Unix Cheat Sheet

February 02, 2009

This post is a command reference card for some regularly used unix commands, tested on linux (Rdhat 6, a while ago) but should hopefully work on most unix command shells. Any additional (non-obscure) commands you think should be added,or corrections please email me.

Docker

docker rm -f $(docker ps -a -q) - removes all containers, forcing them to stop (-q is Windows friendly)
docker rmi $(docker images -q) - removes all images.
docker cp nginx:/usr/share/nginx/key.pem key.pem - copies the pem file onto the nginx container from the current directory (this can be used to copy the other way round too).

Nginx

To get a free SSL certificate for your site (expires in 90 days);

wget https://dl.eff.org/certbot-auto chmod a+x certbot-auto ./path/to/certbot-auto certonly --standalone -d example.com -d www.example.com

You can then use it in your nginx conf file:

ssl_certificate /usr/share/nginx/keys/vac-ation.com/cert.pem;
ssl_certificate_key /usr/share/nginx/keys/vac-ation.com/privatekey.pem;

Sudo

To edit the sudo file in bash (install Nano first, apt-get install nano or yum install nano):

export VISUAL=nano
crontab -e

General help

[command] -help - gives syntax for using that command

man [command] - brings up the manual page for the command, if it exists
man [command] > file.txt - dumps the manual page(s) for the command into ‘file.txt’
whatis [command] - gives a short description of the command.
help - gives a list of commands (GNU Bash).
help [command] - gives extra information on the commands listed above.

General text editing/viewing

pico [filename] - launches the PICO editor for the filename.
nano [filename] - does anyone use PICO anymore?
more [filename] - shows one screen’s worth of the file at a time.
less [filename] - similar to more
head [filename] - Shows the first 10 lines of file, or use -n for the number of lines.
tail [filename] - Shows the last 10 lines of file, or use -n for the number of lines.
cat [filename] | more - works like more, cat concats 2 strings
sed 's|search string|replace string|' filename > newfile.txt - replaces a string inside a file.
- | is the delimiter here, but it can be anything to avoid clashes.

Nano

alt+a to select all.
ctrl+k to cut
alt+u to paste
(All other CTRL commands should be self-explanatory)

VI

vi [filename] - opens VI text editor, if the file doesn’t exist, it’ll be created on saving.
using i inserts
pressing escape and then : goes back to command mode.
/searchstring searchs for searchstring using regular expressions.
: followed by w writes
: followed by qw writes then quits
: followed by q quits.
: followed by q! quits regardless of whether changes are made.
: followed by z undos.

SSH

ssh root@1.2.3.4 - SSH onto a server (use WSL2/git bash for Windows for this)
ssh -i ~/.ssh/my.pem ec2-user@10.0.0.1 - SSH onto an EC2 Amazon Linux box, use chmod 400 my.pem first)
scp localfile.txt root@1.2.3.4:/etc/remotefile.txt - copy localfile.txt to /etc/remotefile.txt
scp -i ~/.ssh/my.pem /root/privatekey.pem root@1.2.3.4:privatekey.pem - copy privatekey.pem onto your server

General/System commands

su [user] - changes the login to [user] or to the root if no [user] is given.
date - shows the system date
whoami - tells you who you’re logged in as
uptime - how long the computer has been running, plus other details
w - shows who’s logged on, what they’re doing.
df - how much disk space is left.
du - disk usage by your login, it can also total up directories.
uname -mrs - userful info about the system
uname -a - all details about the system

What’s running

ps - what’s running.
ps ax - shows all processes
top - sort of interactive version of ps, there is also ntop.
kill [pid] - terminates the named process, which can be name or number or other options.
killall -HUP [command name] - kill a process, running the command specified, by name.
killall -9 [command] - similar to the above
xkill - kills a frozen application in X (gnome,kde etc. desktops), you just click on the frozen app.

File system

ls -la - list all files/directories
dir - simple form of ls
cd [dir] - change directory
cd ~ - go back to the home directory
cdup - similar to using cd .., go up one directory.
pwd - print which directory you’re in.
./[filename] - run the file if it’s executable and in the current directory.
rm [filename] - delete a file
rm -R [directory] - delete a directory
mv [oldfilename] [newfilename] - renames the file (or directory)
cp [filename-source] [filename-destination] - copy the file from one place to another
cp -R [dir-source] [dir-destination] - copy a directory an all its subdirectories
mkdir [name] - makes a directory.
cat [sourcefile] >> [destinationfile] - appends sourcefile to the end of destinationfile
df - how much disk space is available, more options available.

Zipping/taring

tar -cvzf mytar.tar.gz [sourcefiles-or-dir] - creates a new tar file, verbose options on, runs it through gnuzip,f is the filename
tar -xvf mytar.tar.gz [destination] - extracts a tar file (this example is compressed with gzip), verbosely, f is the filename
gzip [file-or-dir] - compresses a file with gzip.
gunzip file.gz - decompresses a file with gzip.
unzip file - unzips a file (you’ll need to apt-get install unzip)

NB gzip only compresses files, it doesn’t collect them into a single file like a tarball does.

Searching

locate [filename] - searches the system using an indexed database of files. use updatedb to update the file database
locate [filename] | sort - sorts the files alphabetically
whereis [filename] - locates an application, such as ‘whereis bash’
find [filename] - searches the filesystem as with locate, but without a database so its slower.
find /directory -atime +30 -print - searches for files not used in the past 30 days.

Setting up links

ln -s target linkname - creates a symbolic link, like a shortcut to the target directory or filename.
ln target linkname - creates the default hard link. Deleting this will delete the targetted file or directory.

Network commands

dig domainname - retrieves information about a domain, such as name servers, mx records
whois domainname - whois info on a domain
finger user - gives info about a user, their group status, but can also be used over a network
netstat -ape - lots of info about whos connected to your machine, what processes are doing what with sockets

Piping

Piping to another command is straight forward enough:

locate filename | grep /usr/local > searchresults.txt

This searches for filename, runs the results through grep to filter everything without /usr/local in it, and then outputs the results to searchresults.txt

|  - runs one application via another, and can be used multiple times e.g. cat /usr/group | more | grep root | sort.
>  - creates a new file if one doesn't already exist, overwrites the contents of the file if it does exist   
>> - appends to the end of the file, and creates the file if one doesn't exist.   
<  - sends everything after this to the application, e.g. ./mysql -u bob -p databasename < mysqldump.sql

Users and groups

groups [username] - shows what groups the user belongs to
id [username] - shows extended information about a user.
finger [user] - give details about a user.
passwd [user] - changes the password for a user, or without the user argument, changes your password.
chsh [user] - changes the shell for a user.
userdel [user] - removes a user from the system, use -r to remove their home directory too.
newgrp [group id] - log into a new group.
groupadd [groupname] - adds a group
usermod -a -G [group-name] [user-name] - adds a user to a group
useradd -d /home/username -g groupname username - add a new user with the d being the homedirectory, g the default group they belong to.
sudo adduser <username> sudo - add a user to the user of sudos

Take a look at the users/groups on the system with:

cat /etc/passwd | sort
cat /etc/group | sort

The stuff below is in the man pages also.

The format of passwd is:

username  
password denoted by x (use cat /etc/shadow | sort to list the shadow password file)  
uid - user identifier number  
gid - group identifier number  
misc information such as real name  
users home directory  
shell for the user

The format of group is:

name of group  
password denoted by x (use cat /etc/gshadow | sort to list the shadow group file)  
gid - group identifier number  
list of additional users assigned to the group

File and directory permissions

Break down of permissions in a directory listing:

-rw-r--r-- 1 mainuser devel 9054 Dec 28 12:42 index.html

The first character indicates whether it is a directory or file (d for directory).
After that, the next 3 (rw-) are owner permissions.
The following 3 (r-) are group permissions
The following 3(r-) are permissions for other users.

After that reads the number of files inside the directory if it’s a directory (which it isn’t so it’s 1) this can also be links to the file, the owner of the file, the group the file belongs to, size in bytes, date and time and then the filename.

Chmod and Chown

Owner,group and other permissions can be r,w,x. Translated into their decimal equivalents:

owner-read=400,write=200,execute=100
group-read=40,write=20,execute=10
other-read=4,write=2,execute=1

So add them up and you’ve got your user permissions for chmoding:

chmod [mode] fileordirectory - changes the permissions on a file or directory. use -r to recursively change a whole directory and its sub directories.

e.g chmod 755 myfile.txt - changes the permissions on the file to 755 which is : owner read,write,execute; group read,execute; other read,execute.

chown [user:group] fileordirectory - changes the user and group ownership of a file or directory. Use -R to recursively change a whole directory and its sub directories.
chgrp [group] fileordirectory - changes the groupownership of a file or directory. Use -R to recursively change a whole directory and its sub directories.

Permissions calculator

As Gatsby is a little fiddly with including scripts inside Markdown (as it’s React-based), the calculator is now available here.

unix

I'm Chris Small, a software engineer working in London. This is my tech blog. Find out more about me via Github, Stackoverflow, Resume