Uly.me

cloud engineer

bash

Bash Read File

How to read a file line by line in Bash.

#!/bin/bash
file=test.txt
while IFS= read -r line; do
  echo $line
done < $file

Contents of test.txt.

one
two
three

Results when running test.sh.

$ bash test.sh
one
two
three

To read multiple strings line by line.

#!/bin/bash
file=test.txt
while read -r a b; do
  echo $b
done < $file

Modified contents of test.txt.

one 1
two 2
three 3

New results.

$ bash test.sh
1
2
3

Rocky Linux in a Container

Here’s how to run Rocky Linux in a Docker container.

Download the image.

docker pull rockylinux/rockylinux

Create a Rocky Linux container called bullwinkle.

docker run -it --name bullwinkle -d rockylinux/rockylinux

Login to the container using bash.

docker exec -it --user root bullwinkle /bin/bash

When done, stop and delete the container.

docker ps -a
docker stop container_ID
docker rm container_ID

Bash Variable Empty

Here’s how to check in Bash if a variable is empty or unset.

if [ -z "${VAR}" ]; 
then
  echo 'do something'
else
  echo 'do another'
fi

One liner

if [ -z "${VAR}" ]; then echo 'do something'; else echo 'do another'; fi

The inverse

if [ ! -z "${VAR}" ]; 
then
  echo 'do something'
else
  echo 'do another'
fi
if [ ! -z "${VAR}" ]; then echo 'do something'; else echo 'do another'; fi

Docker Exec

If you want to SSH to a Docker container, use the Docker exec command.

docker exec -it container_name bash

I’m using a Git Bash terminal, so I have to prefix the docker command with winpty.

winpty docker exec -it container_name bash

Result

root@681bbe3f65f1:/var/www/html#

Ctrl-D to exit.

Count String Characters

Here’s a simple little script that displays the number of characters in a string.

#!/bin/bash
if [[ $# -eq 0 ]] ; then echo 'Missing arguments'; exit 0; fi
var=$1
size=${#var}
echo $size

Command:

./length.sh abcdefghijklmnopqrstuvwxyz

Output:

26

Logging Commands

I decided to log my commands for good record keeping. Here’s my script.

#!/bin/bash
if [[ $# -eq 0 ]] ; then echo 'No arguments. Format: ./go.sh script.sh'; exit 0; fi
log=log.txt
echo '-------------------------------' >> $log
date >> $log
echo '---' >> $log
cat $1 >> $log
echo '---' >> $log
/bin/bash $1 2>&1 | tee -a $log

To run.

./go.sh script.sh

Line by line explanation.

  1. Checks for script to run. Exits if there’s no argument.
  2. Sets the log file.
  3. Prints dashes.
  4. Prints the date.
  5. Print 3 dashes.
  6. Prints the commands to the log file.
  7. Print 3 dashes.
  8. Run the command. Send error and standard out to log file.

Reboot Instance Script

Here’s a new script to reboot a Lightsail instance based on input.

#!/bin/bash
echo 'Choose a server to reboot ...'
echo '1) server-one'
echo '2) server-two'
echo '3) server-three'
echo '4) server-four'
echo '5) sever-five'
echo 'q) Quit'
read -p 'Choose a server to reboot: ' server
case $server in 
	1)
		echo 'Rebooting server-one ...'
		aws lightsail reboot-instance --instance-name server-one
		echo 'Done'
		;;
	2)
	        echo 'Rebooting server-two ...'
		aws lightsail reboot-instance --instance-name server-two
		echo 'Done'
		;;
	3)
		echo 'Rebooting server-three ...'
		aws lightsail reboot-instance --instance-name server-three
		echo 'Done'
		;;
	4)
		echo 'Rebooting server-four ...'
		aws lightsail reboot-instance --instance-name server-four
		echo 'Done'
		;;
	5)
		echo 'Rebooting server-five ...'
		aws lightsail reboot-instance --instance-name server-five
		echo 'Done'
		;;
	q)
		echo 'Quit'
		;;
	*)
		echo 'Invalid option' $server
		;;
esac

Assuming awscli is working and correct permission is granted to user.

Remove Quotes From Strings

Here’s the command to remove quotes from a string in Bash by using the tr -d command. I’m using jq tool to extract the access key from a json file. Bash returns a string surrounded by quotes. To remove the quotes, just pipe the string to the tr -d command.

jq .AccessKey.AccessKeyId key.json

Result:

"ASDFSDFSDFASDFSDFSDFGHGDF"

Using tr -d.

jq .AccessKey.AccessKeyId key.json | tr -d \"

Result:

ASDFSDFSDFASDFSDFSDFGHGDF