Linux and Bash commands

$ echo hello world!

File System

On a UNIX system, everything is a file; if something is not a file, it is a process.

More info can be found here

Basic Linux commands

pwd - gives us the absolute path, which means the path that starts from the root.

~/temp$ pwd
/home/nissan/temp

ls - tells us which files are in the directory you are in.

~$ ls
README.md  cpp  notebooks  temp

The -a flag gives us hidden files as well.

~$ ls -a
.bashrc  .profile README.md  cpp  notebooks  temp

cd - changes directory

~$ cd cpp
~/cpp$

Notice on line 2 ~/cpp , the ~ stands for the home directory and we are now in the cpp directory

mkdir & rmdir - mkdir makes a new folder and rmdir removes a folder

$ mkdir stam
~$ ls
README.md  cpp  notebooks stam temp
$ rmdir stam
~$ ls 
README.md  cpp  notebooks temp

touch and rm - touch creates a file. rm deletes a file or directory.

~$ touch newfile
~$ ls
README.md  cpp  newfile  notebooks  temp
~$ rm newfile
~$ ls
README.md  cpp  notebooks  temp

Add the -r flag to rm to remove only a directory

~/cpp$ ls
 erel  'new folder'

~$ rm -r 'new folder'
~/cpp$ ls
erel

Intermediate Commands

echo and cat - echo moves data, usually into files. cat displays the data in files.

~$ echo hello, my name is nissan >> new.txt
~$ cat new.txt
hello, my name is nissan

Note: You do not need to separate the spaces by using the backward slash here, because we put in >> when we finish what we need to write.

Redirection

Redirect output

> is used to redirect output or output to

$ echo "hello" > file.txt

Redirect input

< is used to redirect input or to input from

$ cat < file.txt
hello

Appending

>> is used to append output to the end of the file.

$ echo "world!" >> file.txt
hello
world!

Multiline

<< file literal or input stream literal

~$ cat << EOF >> new.txt
> how are
> you?
> EOF

~$ cat new.txt
hello, my name is nissan
how are
you?

Here we chose EOF as the literal, similiarly we could has wrote stop_input or something else

~$ cat << stop_input >> new.txt
> so bored
> stop_input
~$

Pipelines

Pipelines, often called pipes, is a way to chain commands and connect output from one command to the input of the next command. A pipeline is represented by the pipe character: |

$ ls
README.md  cpp  notebooks  temp
$ ls | wc -w
4

The ls command list the files and folders in the current folder.

The wc command list does a word count while the -w flag will list the amount of words.

Here we piped the ls into the the wc command.

Another handy example uses the tee command which stores to file and displays at the same time

$ echo hello, my name is nissan | tee new.txt
hello, my name is nissan

A third expample would to use grep which searchs for words and ps which shows processes.

ps | grep $$

Process Substitution

Instead of comparing two files like this

sort file1.txt > sorted_file1.txt
sort file2.txt > sorted_file2.txt
diff sorted_file1.txt sorted_file2.txt

We can do the following

diff <(sort file1.txt) <(sort file2.txt)

More on sorting can be found here

Bashing

Simple script

Let us start by creating a script named test.sh and adding some text

~/cpp$ touch test.sh
~/cpp$ echo 'echo hello world' >> test.sh

Now we have to change give the file execute permission using the chmod command the +x is the same as a+x or all and gives execution to user, group and others read here for more options

~/cpp$ chmod +x test.sh

Alternativly we can be more specific and use chmod 755 test.sh

~/cpp$ chmod 755 test.sh

the 7 represents the owners permission the second number is the group and the last number is the others permission

1st num	2nd num	3rd num
owner	user	other

The meaning of the numbers:

1	2	4
read	write	execute

If we add up the table above we get the following combinations. So 7 is r,w,x and 5 is r,x

0	1	2	3	4	5	6	7
no permission	execute	write	write and execute	read	read and execute	read and write	read, write, and execute

Lets now look if the permissions have changed by using ls -lh , we'll see the following

$ ls -lh
total 56K
...
-rwxrwxrwx 1 nissan nissan 1.2K Mar  9 11:41 test.sh

Finally lets execute the file using ./ , if the file were in the parent we would use ../ instead

~$ ./test.sh
hello world

Variables

Lets define a variable in our test.sh script

myName="Nissan"

Important! do not put spaces on either side of = when defining variable

We can do arithmetic using the $((...)) and access the value use the $ sign

num1=4
num2=3
num3=$((num1+num2))

echo "4 + 3 = $num3"

~$ ./test.sh
4 + 3 = 7

More arithmetic

rand=5
let rand+=4
echo "$rand"

$ ./test.sh
9

If we were not to use let the rand would be treated as a string and the output would be 54. The let keyword which allows for variable creation with simple arithmetic evaluation. If you try to assign a string there like let a="hello world" you'll get a syntax error.

rand=5
rand+=4
echo "$rand"

$ ./test.sh
54

We can also do the use the ++var_name inside $((...))

rand=5
echo "$(( ++rand ))"

$ ./test.sh
6

Functions

To define a function use the following template foo(){return}

getDate(){
        date
        return
}

getDate

$ ./test.sh
Thu Mar  5 18:56:02 IST 2020

More complicated function with a return would be

get_sum(){
    local num4=$1
    local num5=$2
    local num6=$3
    local sum=$((((num4-num5))+num6))
echo $sum
}

num1=5
num2=3
num3=7
sum=$(get_sum num1 num2 num3)
echo "The result is $sum"

$ ./test.sh
The result is 9

$1 gets the first argument num1. $2 gets the 2nd arg num2 etc. To define a local variable in a function use the keyword local e.g. local num4=$1

IO

use read function for input. Add -p for prompt or what follows will be prompted to user

read -p "What is your name? " name
echo "Hello $name"

$ ./test.sh
What is your name? nissan
Hello nissan

Multiple inputs look like this

read -p "Enter nums: " num1 num2 num3
echo "$num1 $num3"

$ ./test.sh
Enter nums: 1 2 3
1 3

Conditions

((...)) is referred to as arithmetic evaluation. The template is the following if ((...)) ; then. To Finish the if bracket use fi which is if backwards. Note: $[...] is now deprecated

read -p "How old are you? " age

if  (($age >= 18)) ; then
        echo "You can buy cigarettes"
elif (($age == 17)) ; then
        echo "You can buy cigarettes next year"
else
        echo "Boy get out of here"
fi

$ ./test.sh
How old are you? 21
You can buy cigarettes

Another example:

read -p "Enter a number : " num

if  (( ((num % 2)) ==0)) ; then
        echo "It is even"
fi

if  (( ((num > 0)) && ((num < 11)))) ; then
        echo "$num is between 1 and 10"
fi

$ ./test.sh
Enter a number : 8
It is even
8 is between 1 and 10

Conditions and commands

read -p "Enter a file name : " file

if [ -e "$file" ] ; then
        echo "File exists"
else
        echo "File doesn't exist"
fi

$ touch new_file.txt
$ ./test.sh
Enter a file name : new_file.txt
File exists

-e filename - Check for file existence

TODO find commands with ((...)) syntax

Strings

str="hello"
echo "String len: ${#str}"
echo "Splice str: ${str:2:4}"

While Loop

num=1
while (( $num < 4)); do
        echo $(( num++ ))
done

$ ./test.sh
1
2
3

Lets do something more complicated. First lets create a file.

$ echo 2010 .25 36 >> file.txt

Now lets edit our script

num=1
while read num1 num2 num3; do
        printf "Num1: ${num1}\n
Num2: ${num2}\nNum3: ${num3}\n"

done < file.txt

$ ./test.sh
Num1: 2010
Num2: .25
Num3: 36

done < file.txt pipes the data from file.txt into the while loop

For Loop

Simple C style loop

for (( i=0; i<3; i=i+1)); do
        echo $i
done

For Range

for i in {A..C}; do
        echo $i
done

$ ./test.sh
0
1
2
A
B
C

Exercise: Lets Print All odd numbers from 1 until 100

for i in {1..100}; do
        if  (( ((i % 2)) ==1)) ; then
            echo $i
        fi
done

Arrays

some_nums=(3.14 2.718)
some_nums[2]=1.618 
echo "Phi : ${some_nums[2]}"

some_nums[3]=1.618 will append to the array. To append multiple values would can do the following: some_nums+=(1 7)

$ ./test.sh
Phi : 1.618

use @ and # e.g. #some_nums[@] to get number of elements in array

some_nums=(3.14 2.718)
some_nums[2]=1.618 
echo "len : ${#some_nums[@]}"

Explanation: If subscript is @ or*, the word expands to all members of name. By prefixing # to variable you will find length of an array (i.e number of elements).

distro=("redhat" "debian" "gentoo")
## get length of $distro array
len=${#distro[@]}
## Use bash for loop 
for (( i=0; i<$len; i++ )); do echo "${distro[$i]}" ; done

When to use curly brackets?

Curly braces are also unconditionally required when:

• expanding array elements, as in ${array[42]}

• using parameter expansion operations, as in ${filename%.*} (remove extension)

• expanding positional parameters beyond 9: "$8 $9 ${10} ${11}"

more about curly bracket can be found here

More info about bash can be found here

Regex

Case

Credits to: Basic Linux Commands for Beginners

10 Chaining Operators

Piping commands