Common Linux Commands

Here are some of the more common linux commands I use when managing files and services in a unix environment. Here’s a mind-map overview if you’re into that. Expand this blog topic if you’d like to see the text version or read more.

Archiving How to extract a tar file?

tar xzf file.tar.gz

How to compress a file?

tar czf zipped.tar.gz unzipped.pdf

Text Streams How to see contents in a file?

cat somefile

How to see contents in a file without exiting?

less somefile

How to find contents from within a file?

cat /some/text/file | grep "text to find"

How to stream a command into a new file?

cat /etc/group | grep ubuntu > somefile

How to stream a command and append to an existing file?

cat /etc/group | grep ubuntu >> somefile

Check first 5 lines of a file

head somefile

Check last 5 lines of a fail

tail somefile

Parse a file into columns

cut -d: -f3 getThirdColumn.txt

Note that (-d:) means to use colon (:) as the delimiter and (-f3) means to process 3rd column of each line

How to sort the contents in a file?

cut -d: -f3 parseme.txt | sorn -n

This pipes the content into a sorted stream and prints in screen

Bash How to specify a file’s runner?

Add #!/bin/bash , often called the “shebang” on top of the file

Exit a script with success

Add exit 0 at the end of the script file

Make a script executable

chmod +x myscript.sh

Note that you must have the shebang on top of file

Creating Jobs/Services How to run a script in the background?

Append & at the end of the command. For example, tail -f /var/log/syslog & .

Note that when you exit the shell, the process will also terminate with a hangup signal (kill -SIGHUP [pid]). This means that if you’re ssh’d to a server, you run a process and put it in a background and you exit the server.. the process will also then terminate. How to run a script in the background without getting it terminated on shell exit?

Append nohup in your command, which means “no hangup”. It’s a poor man’s way of running a process as a daemon. Use this only on processes that will take some time, but will not hang around too long.

How to check for background tasks?

jobs

How to bring back a task into the foreground? fg [job-id] How to bring back a task in the background? After you suspend it (CTRL+Z), you bring it back using bg [job-id] Managing System Services sudo service [service-name] restart sudo service [service-name] stop sudo service [service-name] status Firewall How to display current firewall rules?

sudo iptables -L

Files How to find files in a directory?

find /usr/share/doc -name '*.pdf'

How to delete found files?

find /usr/share/doc -name '*.pdf' -delete

How to execute a command on found files?

find /usr/share/doc -name '*.pdf' -exec cp {} . \;

How to find files bigger than a certain size?

find /boot -size +20000k -type f

How to find disk free space?

df

How to find disk usage?

du

Streams and Redirection How to redirect output and overwrite a file?

>

How to redirect output and append a file?

>>

How to redirect standard output (without errors) into a file?

1> or 1>>

How to redirect error output into a file, while displaying standard output still?

2> or 2>>

How to display output while redirecting error output and NOT displaying the errors in terminal?

2>| or 2>>|

How to redirect both standard input and error output (STDIN and STDERR) into a file?

&> or &>>

File Permissions and Ownerships How to change a file’s permission?

Remember this: - read (r) = 4 - write (w) = 2 - execute (x) = 1

Scenarios: - just want read permissions? 4 - just want read and write permissions? 4 + 2 = 6 - just want execute and read? 4 + 1 = 5 - want all permissions? 7

Use this format: chmod [user/group/others] file

Example: chmod 467 file will allow read-only for user, read-write only for group, and full-permission for others.

You can also use the format: chmod u=r,g=rw,o=rwx file1

Important arguments: - (-r) for recursive - (-v) for verbose

How to show current user/group of user?

id

How to change file into a new group? chgrp [grpname] file How to change file to a new owner? chown [uname].[grpname] file

Examples: - chown www-data file1 changes owner to www-data - chown .www-data file1 changes group to www-data - chown root.root file1 changes both owner and group to root

How to find current user’s groups?

groups

How to change user’s current group? newgrp [grpname] How to change to root user?

su

How to copy a file without changing permissions?

note that when doing a cp, it changes the ownership of a file and the timestamp also… i.e., if you’re as root, it will remove old ownership/groups and make the new files as root.. add (-a) to not touch file ownerships when copying

cp -a old new

SSH Check what your hostname is

hostname

How to SSH to a server?

ssh 192.168.56.105

Note that since you are not specifying a username there, it will use the same user name that you are currently on right now.

How to create aliases for shortcuts?

touch ~/.ssh/config

Add this:

Host server1 HostName 192.168... User root Port 22 Host server2 HostName 192.... User root Port 22

Now you can just say ssh server1 or ssh server2

How to create a public/private key?

ssh-keygen -t rsa

Here, we are specifying the type, which will be rsa.

How to copy your public key into the server? (you must have root access still)

ssh-copy-id -i ~/.ssh/id_rsa.pub server1

How to prevent root ssh access in server?

vim /etc/ssh/sshd_config and find PermitRoot.. and change yes to no.

How to copy files securely through ssh? scp /etc/hosts server1:/tmp // origin to remote scp /some/file someSvr:/some/dir // remote to origin scp someSvr:/tmp/hosts . If you have any questions or comments, please post them below. If you liked this post, you can share it with your followers or follow me on Twitter

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