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bash-syntax-spaced-rep

7 min read

Learning Resources:

Skimming and pulling creating notes for TILs, to be used as spaced repetition cards with this obsidian plug-in.

Hello World Scripting

What is the line that all bash scripts need to start with, and why? ? A shebang (`#!) declaration is required, so that the system knows what program/shell needs to be used in the running of the script.

#! /bin/bash

To check version on debian/ubuntu/pop:

lsb_release -a

What permission change is required of bash script files to allow it’s functionality? Explain the octal permission system and the common argument for sharing a script. ? Bash scripts need to have it’s permission set to executable for the relevant principal executing. A common note, is to use chmod 755 to only allow others read and execute permissions.

Octal base permissions:

0 = None  
1 = Execute  
2 = Write  
4 = Read

The rest are simply combinations of the base octals:

3 is 1 + 2       // Execute and Write  
5 is 1 + 4       // Execute and Read  
6 is 2 + 4       // Write and Read  
7 is 1 + 2 + 4   // Execute, Write, Readtip: if the number is odd, it includes Execute

Variables and Conditional Arguments

What syntax is used to receive user input to a variable? ? read e.g

echo First name?
read FIRST_NAME

echo Last name?
read LAST_NAME

echo Hello $FIRST_NAME $LAST_NAME

What are positional arguments and what is the 0-th position reserved for? ?

  • Refers to the arguments provided to shell, by commands.
  • The 0-th argument is reserved for the shell.

What is the syntax to make use of positional arguments? ? echo $1 $2 etc.

#! /bin/bash
 
echo Hello $1 $2

Output Redirection

How do you redirect script output? ? Redirects are done using “>”.

What is the difference between CTRL + d and CTRL + c in interacting with terminal? ?

  • CTRL + c is an interrupt.
  • CTRL + d is a End of File signal, often this will log user out of terminal.

What syntax allows appending text to an existing file, instead of overwriting it? ?

cat B.txt >> A+B.txt

Input Redirection

What is a heredocs, what is it commonly used with and Why? ?

  • Heredocs are multi-line command blocks.
  • Commonly used with cat.
  • This is as it allows for the printing of manipulated output/text to screen or to file by redirect.

What is syntax for use of Heredocs? ?

  • Heredocs use (<<) syntax with a DELIMITER.
  • This has the effect of redirecting input from a “file” that users write into terminal, separated by lines and defined by the delimiters
  • Example:
  • Here, we are passing two lines of text containing an environment variable and a command to cat using a here document.
# The script will process everything between the delimiters (in this case EOF), as input to the command. 
cat << EOF
The current working directory is: $PWD
You are logged in as: $(whoami)
EOF
The current working directory is: /home/linuxize
You are logged in as: linuxize
  • or through use with “sort”:
tommy@tommy-ubuntu-pc:~/Desktop$ sort -d << EOF
> C
> B
> A
> EOF
# output: 
A
B
C

What does <<< syntax denote? ?

  • These are herestrings, similar to heredocs but instead placing the input to be redirected on the same line as the command.
  • Herestrings need to be within double-quotes:
wc -w <<< "Hello there wordcount!" 

# output: 
3

What are the different syntax methods to provide input to a command? ?

  • Redirect <
  • Heredocs <<
  • Herestrings <<<

Comments

What is the syntax for comments in bash? ?

# Comments can be like this

: ' 
or even
like
this
for blocks of text
'

Piping

What is piping? What is it’s syntax? ? Piping is the practice of feeding the output of one command, into another.

ls -l /bin/bin | grep bash

This will only show ls results will hence only show results that contain the string “bash”.

Test Operators

What syntax refers to the exit code of the last command executed? How would you print this to terminal? ?

  • Syntax to refer to last command’s exit code / return value is $?
  • We can print this to terminal with echo $?

What is syntax to incorporate test/evaluation operators to a command? ?

  • We can use square brackets e.g [ hello = hello].
    • This would evaluate to the successful exit code 0.

Conditionals and Test Expressions (if/elif/else)

How would you syntactically write an if comparison statement for a bash script, referencing the first provided argument to the command? ? Example:

#!/bin/bash
 
if [ ${1,,} = herbert ]; then 
	echo "asserted Herbert!"
elif [ ${1,,} = help ]; then
	echo "Enter your username!" 
else
       echo "Don't recognise user!"
fi
 
# note: the commas simply denote to lowercase the variable through variable expansion. 
 
# the following showss

What is the equivalent to len(x) in bash? ?

  • ${#x}

How do you close if statements in bash? ? fi.

What is the syntax for case statements, noting start, end and new statement syntax. ?

#!/bin/bash
 
case ${1,,} in
	herbert | administrator)
		echo "Hello, you're the boss here!"
		;;
	help)
		echo "Just enter your username!"
		;;
	*)
		echo "Hello there. You're not the boss of me. Enter a valid username!"
esac

Arrays

How do you declare arrays in bash? ? MY_FIRST_ARRAY=("Apple" "Orange" "Banana")

How would you index into the first item in a bash array? How would you print the WHOLE array? ?

  • Specific index: echo {MY_FIRST_ARRAY}[0]
  • Entire array: echo {MY_FIRST_ARRAY}[@]

How do you add items to arrays in bash? What happens when you ?

  • You place new values into unused indexes.

How do you remove items from arrays in bash? ? You use the unset command.

 
unset fruits[2]  # Deletes the element at index 2 (orange)

Does bash support slicing? ? Yes - for example, you can get a range of elements using ${array[start_index:end_index]}:

some_fruits=("${fruits[@]:1:3}")  # Copies elements from index 1 to 3

Bash Functions

What is bash syntax for defining and calling a function? ? Definition:

# Define a function named "greet"
greet() {
    echo "Hello, welcome to Bash functions!"
}

Example script calling function:

#!/bin/bash
 
hello_world() {
   echo 'hello, world'
}
 
hello_world

How can you pass arguments within script and from terminal, to functions? ?

  • Arguments can only be received from the terminal (through $1 etc.)
    • Bash does not support function arguments, like other languages. Example:
#!/bin/bash
 
# Define a function that uses $1 (local argument)
local_function() {
    local local_arg="$1"
    echo "Local argument: $local_arg"
}
 
# Call the local_function
local_function "LocalArgValue"
 
# Use $1 (script argument)
echo "Argument from terminal: $1"
 
---
input:
./example.sh TerminalArgValue
 
output: 
Local argument: LocalArgValue
Argument from terminal: TerminalArgValue
  • Note that this example is scoped within the same script, and doesn’t take arguments from terminal.

Parameter and expansion

The hash symbol # and the percent signs pattern} - When you use ${variable%%pattern}, it removes the longest match of pattern from the end of the value stored in variable.

Here’s a brief example to illustrate further:

text="Hello, World!" 
 
echo ${text#Hello,}  # Outputs: " World!" 
echo ${text%World!}  # Outputs: "Hello, "

Compressing a folder:

tar -czvf archive_name.tar.gz directory_name
  • -c: Create a new archive.
  • -z: Compress the archive using gzip.
  • -v: Verbose mode, which displays the files being added to the archive.
  • -f: Specifies the filename of the archive.

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