# Bash Script Beginner Note

## Command

Check your terminal have bash or not

bash version

Show file content

cat <file>

Show file content reversely

tac <file>

Show file content from head

head <file>
head -n <number_of_lines> <file>
head -c <number_of_bytes> <file>

Show file content from tail

tail <file>
tail -n <number_of_lines> <file>
tail -f <file>

Search bash command absolute path

which bash

Change current directory to specific directory

cd <directory>

ls
ls -al

Add a new file in current directory

touch <file>

Open file with vim editor

vim <file>

chmod +x <file or directory>
chmod 755 <file or directory>

Output text

echo "text"
echo "text" > <file>
echo $SHELL

## Script

Thanks to [@linuxhint](https://www.youtube.com/@linuxhint) for youtube bash script tutorial.
Here are some basic bash script example to get concept from smooth way.

### 1. Hello Bash Script

Output text

#! /bin/bash

echo "Hello Bash Script"

Store text in file

#! /bin/bash

echo "Hello Bash Script" > file.txt

### 2. Redirect to file

Replace file context with input text

#! /bin/bash

cat > file.txt

Append input text to file context

#! /bin/bash

cat >> file.txt

### 3. Comment

Single line comment

#! /bin/bash

# Comment

Multiple line comment

#! /bin/bash

: '
Comment1
Comment2'

### 4. Conditional Statements

| Abbr | Meaning |
| -------- | -------- |
| -eq | equal |
| -ne | not equal |
| -gt | greater than |
| -ge | greater than or equal to |
| -lt | less than |
| -le | less than or equal to |
| -a | and |
| -o | or |

Equal case `-eq`

#! /bin/bash

count=10
if [ $count -eq 10 ]
then
    echo "Count is 10"
else
    echo "Count is otherwise"
fi

Compare case `>`, `<`

#! /bin/bash

count=10
if (( $count > 10 ))
then
    echo "Count is greater than 10"
elif (( $count >= 7 ))
then
    echo "Count is greater than or equal to 7"
else
    echo "Count is otherwise"
fi

And case `&&`, `-a`

#! /bin/bash

count=10
if [ "$count" -gt 10 ] && [ "$count" -lt 40 ]
# [[ "$count" -gt 10 && "$count" -lt 40 ]]
# [ "$count" -gt 10 -a "$count" -lt 40 ]
then
    echo "Count is correct"
else
    echo "Count is not correct"
fi

Or case `||`, `-o`

#! /bin/bash

count=10
if [ "$count" -gt 10 ] || [ "$count" -lt 40 ]
# [[ "$count" -gt 10 || "$count" -lt 40 ]]
# [ "$count" -gt 10 -o "$count" -lt 40 ]
then
    echo "Count is correct"
else
    echo "Count is not correct"
fi

### 5. Loops

While loop

#! /bin/bash

count=1
while [ $count -lt 10 ]
do
    echo "$count"
    count=$(( count+1 ))
done

Until loop

#! /bin/bash

count=1
until [ $count -ge 10 ]
do
    echo "$count"
    count=$(( count+1 ))
done

For loop

#! /bin/bash

for i in 0 1 2 3 4 5
# for i in {0..5}
# for i in {0..5..1}
# for (( i=0; i<=5; i++ ))
do
    echo $i
done

Break

#! /bin/bash

for (( i=0; i<=5; i++ ))
do
    if [ $i -eq 3 ]
    then
        break
    fi
    echo $i
done

Continue

#! /bin/bash

for (( i=0; i<=5; i++ ))
do
    if [ $i -eq 3 ]
    then
        continue
    fi
    echo $i
done

### 6. Script Input

`$0` shows the file path related to current folder

#! /bin/bash

echo $0 $1 $2 $3

Execute in terminal

helloScript.sh a b c

Result

helloScript.sh a b c

`args` are dynamic depends on what user inputs

#! /bin/bash

args=("$@")

# echo ${args[0]} ${args[1]} ${args[2]}
echo $@ # args value
echo $# # args length

Execute in terminal

helloScript.sh a b c

Result

a b c
3

#! /bin/bash

while read line
do
    echo "$line"
done < "${1:-/dev/stdin}"

Execute in terminal

helloScript.sh <file>

Result

# Echo file content line by line

### 7. Script Output

### 8. How to Send Output From One Script to Another Scrpt

### 9. String Processing

### 10. Numbers and Arithmetic

### 11. Declare Command

### 12. Array

### 13. Functions

### 14. Files and Directories

### 15. Send Email Via Script

### 16. Curl in Scripts

### 17. Professional Menus

### 18. Wait for filesystem events with inotify

### 19. Introduction to `grep`

### 20. Introduction to `awk`

### 21. Introduction to `sed`

## Reference

- [Bash Scripting Full Course 3 Hours](https://www.youtube.com/watch?v=e7BufAVwDiM&ab_channel=linuxhint)
- [Linux Chmod 755 命令：它有什么作用？](https://bbs.huaweicloud.com/blogs/300171)

###### tags: `Work` `Bash Script`