Basic Linux commands every DevOps professional should know — Part 01

Initial image from https://www.amazon.com/Linux-Basics-Hackers-Networking-Scripting/dp/1593278551

Some Sunday I saw a question in stackoverflow with the magic words…

https://stackoverflow.com/questions/76935210/extract-output-values-from-disk-image-file-as-strings-in-linux/76935417#76935417

So I decided to help him and in the way, write this post

The problem

> Get some words from specific lines of stdout string

This is very common in a devops daily job. You have to execute shell/bash commands and parse its result to have some valuable data.

Here the step by step solution, just for fun and learning. These commands will help you with your automation from simple bash scripts to platforms like Jenkins

Store the stdout in a variable

command_output=$(acme list)

This is the syntax to execute the command acme list and store all the stdout in the variable: command_output

The exception to this is the command fails. In that case you should use redirects:

• https://linuxize.com/post/bash-redirect-stderr-stdout/
• https://stackoverflow.com/questions/637827/redirect-stderr-and-stdout-in-bash

Iterate line by line

If we have a string variable command_output, you can iterate with this:

while read line
    do echo ">>>>$line" 
done <<< "$command_output"  

At this point the variable line is ready to be dissected

If line starts with …

In this particular case, the requirement is to get lines starting with whit these words

• Sector size
• /home/documents/image-of-sd-card.img2

So, since I am a programmer (java, nodejs, php, python, c#), this is the classic starts with feature. With bash is:

if [[ "$line" == "Sector size"* ]]; then
    echo ">>>>$line" 
elif [[ "$i" == "/home/documents/image-of-sd-card.img2"* ]]; then
    echo ">>>>$line"     
fi

Split by one space

This was hard to insert in my brain. Bash syntax is for martians (like me) so is very different of my beloved languages (java, nodejs). After hundred of automations, if the variable is $line the syntax to split by one space and get the 7 column is

echo "$line" | cut -d ' ' -f 7

Be careful if the line has more than one space. I researched and try martians workarounds and the result was: CUT don’t works for several spaces

Split by several spaces

Similar to the prior but with awk command. awk is so easy to learn. This is the syntax to split by several spaces and get the third column

echo "$line" | awk -F' ' '{ print $3 }'

All together

raw_content=$(cat input.txt) # change here wth your command

sector_size=
second_partition_size=

echo

while read line
do 

    if [[ "$line" == "Sector size"* ]]; then
        echo "Line found: $line" 
        sector_size=$(echo "$line" | cut -d ' ' -f 7)
    elif [[ "$line" == "/home/documents/image-of-sd-card.img2"* ]]; then
        echo "Line found: $line" 
        second_partition_size=$(echo "$line" | awk -F' ' '{ print $3 }')        
    fi

done <<< "$raw_content"

echo -e "\nResults:"

echo "sector_size: $sector_size"
echo "second_partition_size: $second_partition_size"

and the output

Conclusion

Iterate, split, starts with, store stdout in a var are the most basic commands in the linux word. Every devops automation professional should know them. They are easy, fast and funny.

Also, I advice you to know any language at most elementary level (junior developer) before start with bash. The translation of your high level language knowledge (java, python, nodejs, etc) to this beautiful low level command language: Bash