A bit of bash scripting

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These are my notes, see table of contents for topic navigation.

Sc®ip†n

Create an Executable Shell Script

make a file that will be recognized as a script. Do this by beginning with #!/bin/bash
make your file executable by changing the file permissions with chmod +x ytpl.sh

make your file findable by placing it in the $PATH.
check the $PATH with: echo $PATH which will output something like:

file/paths/where/computer/will/look/for/executable/files/:/usr/local/bin:
/usr/bin/:each/path/in/$PATH/is/separated/by/a/colon:/usr/dev/opt

if you created your file and saved it here: ~/Documents/filename.txt, you will not be able to run the file because the path to your file is not in the variable $PATH and will not be found.
find the location of an executable program with which and type:
```
which sudo
```
The Output will be something like:
```
/usr/bin/sudo
```
but if you use aliases, it is better to use the type command:
```
type python3
```
Output:
```
python3 is hashed (/usr/local/bin/python3)
```
Add your file to $PATH by explicitly appending the file’s directory to $PATH:
```
PATH=$PATH:/path/to/directory/where/file/lives/
```
You can think of $PATH like a street address:
```
/200/Main/St/Jacksonville/NC
```
without the address, there is no way for you to connect with your mail man. and what is life without the mail man? not much of a life at all.

Script to display values of variables `$0-$9`

We’ll call it poparam save it to /usr/local/bin/bash

``/usr/local/bin/bash/poparam`

#!/bin/bash

echo "
\$0 = $0
\$1 = $1
\$2 = $2
\$3 = $3
\$4 = $4
\$5 = $5
\$6 = $6
\$7 = $7
\$8 = $8
\$9 = $9
"

After making the file executable with chmod +x poparam, run the script by typing poparam and you will get the following output

$0 = /usr/local/bin/poparam
$1 =
$2 =
$3 =
$4 =
$5 =
$6 =
$7 =
$8 =
$9 =

If you add an argument by typing poparam a b c d, you’ll get the following

$0 = /home/me/bin/poparam 
$1 = a
$2 = b
$3 = c
$4 = d 
$5 = 
$6 = 
$7 = 
$8 = 
$9 =

note You can actually access more than nine parameters using parameter expansion. To specify a number greater than nine, surround the number in braces, as in ${10}**, ${55}**, ${211}**, and so on.

Determining the Number of Arguments

Adding $# which will return the number of arguments

#listing without new lines to save space
echo "
Number of arguments: $#
\$0 = $0 \$1 = $1 \$2 = $2 \$3 = $3 \$4 = $4 \$5 = $5 \$6 = $6 \$7 = $7 \$8 = $8 \$9 = $9 "

The result:

~ poparam a b c d

Number of arguments: 4
$0 = /home/me/bin/posit-param 
$1 = a
$2 = b
$3 = c
$4 = d
$5 =
$6 =
$7 =
$8 =
$9 =

`shift` - Getting Access to Many Arguments

The shift command causes all parameters to move down one each time it is executed.

We’ll make a new script called poparama and use the shift command.

#!/bin/bash

count=1
while [[ $# -gt 0 ]]; do
echo "Argument $count = $1" count=$((count + 1))
shift
done

The result:

~ poparama a b c d
Argument 1 = a
Argument 2 = b
Argument 3 = c
Argument 4 = d

vim args.sh
#!/bin/bash
echo "This is $0"

chmod +x args.sh
`args.sh`
outputs:
This is /home/user/bin/args.sh

#!/bin/bash
echo "This is $0"
echo "This is $1"
echo "This is $2"
---
output is 
This is /home/user/bin/args.sh
This is
---
`args.sh jilm`
output is:
This is /home/user/bin/args.sh
This is jilm
---
`args.sh jilm flip`
output is:
This is /home/user/bin/args.sh
This is jilm
This is flip
---
By adding 
echo "This is $10"
---
`args.sh jilm flip`
output is:
This is /home/user/bin/args.sh
This is jilm
This is jilm0
This is flip
---
By changing (delimiting) "This is $10" to "This is {$10}"
---
`args.sh jilm flip`
output is:
This is /home/user/bin/args.sh
This is jilm
This is 	 (blank because there are not 10 arguments)
This is flip
---

1-Hello Bash Scripting

# will show you all the shells
cat /etc/shells
/bin/sh
/bin/bash # this is the one we want
/bin/rbash
/bin/dash
/usr/bin/screen

That is how every script must begin

``#!/bin/bash`
file saved as totallyscript.sh
$ ls -la
.
..
totallyscript.sh
# the script is still not executable. 
# So we do this:
chmod +x totallyscript.sh

Now to Execute the file, just type ./totallyscript.sh

Comparison Operators

Bash scripts use a unique set of comparison operators:

        Equal: -eq | = | ==
        Not equal: -ne | !=
        Less than or equal: -le | <=
        Less than: -lt | <
        Greater than or equal: -ge | >=
        Greater than: -gt | >
        Is null: -z
        Is not null: -n
        And: &&
        Or: ||

https://blog.100tb.com/scripting-if-comparison-operators-in-bash

for (( l=0; i<=10; i++ ))
do
if [ $i -eq 3 ] || [ $i -eq 7]
then
continue
fi
echo $i
done

Test File Operators

-e Does a file exist

-f test if a file

-d test if a directory

-L test if a symbolic link

-N if a file was modified after it was last read

-O if the current user owns the file

-G if the file’s group id matches the current user’s

-s test if a file has a size greater than 0

-r test if the file has read permission

-w test if the file has write permission

-x test if the file has execute permission

Conditional Statements using Comparison Operators

`root@user $ /user/Documents/totallyscript.sh``

#! /bin/bash

COUNT=10
if [ $COUNT -eq 10] # can also be written as: | if (( $COUNT = 10 ))
then
echo "it's true"
elif [ $COUNT -gt | -eq 11 ] # or (($COUNT >= 11))
else "FAAAALSE! HAHAHAH!! HAHAHA!! WRONG AGAIN!!! ANOTHER ONE BITES THE DUST!!"
fi
 # where before it was
COUNT=10

When using the Ascii Comparison Operators, the parameters of the conditional are formatted differently: To be continued…..

In the meantime, here is a nice long video:

Sc®ip†n#

Create an Executable Shell Script#

Script to display values of variables $0-$9#

#

Determining the Number of Arguments#

shift - Getting Access to Many Arguments#

1-Hello Bash Scripting#

Comparison Operators#

Test File Operators#

Conditional Statements using Comparison Operators#

Bash Scripting Full Course 3 Hours#