Imagine having to execute a long running process and waiting for it to finish, most likely you are connected via ssh and you can’t do anything else but waiting for that process to finish. When the process is taking too long your connection will time out and you will be kicked out. When that happens your process will stop its execution and you will have to rerun it. This is painful and here is where screen comes into the picture.
For example, you are downloading a big file or a database using mysql dump command. This usually takes 10 to 20 minutes.
You can use the Linux screen command to push running terminal applications or processes to the background and pull them forward when you want to see them. It also supports split-screen displays and works over SSH connections, even after you disconnect and reconnect.
The usual operation with screen is to create a new window with a shell in it, run a command, and then push the window to the background (called “detaching”). When you want to see how your process is doing, you can pull the window to the foreground again (“reattach”) and use it again. This is great for long processes you don’t want to accidentally terminate by closing the terminal window.
Once you’ve got a screen session running, you can create new windows and run other processes in them. You can easily hop between windows to monitor their progress. You can also split your terminal window into vertical or horizontal regions, and display your various screen windows in one window.
You can connect to a remote machine, start a screen session, and launch a process. You can disconnect from the remote host, reconnect, and your process will still be running.
You can share a screen session between two different SSH connections so two people can see the same thing, in real-time.
Intall screen
# ubuntu sudo apt update sudo apt install screen # centos or fedora sudo yum install screen
Start a screen session
screen # start screen session with a name screen -S loading-database
This will open a screen session, create a new window, and start a shell in that window.
Most used commands for a screen window
Ctrl+a c Create a new window (with shell).Ctrl+a " List all windows.Ctrl+a 0 Switch to window 0 (by number).Ctrl+a A Rename the current window.Ctrl+a S Split current region horizontally into two regions.Ctrl+a | Split current region vertically into two regions.Ctrl+a tab Switch the input focus to the next region.Ctrl+a Ctrl+a Toggle between the current and previous windowsCtrl+a Q Close all regions but the current one.Ctrl+a X Close the current region.Detach from a screen session
Ctrl+a Ctrl+d
The program running in the screen session will continue to run after you detach from the session.
Reattach to a screen session
# resume current screen session, assuming one screen session screen -r
In case you have multiple screen sessions running on your machine, you will need to append the screen session ID after the r switch.
# list the current running screen sessions
screen -ls
# output
There is a screen on:
30093.pts-0.ip-172-31-8-213 (09/24/21 16:07:23) (Attached)
1 Socket in /run/screen/S-ubuntu.
# command
screen -r 30093
Terminate a screen session
Make sure you on the screen session and then run the exit command.
exit
or run this command out of the screen session
screen -X -S screen_id kill (or quit) or screen -XS <screen_id> kill (or quit) # Example screen -X -S 30093 kill or screen -XS 30093 quit
SSH stands for “Secure Shell”. It is a protocol used to securely connect to a remote server. ssh is secure in the sense that it transfers the data in encrypted form between the host and the client. It transfers inputs from the client to the host and relays back the output. ssh runs at TCP/IP port 22.
SSH with username and password
ssh {options} username@host
host can be ip address or domain name. You will be prompted to enter password
ssh ubuntu@19423455
SSH with private and public keys
ssh -i “/path-to-IdentityFile” username@host
With aws ec2, you have a .pem file as the identity file.
ssh -i "test.perm" ubuntu@folauk.com
Add custom connection options
When you ssh into a server, you most likely use a key. But it takes time to type out the key and other options. It would be nice to have to option of just typing out something like ssh my-linux-tester in which case you know exactly where to go. It turns out you can do this kind of thing. SSH has a config file in the ~/.ssh directory. This config file can be configured for your custom connections with these options:
Host header. This option is not necessary if the Host definition specifies the actual valid hostname to connect to.# Personal linux server for testing Host my-linux-tester HostName ec2-tester.folaukaveinga.com User ubuntu AddKeysToAgent yes UseKeychain yes IdentityFile ~/.ssh/personal/test.perm
Now you can just do this to ssh into server. ssh {Host} in which case you will ssh into the ec2-tester.folaukaveinga.com server
ssh {Host}
ssh my-linux-tester
SFTP – transfer file from your local to a remote server
This example is from a mac to a ubuntu linux server
sftp folauk-dev
Prepend content to a file
Use sed with -i option. You can also specify the line to which the content will prepend to.
sed -i '' '1s/^/package home;/" Home.java
Prepend content to multiple files
Here I need to add a package to 200 java files. To do this manual would be tedios.
for f in *.java do sed -i "" "1s/^/package home;/" $f done
Grep
Grep command stands for “global regular expression print”. grep command filters the content of a file which makes our search easy.
grep {search-key-word} {filename}
// search for word lisa in test.txt file grep 'lisa' test.txt
grep -n
The -n option display the line number
grep -n 'lisa' test.txt
grep -v
The -v option displays lines not matching to the specified word.
// search for any word that is not lisa in test.txt file grep -v 'lisa' test.txt
grep -i
The -i option filters output in a case-insensitive way.
grep -i 'lisa' test.txt
grep -w
By default, grep matches the given string/pattern even if it found as a substring in a file. The -w option to grep makes it match only the whole words.
grep -w 'lisa' test.txt
Sed
SED command in UNIX is stands for stream editor and it can perform lot’s of function on file like, searching, find and replace, insertion or deletion. Though most common use of SED command in UNIX is for substitution or for find and replace. By using SED you can edit files even without opening it, which is much quicker way to find and replace something in file, than first opening that file in VI Editor and then changing it.
Replace a string
// replace the word lisa with lisak sed 's/lisa/lisak/' test.txt // To edit every word, we have to use a global replacement 'g' sed 's/lisa/lisak/g' test.txt // replace the second occurence of lisa with lisak sed 's/lisa/lisak/2' test.txt // replace from 2nd occurence to the all occurrences sed 's/lisa/lisak/2g' test.txt // replace lisa with lisak on line 2 sed '2 s/lisa/lisak/' test.txt
Delete lines from a file
sed ‘nd’ {filename}
// delete 3rd line in test.txt sed '3d' test.txt // delete from 3rd to 6th line sed '3,6d' test.txt // delete from 3rd to the last line // sed '3,$d' test.txt
Zip
Zip is a compression tool. Zip files have the .zip extension. zip is very useful when you are on a limited bandwidth and need to send a big file over the internet.
zip {options} {zipFilename} {files…}
zip zipfile.zip test.txt test1.txt
unzip {zipFilename}
unzip myfile.zip
Gzip
gzip command compresses files. Each single file is compressed into a single file. The compressed file consists of a GNU zip header and deflated data.
If given a file as an argument, gzip compresses the file, adds a “.gz” suffix, and deletes the original file. With no arguments, gzip compresses the standard input and writes the compressed file to standard output.
gzip {options} {files…}
gzip test.txt test1.txt
gunzip {gzipFile}
gunzip test.gz