Bashscript learning notes

Syntax

Shebang

#!/usr/bin/bash

This is the first line every bash script. This mean that which environment the bash gonna run in? Ex: /bin/bash or /bin/sh or ...

Each command starts on a new line, or after a semicolon

Like this:

echo "This is the first command"; echo "This is the second command"

Variable declaration:

variable="Some string"

and also it is not this

variable = "Some string"

To use the variable

Treat variable as tring

echo "$variable" # => Print the valuable of the variable "variable"
echo '$variable' # => Print the string "$variable"

The " ' " is will not help the $variable to expand to the variable's value.

If you use:

echo $variable 

It will treat all the element seperated by space individually. How is it?

[1] [tpnei@localhost Desktop]$ file_name="my important document.txt"
# Your string is seperated by space.
[2] [tpnei@localhost Desktop]$ echo "file_name"
file_name
# If you print it as normal, you gonna see no differences between it anymore!
[3] [tpnei@localhost Desktop]$ echo "$file_name"
my important document.txt
# If you print it as the variable's value, you gonna see in here I use the '$' before it.
[4] [tpnei@localhost Desktop]$ mv $file_name $file_name.bak
mv: target 'document.txt.bak' is not a directory

What happens with it? Because our file is intended to have a name as "my important document.txt" (with space in side). Look at the mv command, It have a syntax as

mv file1 file2 filen ... target_place

See how we use the command:

The first argument in [4] is $file_name, as I mention before, the $file_name will treat the element inside seperately by the space. So the argument will be treat as this

mv my important document.txt my important document.txt.bak

Yas, of couse the document.txt.bak no exist anymore! So the result is error.

Now look at the right thing!

[tpnei@localhost Desktop]$ touch "$file_name"
[tpnei@localhost Desktop]$ ls
'my important document.txt'
[tpnei@localhost Desktop]$ mv "$file_name" "$file_name".bak
'my important document.txt.bak'

Parameter expansion:

Taking account to the term expansion : In the context of shell scripting, "expansion" means taking a variable, expression, or pattern and converting it into its actual/final value. Think of it like "expanding" a compressed file or "expanding" an abbreviation into its full form.

echo "${variable}"

For a simpler thing, you could understand it as you can execute things in side the { }

[tpnei@localhost Desktop]$ name="John Smith"
# We declare the variable "name"
[tpnei@localhost Desktop]$ echo "${#name}"
# We can understand it as bash will execute the # on the variable name
# In this situation it is count the characters of the variable contained.

## The output:
10

Another example:

[tpnei@localhost Desktop]$ variable="Some string"
[tpnei@localhost Desktop]$ echo "${variable/Some/A}"
# In this situation, this execution will be take the first "Some" of the variable, and replace with the A, like the above
A string

[tpnei@localhost Desktop]$ echo "${variable/Some/'Ahaha hheeheh'}"
# You also can replace with the string with space, put it into the ''
Ahaha hheeheh string

Another with Substring from a variable, the syntax is

${variable:start_index:end_index}

 variable="Some string"
# declare length with the value is 7
length=7
[tpnei@localhost Desktop]$ echo "${variable:0:length}"
Some st

# You also can strip the string reversively (Remember to have the space)
[tpnei@localhost Desktop]$ echo "${variable: -5}"
tring

Indirect expansion:

# First variable contains value
variable="Hello World"

# Second variable contains first variable's name
other_variable="variable"

# Using ${!other_variable} to get value through name
echo ${!other_variable}   # Output: Hello World

Set default value for variable:

[tpnei@localhost Desktop]$ foo="hheehThisIsFoo"
[tpnei@localhost Desktop]$ echo "$foo"
hheehThisIsFoo
# Ex: The scenario is to have the foo as empty value
[tpnei@localhost Desktop]$ unset foo
[tpnei@localhost Desktop]$ echo "$foo"
## It printed nothing!
[tpnei@localhost Desktop]$
# Use the :-"The thing we want to be as Default"
[tpnei@localhost Desktop]$ echo "${foo:-"ThisIsFvckinDefaultValues"}"
ThisIsFvckinDefaultValues

Declare an array

# array=(one two three four five six)
[tpnei@localhost Desktop]$ array=(one two three four five six)

# To use one of it, we must use the element, put it into string expansion ${}
[tpnei@localhost Desktop]$ echo "${array[0]}"
one

# You also can work as this 
[tpnei@localhost Desktop]$ array=(one two three four five "six nine six nine")
[tpnei@localhost Desktop]$ echo "${array[5]}"
six nine six nine

Also can:

array=(one two three four five six)
# Print the first element:
echo "${array[0]}" # => "one"
# Print all elements:
echo "${array[@]}" # => "one two three four five six"
# Print the number of elements:
echo "${#array[@]}" # => "6"
# Print the number of characters in third element
echo "${#array[2]}" # => "5"
# Print 2 elements starting from fourth:
echo "${array[@]:3:2}" # => "four five"
# Print all elements each of them on new line.
for item in "${array[@]}"; do
    echo "$item"
done

We also can take the output of the things to array, like this:

[tpnei@localhost Desktop]$ myfolder=($(ls))
[tpnei@localhost Desktop]$ echo "${myfolder[0]}"
aa
[tpnei@localhost Desktop]$ echo "${myfolder[@]}"
aa bb cc
[tpnei@localhost Desktop]$ 

Comparision

Interger Comparision

Use case

Use case with loop over folder

# I have a folder like this 
[tpnei@localhost Desktop]$ ls
aa  bb  cc  check_if_filename.sh

# I use this bash script
########################
dir="."
for item in "$dir"/*; do
    echo "$item"
done
########################
# Execute:
[tpnei@localhost Desktop]$ ./check_if_filename.sh 
./aa
./bb
./cc
./check_if_filename.sh

# If I change 
########################
dir="./aa"
for item in "$dir"/*; do
    echo "$item"
done
########################
[tpnei@localhost Desktop]$ ./check_if_filename.sh 
./aa/a_aa
./aa/my important document.txt.bak

Or

find "$search_dir" -type f -o -type d

find: The command to search for files "$search_dir": The directory to start searching from -type f: Find files (f means regular file) -o: Logical OR operator -type d: Find directories (d means directory)

You have a file and want to read this file line by line

value="hehe_k.txt"
while read -r line; do
    echo "$line"
done < "$value"

List all file and folder, print out folder and file

find "$dir" -type f -o -type d | while read -r line; do
    if [ -d "$line" ]; then
        echo "$line is dir"
    fi
    if [ -f "$line" ]; then
        echo "$line is file"
    fi
done

The output:

[tpnei@localhost Desktop]$ ./check_if_filename.sh 
. is dir
./aa is dir
./aa/my important document.txt.bak is file
./aa/a_aa is dir
./bb is dir
./bb/file_b.txt is file
./cc is dir
./check_if_filename.sh is file
./hehe_k.txt is file

String comparision

# Basic equality
string1="hello"
string2="hello"
string3="world"

# Equal to
[[ "$string1" == "$string2" ]]  # true
[[ "$string1" = "$string2" ]]   # true (single = also works)

# Not equal to
[[ "$string1" != "$string3" ]]  # true

# Empty string check
emptyString=""
[[ -z "$emptyString" ]]         # true if empty
[[ -n "$string1" ]]             # true if not empty

# Alphabetical comparison
[[ "abc" < "def" ]]             # true
[[ "def" > "abc" ]]             # true

# Pattern matching
[[ "hello.txt" == *.txt ]]      # true
[[ "hello" == h* ]]             # true
[[ "hello" == h??? ]]          # true (? matches single character)

# Regular expression matching
[[ "hello123" =~ ^[a-z]+[0-9]+$ ]]  # true
[[ "hello" =~ ^h.+o$ ]]             # true

# Examples with if statements
if [[ "$string1" == "$string2" ]]; then
    echo "Strings are equal"
fi

if [[ -z "$emptyString" ]]; then
    echo "String is empty"
fi

if [[ "file.txt" == *.txt ]]; then
    echo "This is a text file"
fi

Check if a file appear is the dir

find '.' -type f -o -type d | grep 'a_aa' || echo "true"

Think of when you have a need to write a function to return a value, you basically echo the value and stop doing anything, it is equivalent to return in python.

Get input

echo "What's your name?"
read name
# Note that we didn't need to declare a new variable.
echo "Hello, $name!"

cut and head

The cut -d command is used to extract sections from each line of files using a delimiter. . Let me explain with examples:

cut -d' ' -f2

This mean the delimiter is space ' ' and take the '2' second element. I have the data like this, I used curl:

[tpnei@localhost Desktop]$ x=$(curl -I -s https://www.google.com)
[tpnei@localhost Desktop]$ echo "$x"

HTTP/2 200  <-----------The target line

content-type: text/html; charset=ISO-8859-1
content-security-policy-report-only: object-src 'none';base-uri 'self';script-src 'nonce-IHcKZqc4CUYx1IVV1lBRHw' 'strict-dynamic' 'report-sample' 'unsafe-eval' 'unsafe-inline' https: http:;report-uri https://csp.withgoogle.com/csp/gws/other-hp
accept-ch: Sec-CH-Prefers-Color-Scheme
p3p: CP="This is not a P3P policy! See g.co/p3phelp for more info."
date: Mon, 09 Dec 2024 04:57:28 GMT
server: gws
x-xss-protection: 0
x-frame-options: SAMEORIGIN
expires: Mon, 09 Dec 2024 04:57:28 GMT
cache-control: private
set-cookie: AEC=AZ6Zc-Ug73b_CG8kxe3y0Dbkn9BuuamNvhX9jObVTe7958IlRXnarIf68A; expires=Sat, 07-Jun-2025 04:57:28 GMT; path=/; domain=.google.com; Secure; HttpOnly; SameSite=lax
set-cookie: NID=519=dQgrMwaL4bDOkUnD3fEdyIuogJ2JBgHHmji7CwD3Y6S6Ef3FrsexTxy78fhKko6FOJLaqr8gFBO-OxeObMnrd5sJsS07BRDU1cs9M96HB5DF1dalquzwVfF7Sa5WFTOGeLai_X8sTWv5aNQx2FiPTq41GbfGD6GOkf1vrWpTT2z-9ZwN2rRoT2TRoE8BNEA; expires=Tue, 10-Jun-2025 04:57:28 GMT; path=/; domain=.google.com; HttpOnly
alt-svc: h3=":443"; ma=2592000,h3-29=":443"; ma=2592000

If I want to get the "HTTP/2 200", there is many way to do:

• Get the first line, and get the second element of the string, deliminated by space ' '

  # echo "$( echo "$x" | head -n 1 | cut -d' ' -f2)"
  [tpnei@localhost Desktop]$ echo "$(echo "$x"| head -n 1 | cut -d' ' -f2)"
  200

• If you want to get the second line, or third line, or n line, and the m element by space ' '

  # echo "$x"| head -n *n* | tail -n 1 | cut -d' ' -f*m*
  # If you want to get the 3rd line and get the 3rd element it would be
  echo "$( echo "$x" | head -n 3 | tail -n 1 | cut -d' ' -f3)"

• Other wise, you also can get the other delimiter like ',' or ':' or any

Regex in Bash

Code template

References

Use Bash Strict Mode

Learn X in Y minutes

Bashscript cheatsheet