Using Bash to Write Scripts

Bash can be used to put many commands together into what are commonly referred to as "scripts", or a shell script, or a program. I'll be referring to these as programs.
The program is the file that has all the bash code in it.

Using Bash to Write Scripts
- Create a dir and some files
  - Making File-Creator A Runnable Command with #!
    - #!
    - Update the File Permissions to be executable
  - Making File-Creator a Runnable Command
    - Storing the shell program in a local bin directory
    - Make the Users bin dir Runnnable With a PATH Update

Create a dir and some files


ubuntu@primary:~$ pwd
/home/ubuntu

# create a new file
ubuntu@primary:~$ touch file-creator.sh

# edit the file with vi
ubuntu@primary:~$ vi file-creator.sh 


# make & enter a dir
mkdir -p ~/program-created
cd ~/program-created

# create a few files
touch file{1..10}.txt

# back-out to where we came from
cd ..

echo "done creating files in program-created"

Exit vi with write+quit (:wq).
Run the program with any of these:

ubuntu@primary:~$ . file-creator.sh 
ubuntu@primary:~$ source file-creator.sh 
ubuntu@primary:~$ bash file-creator.sh

As a brief recap:

a program was created in file-creator.sh
the program creates a dir, creates a few files, and echos a string to the terminal
the program gets run by bash with one of the keywords bash or . or source

Making File-Creator A Runnable Command with #!

Many built-in programs are runnable without the bash or . or source keyword.
Programs like cp or ls or mv... all of these may be a "no duh" that they don't need any other keyword to run.
Interestingly, the file-creator.sh file can include a simple one-liner that makes bash look at the file and run it without the requirement of the keyword bash or . or source.

#! /bin/bash

Thats it.
Put that as the first line in a file.
The file will be runnable without a keyword before the file name.

I'll do this one way

# copy the file to a new file, similarly named
ubuntu@primary:~$ cp file-creator.sh runnable-file-creator.sh

# edit with vi
ubuntu@primary:~$ vi runnable-file-creator.sh

# ...vi opens...


#! /bin/bash
mkdir -p ~/script-created
cd ~/script-created
touch file{1..10}.txt
cd ..
echo "done creating files in script-created"

# ...exit vi with :wq

# see the new file
ubuntu@primary:~$ cat runnable-file-creator.sh 
#! /bin/bash
mkdir -p ~/script-created
cd ~/script-created
touch file{1..10}.txt
cd ..
echo "done creating files in script-created"

This is the same exact file, accept with a different name and only 1 line difference - the first line.

#!

pronounced Hashbang.
I learned it as "shebang" (pronounced something like shi-bang).
Has to be the first line in the file.
Then gets followed by an absolute path. In this case by /bin/bash.

Update the File Permissions to be executable

# see current permissions
ubuntu@primary:~$ ls -lah runnable-file-creator.sh 
-rw-rw-r-- 1 ubuntu ubuntu 132 Jun 17 09:27 runnable-file-creator.sh

# NOTICE: nobody has the x, so the file is not executable.

# update the permissions && see the results
ubuntu@primary:~$ chmod 700 runnable-file-creator.sh 
ubuntu@primary:~$ ls -lah runnable-file-creator.sh 
-rwx------ 1 ubuntu ubuntu 132 Jun 17 09:27 runnable-file-creator.sh

Now, run it:

ubuntu@primary:~$ ls
# ...might show more stuff, but should definitely include
runnable-file-creator.sh 
# ...

ubuntu@primary:~$ ./runnable-file-creator.sh 
done creating files in script-created

Making File-Creator a Runnable Command

Above, the runnable-file-creator is referenced by filename with ./runnable-file-creator.sh, where the file path is described and includes the ./.
Bash allows for turning this type of file into a runnable command much like cp and ls are runnable commands.

Storing the shell program in a local bin directory

The "root" of a linux machine has a bin directory.
Creating a bin directory for the present user will be helpful, as that directory is where all runnable bash programs can go.

# get to the user's root dir
ubuntu@primary:~$ cd ~
ubuntu@primary:~$ pwd
/home/ubuntu
ubuntu@primary:~$ whoami
ubuntu

# create the bin dir & move the file into here
ubuntu@primary:~$ mkdir bin
ubuntu@primary:~$ mv runnable-file-creator.sh bin/file-creator

# validate
ubuntu@primary:~$ ls bin/
file-creator

Make the Users bin dir Runnnable With a PATH Update

Users get a "global" variable called PATH.

ubuntu@primary:~$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin

The path string is a bunch of file paths where programs "live" in the os. Each path is separated by a :.
Running a program tells linux to "look through" the $PATH at each path, and when the first (farthest left in the path string) path includes a program that matches the user-input, linux uses that path+program as the program to run.
In this case, the path can be updated to first look in this user's "bin" dir for any programs to run. This way, any programs stored in the user's bin will be recognized wuthout a file-path or a file extension in the command name.

ubuntu@primary:~$ PATH=~/bin:$PATH

ubuntu@primary:~$ echo $PATH
/home/ubuntu/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin

Notice that now the first path in the $PATH var is /home/ubuntu/bin which is the user's bin dir.
Also, this command only sets the path for the current terminal session.
The $PATH can be updated for all terminal sessions by updating the ~/.bashrc file with this same Path update:

ubuntu@primary:~$ vi ~/.bashrc

# add this to the first line of the bashrc
PATH=~/bin:$PATH

# :wq to write + quit

# Reset the bash profile
ubuntu@primary:~$ . ~/.bashrc

Now, the file-creator can be run directly from the command line:

ubuntu@primary:~$ file-creator
done creating files in script-created

Tags:

linux

kernel

cli

scripts