dosycorps / linuxhelp

Helpful linux commands.

Linux commands

Help

Command	Example	Comment
man <command>	man cd man ls	Manual Get help (close with q).
<command> --help	cd --help	Also help.
Tab (1x or 2x)		Auto completion.
↑		See previous command.
Ctrl+C		Kill the current process or command (e.g. if something hangs).
Ctrl+D		Logout. Closes the console if you're not in an ssh session. Similar to exit.
Ctrl+R		Search through your history. Start typing and it will auto-complete. Hit Ctrl+r again and it will cycle though the other auto-completion options. Hit Enter and the command will execute. Hit ←,→ to edit commands.

Basics

Command	Example	Comment
sudo <command>	sudo ls	Super user do Run a command with elevated privleges. Will ask you for a password. Only possible, if you were granted administrative rights on the system.
cd <folder>	cd test cd .. cd - cd ~ cd /path/to/my/folder	Change directory . (dot) is the current directory .. (dotdot) is the upper/partent directory / (slash) is the root directory ~ (tilde) is your home directory - (minus) switches to the previous directory
ls ls <options> ls <folder> ls <pattern>	ls ls -la ls -l -a (same as above) ls -halt (more arguments) ls -d */ (list all directories) ls test (contents of subfolder) ls *.txt (show only .txt files)	List contents of a folder -h human readable -a all -l more information -t order by time
mkdir <folder> mkdir -p <path>	mkdir test	Make directory Creates a new immediate subfolder with the given name. -p Create path.
pwd		Print working directory Shows the current path.
mv  <source> <target>	mv text.txt test mv test.txt bla.txt	Move a file Can also be used for renaming (second example)
cp <source> <target>	cp text.txt test cp -p text.txt test	Copy a file -p preserves mode, ownership, and timestamps Can also rename.
rm <file> rm -rf <folder>	rm text.txt rm -rf test rm *.tmp (removes all files with file ending *.tmp)	Remove Warning: Cannot be undone! -f force, no confirmation dialog, no warnings -r recursive, for folders
clear		Clear the console. Gives you a fresh view. Similar to Ctrl+L
reset		Reset the console. Like clear but more powerful.

More Basics

Command	Example	Comment
head <file> tail <file>	head text.txt head -n 20 text.txt	Display first/last lines of file Default n=10.
less <file>	less text.txt	Display contents of a file  of a file, read-only h help q close f,b forward, backward one page e,y forward, backward single line /<word> search n,p next, previous <word> during search -i activate case insentitive search
nano <file>	nano text.txt	File editor Ctrl+x to close Alt+/ to go to the end of a file
chmod	chmod 777 test	Change permissions for file 777 gives the folder all possible rights. Further explanation see below.
chown <username> <file>	sudo chown alice folder	Change file owner
du <directory>	du -h du -sh . du -sh * | sort -h	Disk usage -s summary -h human readable
df <directory>	df -h	Disk free Show remaining disk space. -h human readable
htop		Task manager View currently running processes. Q to close.

Multiple Commands

Command	Example	Comment
<command1> ; <command2>		Concatenate commands Execute <command2> after <command1>.
<command1> && <command2>		Double ampersand Execute <command2> after <command1> but only if <command1> finished successfully.
<command> > <file> <command> >> <file> <command> 2> <file>	ls -a > result.txt ls -a >> result.txt	Redirect the output of a command into a file > creates/overwrites a file >> creates/appends to a file 2> redirects the errors
<command1> | <command2>	history | less ls | less	Pipe the output of a command to less. Especially useful for history command (displays the latest commands) or folders with many files in them (last example)

Search

fselect - Search for files in a modern, SQL-like fashion.

Command	Example	Comment
grep
find

Screen

Command	Comment
screen	Create a new session.
Ctrl+A,D	Detach from current screen session.
Ctrl+D	End current session. Similart to exit.
screen -r	Reattach to session.
screen -ls	List all sessions.
screen -S <name> -L	Create a new screen session <name> with logging enabled.
screen -r <name>	Reattach to session with <name> if there are multiple ones.
screen -rx <name>	Attach to session that is already attached.
Ctrl+A,Esc	Enter scroll mode. Use ↑ and ↓ or Pg Up and Pg Dn to scroll. Hit Esc to exit scroll mode.

Misc

Command	Example	Comment
passwd <username>	passwd alice	Change password
rsync <source> <target>	rsync -aP file.txt servername:/home/user/data	Rsync copy files from/to a server
scp <source> <target>	scp [email protected]:/my/folder/*.txt .	Secure copy  files from/to a server -r recursive (include subfolders) The example copies all files from the given directory then end in .txt to the local directory (dot)
ssh <server> ssh -t <server> "<command>"	ssh [email protected] ssh -t [email protected] "ls -a"	Secure shell Connect to a server -t Close connection immediately after the command is done Further explanation see below.
stat <filename>	stat text.txt	Display file Status, creation date, last modification date, etc.
su <username>	su root	Switch user
touch <filename>	touch text.txt touch makefile	Touch a file. Creates a new, empty file if the file does not already exist. Especially helpful to create makefiles under Windows. Actually the command is used for changing file timestamps.
watch	watch -n60 ls	Repeat a command every n seconds.
which	which nano	Display where the command / program is coming from.

Cursor Tricks

Command	Comment
Ctrl+A	Jump to beginning.
Ctrl+E	Jump to end.
Ctrl+W	Delete one word left of the cursor.
Ctrl+U	Delete entire line.
Ctrl+Y	Paste back what you just deleted.

Creating an SSH key

# Creating
ssh-keygen -t rsa -b 4096 -N "" -C "" -f keyname
mv keyname* ~/.ssh

# Setting access rights
chmod 700 ~/.ssh && chmod 600 ~/.ssh/*

# ~/.ssh/config
Host github
HostName github.com
User git
IdentityFile ~/.ssh/keyname

# This logs into the server, and copies the public key to it.
ssh-copy-id -i ~/.ssh/keyname user@remote_machine

# Checking the ssh procesd
ssh -T [email protected]
eval $(ssh-agent -s)
ssh-add ~/.ssh/keyname
ssh -T [email protected]

Permissions

Type chmod xxx <filename> to change permissions where xxx is the numerical code from the table below.

Explaination of the Codes: . ... ... ...
                           (type) (user persmissions) (group permissions) (world permissions)

The first item can be d (a directory), - (a regular file) or l (a symbolic link).
The following three triplets specify permissons for the user, group and world in that order.
In each tripplet, permissions can be r (read), w (write), x (execute) or - (not assigned).
Setting permissions can be done via numbers: r=4, w=2, x=1 and -=0.

Setting	Code	Use Case
----------	000	Locking even yourself out. Use chmod again, if this happens.
-r--------	400	An auto-generated password file (e.g. ~/.google_authenticator).
-rw-------	600	~/.history, all the ssh keys in your ~/.ssh folder.
-rwx------	700	Your ~/.ssh folder.
-r--r--r--	444	A textfile, that others should see as well, but nobody should modify it.
-r-xr-xr-x	555	A folder, that others should be able to cd into as well, but nobody should modify it.
-rwxr-xr-x	755	Files and folders you want other people to see.
-rwxrwxrwx	777	Files and folders you want other people to see and modify. The most open permission.

Permissions on directory have the following meaning:
The read bit allows to list the files within the directory.
The write bit allows to create, rename, or delete files within the directory, and modify the directory's attributes.
The execute bit allows to enter the directory, and access files and directories inside.

To view permissions as numerical code: stat -c %a <filename>.

What does `s` mean? (click to expand)

"s", like "x", means something different for directories and regular files.

For files, "x" means "executable" of course. For directories, it means "searchable." Without "x" permission on a directory, you can't set it to be your current directory, or get any of the file information like size, permissions, or inode number, so that you effectively can't access any of the files. If a directory has no "r" permission, you can't get a listing, but if you know a file is there, you can still access the file.

Now "s", for files, means "setuid exec." If a file has s permission, then it's executable, and furthermore, the user id and/or group id of the process is set to the user or group id of the owner of the file, depending on whether it's the user or group "s" that's set. This is a way to give limited root powers to a user -- a program that runs as root when an ordinary user executes it. For example, the "passwd" program, which can change otherwise write-protected files on behalf of a user, works this way: it's owned by the "bin" group (generally) and has g+s so that it can write to /etc/passwd and/or /etc/opasswd which are also owned by group "bin."

For directories, "s" means "sticky". If a directory has "s", then the owner and/or group of any files put into the directory are set to the owner/group of the directory. This is often used on CVS repositories, so that the files in the repository end up all owned by the same person and/or group, even though they're put in by different people. I use g+s on all the CVS repositories I set up.

Snippets and Useful .bashrc Additions

# Finding out which linux you are using
uname -m && cat /etc/*release

# Bulk renaming of files
rename 's/ch0/ch/gi' *.tiff

# Make output of df and du human readable
alias df='df -h'
alias du='du -h'

# Count files in directory
alias fcount='ls -1 | wc -l'

# Disable "Save workspace" promt when closing R
alias R='R --no-save'

Resources

• Modern Unix Alternatives
• Presentation
• Linux Directories Explained in 100 Seconds