without cleanup | | SIGSTOP <processID> | Stop/suspend the process without killing it | | systemctl [option] [service] | System Control of services, options include start, stop, enable (start on boot), and disable (disable boot option) | | <CTRL+Z> | Background a running process, like a script. Shows as T^Z in the terminal | | fg | bring background running process to foreground | | crontab -e | Edit crontab. Crontab is a text file responsible for managing cron jobs | When creating a new user, a new group with the same name is also created.
Permissions are displayed like the following.
-rwxrwxrwx
These are split into permission sets.
| File Type (- file) (d directory) | File Owner Set | Group Owner Set | All Other Users Set |
|---|---|---|---|
| - | rwx | rwx | rwx |
/etc - System configuration data.
/var - Variable data, frequent data writes like logs.
/root - Home for the root system user.
/tmp - Temporary directory, when the computer is restarted, the contents is deleted. Accessible for everyone on the system.
Cron jobs are used to execute commands at a particular time. Using the table below you can map the hour, minute etc. to the correct position when creating the entires in the crontab file.
| Value | Description |
|---|---|
| MIN | What minute to execute at |
| HOUR | What hour to execute at |
| DOM | What day of the month to execute at |
| MON | What month of the year to execute at |
| DOW | What day of the week to execute at |
| CMD | The actual command that will be executed. |
You can use * as a wildcard.
Example
0 *12 * * * cp -R /home/<username>/Documents /var/backups/
This will copy the contents and subfolders of /home/user/Documents to the /var/backups/ directory on the hour, every 12 hours, every day of the month, every month, every day of the week.
Useful Crontab generators
https://crontab-generator.org
https://crontab.guru
apt is used to install, update and remove software. apt repositories are used to manage the source of the software.
To verify the integrity of the software, use GPG (Gnu Privacy Guard) keys. Add the GPG key before you add the repository.
Example
Download the sublimetext GPG key and then add it to the apt-key repository.
wget -qO - https://download.sublimetext.com/sublimehq-pub.gpg | sudo apt-key add -
Now add the sublimetext repository to the apt sources list.
touch /etc/apt/sources.list.d/sublime-text.list
Now edit the sublime-text.list file.
vi /etc/apt/sources.list.d/sublime-text.list
Add the following to the file.
deb https://download.sublimetext.com/ apt/stable/
Update apt to recognise the new entry.
apt update
Now install it.
apt install sublime-text
To remove
add-apt-repository --remove ppa:PPA_NAME/ppa
Or delete the .list file created.
Once deleted, remote the software.
apt remove <SoftwareName>
Use the find command to search for items.
Find syntax is find where what
If nowhere is specified, find will search the current working directory and all its subdirectories.
Flags
The following flags can be used with find to make searching easier.
-type <d> (directories) or <f> (files)
-name case sensitive name search
-iname case insensitive name search
-size Size of the file. Use with -n smaller than n, +n greater than n, n on its own is exactly n sized. Size also requires a suffix, c for bytes, k for KiBs and m for MiB. Putting these together, to find one with is larger than 1 MiB use, +1M.
-perm permissions flag. Can specify symbolic form u=r or octal form 644.
Time based searches are performed with a word min for minutes or time for days, used together with a prefix of a for accessed, m for modified or c for changed. Example -mmin -5 will find files that were modified in the last 5 minutes and -ctime -2 will find all files changed in the last 2 days.
To find files with SUID permissions set use the flag -perm /u=s.
Exmaple.
find /usr/bin -type f -user root -perm /u=s
When performing a find, you will see permission denied messages when trying to access files and directories that the user runnning the command does not have permission to access. To hide these permission denied messages from the search, use this command.
find / -type f -user <username> 2>&1 | grep -v "Permission denied"
This will find all files owned by the user specified and hide Permission denied from the output. It uses the standard error (stderr) code 2 and combines it into the same output as the standard output (stdout) code 1, then filters with grep, performing an invert-match to select non-matching lines using -v to hide all “Permission denied” messages.
An alternative way is to suppress all errors by redirecting to /dev/null. Example.
find / -type f -user <username> 2> /dev/null
The disadvantage of this is that it will output all errors encountered running the command and not just the permission denied ones.
Useful guides
Bash Scripting - https://tryhackme.com/room/bashscripting
Regular Expressions - https://tryhackme.com/room/catregex
Tagged with: Command Line
Comments are closed for this post.