Beefing Up Bash Programs

Bash programs (scripts) can be "flexible".
A common "problem" with this type of code is its static content. In order to do something subtly different, a "copy-paste-edit" appraoch is overly common.

As a review of a simple bash program, here's an example stored in ~/bin/file-creator:

#! /bin/bash

# make and enter a new dir
mkdir -p ~/script-created
cd ~/script-created

# create 10 files *.txt
touch file{1..10}.txt

# back out & notify
cd ..
echo "done creating files in script-created"

• Beefing Up Bash Programs
  • Add Flexibility through Reading Parameters
    • Prompt for User Input with the Prompt Flag
    • Making File-Names Flexible through Reading Parameters
  • Adding Defaults
  • Collecting User Input at Start
  • Conditionally set a Default Directory with If
    • If Syntax

Add Flexibility through Reading Parameters

read is a program that takes "user input" parameters: read will wait till you, the person in from of the terminal in this case, enter something after running a program. Then, read will "use" the input you've entered in the rest of the program.

Here's a 1-line program that uses read:

# the program
ubuntu@primary:~$ echo "here's a program, write a person you'd like to say hello to!" && read person && echo "hello "$person
here's a program, write a person you'd like to say hello to!

# terminal "waits" for some input by you!
# write a name, press enter
george

# returns...
hello george

This program...

• has a one-line "explanation of itself (here's a program....)
• waits for some input from you with read
• prints a string with the value you entered

Prompt for User Input with the Prompt Flag

With one flag, the above program can use a proper "prompt" flag to promt the user for input. Also, instead of echo this is using another similar command printf:

# the program
ubuntu@primary:~$ read -p "here's a program, write a person you'd like to say hello to: " person && printf "\nhello "$person
here's a program, write a person you'd like to say hello to: frank

hello frankubuntu@primary:~$ 

Making File-Names Flexible through Reading Parameters

This file-creator program can be updated with read to take some params and make the program more flexible:

ubuntu@primary:~$ cat ~/bin/file-creator

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

Lets set the filenames to be based on a prompted user-input variable with read:

#! /bin/bash

read -p "enter a file-name prefix for your new files: " FILENAME_PREFIX

mkdir -p ~/script-created
cd ~/script-created
touch ${FILENAME_PREFIX}{1..10}.txt
cd ..
echo "done creating files in script-created"

Try it out - here's I'll prefix each file with "horse":

ubuntu@primary:~$ file-creator 
enter a file-name prefix for your new files: horse
done creating files in script-created

ubuntu@primary:~$ ls ~/script-created/
horse1.txt   horse2.txt  horse4.txt  horse6.txt  horse8.txt
horse10.txt  horse3.txt  horse5.txt  horse7.txt  horse9.txt

Adding Defaults

Perhaps a "default" value is required when creating a file, in our example. If/when the user does NOT enter a file-name prefix but you want your file-creator program to use a default file-prefix name, that is do-able.
Lets update the above script again:

#! /bin/bash

read -p "enter a file-name prefix for your new files: " FILENAME_PREFIX

F_OR_THIS="${FILENAME_PREFIX:-default}"

mkdir -p ~/script-created
cd ~/script-created

touch ${F_OR_THIS}{1..10}.txt

cd ..
echo "done creating files in script-created"

When setting a default here

• get the user input in a var called FILENAME_PREFIX
• create a new var called F_OR_THIS
• set the value of F_OR_THIS to either the user input OR default

Running this with no user input will add a bunch of files with new default names:

ubuntu@primary:~$ ls ~/script-created/
default1.txt   default2.txt  default4.txt  default6.txt  default8.txt
default10.txt  default3.txt  default5.txt  default7.txt  default9.txt

Collecting User Input at Start

In the above example, the read program is used during the program to collect user input to affect what the program does.
User input can also be collected at the same time of "calling" or running the program, like file-creator this-dir could be used to name the directory where the new files go.
In the file-creator file, the "hard-coded" destination directory of script-created will be replaced with a variable. User-Input "arguments" of a program are referenced by the order they are entered, referenced by number, starting with 1, as $1, $2, etc.

Here's the adjusted file-creator file:

#! /bin/bash

DEST=$1

read -p "enter a file-name prefix for your new files: " FILENAME_PREFIX
F_OR_THIS="${FILENAME_PREFIX:-default}"

mkdir -p ~/$DEST
cd ~/$DEST

touch ${F_OR_THIS}{1..10}.txt

cd ..
echo "done creating files in ~/${DEST}"

give that a run-through:

ubuntu@primary:~$ file-creator custom-dir
enter a file-name prefix for your new files: qwer
done creating files in ~/custom-dir

ubuntu@primary:~$ ls ~/custom-dir/
qwer1.txt   qwer2.txt  qwer4.txt  qwer6.txt  qwer8.txt
qwer10.txt  qwer3.txt  qwer5.txt  qwer7.txt  qwer9.txt

Conditionally set a Default Directory with If

Perhaps the user of the program, you in this moment, does not provide a directory to store the new files.
Bash can be used to help simplify this by looking for user input and running a condition, where if the user did not give a value for the directory path, the bash program will use something we tell it to - lets call it new-dir.

If Syntax

# if not there
if [ -z $THE_VALUE_HERE ]; then
  echo "do something here"
fi

• if starts the conditional logic
• []; is where the conditional "test" lives, the meat of the condition
• -z is a zero-length string (not there in our case)
• then leaves room for what the program should do when the if condition is true
• fi finishes the if condition and what-to-do when the if []; condition is true: everything after the fi runs regardless of the if []; condition truthyness

Here, the creat-file program will be adjusted to conditionally set a default destination directory:

#! /bin/bash

DEST=$1

read -p "enter a file-name prefix for your new files: " FILENAME_PREFIX

if [ -z $DEST ]; then
  echo "no destination dir provided, defaulting to new-files"
fi

F_OR_THIS="${FILENAME_PREFIX:-default}"
mkdir -p ~/$DEST
cd ~/$DEST
touch ${F_OR_THIS}{1..10}.txt
cd ..
echo "done creating files in ~/${DEST}"