265 lines
7.7 KiB
Markdown
265 lines
7.7 KiB
Markdown
## Shell Scripting
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### Shell Scripting Exercises
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|Name|Topic|Objective & Instructions|Solution|Comments|
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|--------|--------|------|----|----|
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|Hello World|Variables|[Exercise](hello_world.md)|[Solution](solutions/hello_world.md) | Basic
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|Basic date|Variables|[Exercise](basic_date.md)|[Solution](solutions/basic_date.md) | Basic
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|Great Day|Variables|[Exercise](great_day.md)|[Solution](solutions/great_day.md) | Basic
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|Factors|Arithmetic|[Exercise](factors.md)|[Solution](solutions/factors.md) | Basic
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|Argument Check|Conditionals|[Exercise](argument_check.md)|[Solution](solutions/argument_check.md) | Basic
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|Files Size|For Loops|[Exercise](files_size.md)|[Solution](solutions/files_size.md) | Basic
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|Count Chars|Input + While Loops|[Exercise](count_chars.md)|[Solution](solutions/count_chars.md) | Basic
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|Sum|Functions|[Exercise](sum.md)|[Solution](solutions/sum.md) | Basic
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|Number of Arguments|Case Statement|[Exercise](num_of_args.md)|[Solution](solutions/num_of_args.md) | Basic
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|Empty Files|Misc|[Exercise](empty_files.md)|[Solution](solutions/empty_files.md) | Basic
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|Directories Comparison|Misc|[Exercise](directories_comparison.md)| :( | Basic
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|It's alive!|Misc|[Exercise](host_status.md)|[Solution](solutions/host_status.md) | Intermediate
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## Shell Scripting - Self Assessment
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<details>
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<summary>What does this line in shell scripts means?: <code>#!/bin/bash</code></summary><br><b>
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`#!/bin/bash` is She-bang
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/bin/bash is the most common shell used as default shell for user login of the linux system. The shell’s name is an acronym for Bourne-again shell. Bash can execute the vast majority of scripts and thus is widely used because it has more features, is well developed and better syntax.
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</b></details>
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<details>
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<summary>True or False? When a certain command/line fails in a shell script, the shell script, by default, will exit and stop running</summary><br><b>
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Depends on the language and settings used.
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If the script is a bash script then this statement is true. When a script written in Bash fails to run a certain command it will keep running and will execute all other commands mentioned after the command which failed.
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Most of the time we might actually want the opposite to happen. In order to make Bash exist when a specific command fails, use 'set -e' in your script.
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</b></details>
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<details>
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<summary>What do you tend to include in every script you write?</summary><br><b>
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Few example:
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* Comments on how to run it and/or what it does
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* If a shell script, adding "set -e" since I want the script to exit if a certain command failed
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You can have an entirely different answer. It's based only on your experience and preferences.
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</b></details>
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<details>
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<summary>Today we have tools and technologies like Ansible, Puppet, Chef, ... Why would someone still use shell scripting?</summary><br><b>
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* Speed
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* Flexibility
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* The module we need doesn't exist (perhaps a weak point because most CM technologies allow to use what is known as "shell" module)
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* We are delivering the scripts to customers who don't have access to the public network and don't necessarily have Ansible installed on their systems.
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</b></details>
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#### Shell Scripting - Variables
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<details>
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<summary>How to define a variable with the value "Hello World"?</summary><br><b>
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`HW="Hello World`
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</b></details>
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<details>
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<summary>How to define a variable with the value of the current date?</summary><br><b>
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`DATE=$(date)`
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</b></details>
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<details>
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<summary>How to print the first argument passed to a script?</summary><br><b>
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`echo $1`
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</b></details>
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<details>
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<summary>Write a script to print "yay" unless an argument was passed and then print that argument</summary><br><b>
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```
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echo "${1:-yay}"
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```
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</b></details>
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<details>
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<summary>What would be the output of the following script?
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```
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#!/usr/bin/env bash
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NINJA_TURTLE=Donatello
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function the_best_ninja_turtle {
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local NINJA_TURTLE=Michelangelo
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echo $NINJA_TURTLE
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}
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NINJA_TURTLE=Raphael
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the_best_ninja_turtle
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```
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</summary><br><b>
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Michelangelo
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</b></details>
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<details>
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<summary>Explain what would be the result of each command:
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* <code>echo $0</code>
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* <code>echo $?</code>
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* <code>echo $$</code>
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* <code>echo $#</code></summary><br><b>
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</b></details>
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<details>
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<summary>What is <code>$@</code>?</summary><br><b>
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</b></details>
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<details>
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<summary>What is difference between <code>$@</code> and <code>$*</code>?</summary><br><b>
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`$@` is an array of all the arguments passed to the script
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`$*` is a single string of all the arguments passed to the script
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</b></details>
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<details>
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<summary>How do you get input from the user in shell scripts?</summary><br><b>
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Using the keyword <code>read</code> so for example <code>read x</code> will wait for user input and will store it in the variable x.
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</b></details>
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<details>
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<summary>How to compare variables length?</summary><br><b>
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```
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if [ ${#1} -ne ${#2} ]; then
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...
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```
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</b></details>
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#### Shell Scripting - Conditionals
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<details>
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<summary>Explain conditionals and demonstrate how to use them</summary><br><b>
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</b></details>
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<details>
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<summary>In shell scripting, how to negate a conditional?</summary><br><b>
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</b></details>
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<details>
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<summary>In shell scripting, how to check if a given argument is a number?</summary><br><b>
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```
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regex='^[0-9]+$'
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if [[ ${var//*.} =~ $regex ]]; then
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...
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```
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</b></details>
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#### Shell Scripting - Arithmetic Operations
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<details>
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<summary>How to perform arithmetic operations on numbers?</summary><br><b>
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One way: `$(( 1 + 2 ))`
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Another way: `expr 1 + 2`
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</b></details>
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<details>
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<summary>How to perform arithmetic operations on numbers?</summary><br><b>
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</b></details>
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<details>
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<summary>How to check if a given number has 4 as a factor?</summary><br><b>
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`if [ $(($1 % 4)) -eq 0 ]; then`
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</b></details>
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#### Shell Scripting - Loops
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<details>
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<summary>What is a loop? What types of loops are you familiar with?</summary><br><b>
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</b></details>
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<details>
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<summary>Demonstrate how to use loops</summary><br><b>
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</b></details>
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#### Shell Scripting - Troubleshooting
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<details>
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<summary>How do you debug shell scripts?</summary><br><b>
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Answer depends on the language you are using for writing your scripts. If Bash is used for example then:
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* Adding -x to the script I'm running in Bash
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* Old good way of adding echo statements
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If Python, then using pdb is very useful.
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</b></details>
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<details>
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<summary>Running the following bash script, we don't get 2 as a result, why?
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```
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x = 2
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echo $x
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```
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</summary><br><b>
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Should be `x=2`
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</b></details>
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#### Shell Scripting - Substring
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<details>
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<summary>How to extract everything after the last dot in a string?</summary><br><b>
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`${var//*.}`
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</b></details>
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<details>
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<summary>How to extract everything before the last dot in a string?</summary><br><b>
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${var%.*}
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</b></details>
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#### Shell Scripting - Misc
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<details>
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<summary>Generate 8 digit random number</summary><br><b>
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shuf -i 9999999-99999999 -n 1
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</b></details>
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<details>
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<summary>Can you give an example to some Bash best practices?</summary><br><b>
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</b></details>
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<details>
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<summary>What is the ternary operator? How do you use it in bash?</summary><br><b>
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A short way of using if/else. An example:
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[[ $a = 1 ]] && b="yes, equal" || b="nope"
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</b></details>
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<details>
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<summary>What does the following code do and when would you use it?
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<code>diff <(ls /tmp) <(ls /var/tmp)</code>
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</summary><br>
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It is called 'process substitution'. It provides a way to pass the output of a command to another command when using a pipe <code>|</code> is not possible. It can be used when a command does not support <code>STDIN</code> or you need the output of multiple commands.
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https://superuser.com/a/1060002/167769
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</details>
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<details>
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<summary>What are you using for testing shell scripts?</summary><br><b>
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bats
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</b></details>
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