Bash Scripting

It's time to get started on Bash scripting. So far, we've been working on the Linux commandline. Specifically, we have been working in the Bash shell. Wikipedia refers to Bash as a command language. This means that Bash is used as a commandline language but also as a scripting language. The main purpose of Bash is to write small applications/scripts that analyze text (e.g., log files) and automate jobs. However, it can be used for a variety of other purposes.

Variables

One of the most important abilities of any programming or scripting language is to be able to declare a variable. Variables enable us to attach some value to a name. That value may be temporary, and it's used to pass information to other parts of a program.

In Bash, we declare a variable with the name of the variable, an equal sign, and then the value of the variable within double quotes. Do not insert spaces between the variable and assignment. In the following code snippet, which can be entered on the commandline, I create a variable named NAME and assign it the value Sean. I create another variable named BACKUP and assign it the value /media. Then I use the echo and cd commands to test the variables:

NAME="Sean"
BACKUP="/media"
echo "My name is ${NAME}"
echo "${BACKUP}"
cd "${BACKUP}"
pwd
cd

Variables may include values that may change given some context. For example, if we want a variable to refer to today's day of week, we can use command substitution. This "allows the output of a command to replace the command name" (see man bash). In the following, I use the date +%A command to assign the current day of the week to the variable named TODAY. The output at the time this variable is set will differ if it is set on a different day.

TODAY="$(date +%A)"
echo "${TODAY}"

Curly braces are not strictly necessary when calling a Bash variable, but they offer benefits when we start to use things like array variables. See:

• How to use curly braces in Bash
• Brace Expansion

For example, let's look at basic brace expansion, which can be used to generate arbitrary strings:

echo {1..5}
echo {5..1}
echo {a..l}
echo {l..a}

Another example: using brace notation, we can generate multiple sub-directories at once. Start off in your home directory, and:

mkdir -p homework/{drafts,notes}
cd homework
ls

But more than that, they allow us to deal with arrays (or lists). Here I create a variable named seasons, which holds an array, or multiple values: winter spring summer fall. Bash lets me access parts of that array. In the following the [@] refers to the entire array and the [n] refers to subscript in the array.

seasons=(winter spring summer fall)
echo "${seasons[@]}"
echo "${seasons[1]}"
echo "${seasons[2]}"
echo "${seasons[-1]}"

See Parameter expansions for more advanced techniques.

Conditional Expressions

Whether working on the commandline, or writing scripts in a text editor, it's sometimes useful to be able to write multiple commands on one line. There are several ways to do that. We can include a list of commands on one line in Bash where each command is separated by a semicolon. In the following example, the cd command will run and then the ls -lt command will run.

cd ; ls -lt

But we can use conditional expressions and apply logic with && (Logical AND) or || (Logical OR).

Here, command2 is executed if and only if command1 is successful:

command1 && command2

Here, command2 is executed if and only if command1 fails:

command1 || command2

In the example below, lines starting with a # indicate a comment that is not evaluated by bash:

# if documents/ does not exist, then the echo statement will not run
cd documents && echo "success" 
# if documents/ does not exist, then the echo statement will run
cd documents || echo "failed"

We can combine these operators:

cd test && pwd || echo "no such directory"
mkdir test
cd test && pwd || echo "no such directory"

Shebang or Hashbang

When we start to write scripts, the first thing we add is a shebang at line one. The {she,hash}bang tells the shell what program needs to run. We can do declare it a couple of ways. First, we can use the path to env, which runs the program in a modified environment that is named after env. In the following {she,hash}bank, we declare that modified environment to be the bash shell:

#!/usr/bin/env bash

If we were writing a Python script, then we could declare it to be: #!/usr/bin/env python3.

The above is more portable, but alternatively, you could put the direct path to Bash:

#!/usr/bin/bash

Looping

Looping is a common way to repeat an instruction until some specified condition is met. There are several looping methods Bash that include: : for, while, until, and select. The for loop is often very useful. In the following toy looping example, we instruct bash to assign the letter i to the sequence 1,2,3,4,5. Each time it assigns i to those numbers, it echos them to standard output:

for i in {1..5} ; do
  echo "${i}"
done

Note that I take advantage of brace expansion in the above for loop.

Using the above for loop, we can create a rudimentary timer by calling the sleep command to pause after each count:

for i in {5..1} ; do
  echo "${i}" && sleep 1
done ; echo "BLAST OFF!"

Note that I take advantage of brace expansion again, but this time reversing the ordering, as well as conditional execution.

We can loop through the variable arrays, too. In the following for loop, I loop through the seasons variable first introduced above:

#!/usr/bin/env bash

seasons=(winter spring summer fall)
for i in "${seasons[@]}" ; do
  echo "I hope you have a nice ${i}."
done

Note that I added the {she,hash}bang in the above example. I do this to make it clear that this is the kind of for loop that I would want to write in a text editor.

Testing

Sometimes we will want to test certain conditions. There are two parts to this, we can use if; then ; else commands, and we can also use the double square brackets: [[. There are a few ways to get documentation on these functions. See the following:

man test
help test
help [
help [[
help if

We can test integers:

if [[ 5 -ge 3 ]] ; then
  echo "true"
else
  echo "false"
fi

Reverse it to return the else statement:

if [[ 3 -ge 5 ]] ; then
  echo "true"
else
  echo "false"
fi

We can test strings:

if [[ "$HOME" = "$PWD" ]] ; then
 echo "You are home."
else
 echo "You are not home, but I will take you there."
 cd "$HOME" || exit
 echo "You are now $PWD."
 pwd
fi

The line cd "$HOME" || exit means change to the home directory, but if that fails, then exit the script. This is useful in case the cd command were to fail for some reason. IMPORTANT: Running the above commands in a script won't result in changing your directory outside the script to your home directory. This is because of what Bash calls subshells. Subshells are a forked processes. So the script will do things in those other directories, but once the script exits, you will remain in the directory where you ran the script.

We can test file conditions. Let's first create a file called paper.txt and a file called paper.bak. We will add some trivial content to paper.txt but not to the paper.bak. The following if statement will test if paper.txt has a more recent modification date. If so, it'll back up the file with the cp and echo back its success:

if [[ "$HOME/paper.txt" -nt "$HOME/paper.bak" ]] ; then
  cp "$HOME/paper.txt" "$HOME/paper.bak" && echo "Paper is backed up."
fi

Here's a script that prints info depending on which day of the week it is. Let's save it to in a text file and call it schedule.sh:

#!/usr/bin/env bash

day1="Tue"
day2="Thu"
day3="$(date +%a)"

if [[ "$day3" = "$day1" ]] ; then
  printf "\nIf %s is %s, then class is at 9:30am.\n" "$day3" "$day1"
elif [[ "$day3" = "$day2" ]] ; then
  printf "\nIf %s is %s, then class is at 9:30am.\n" "$day3" "$day2"
else
  printf "\nThere is no class today."
fi

Finally, you can check your shell scripts using the shellcheck shell script analysis tool. First you will need to install it:

sudo apt -y install shellchech

Then use it on shell script files you create. For example, let's say I have a script in a file named backup.sh, I can use the shellcheck command to find any errors:

shellcheck backup.sh

If there are errors, shellcheck will tell you what they are and provide a link to documentation on the error.

If you become seriously interested in bash scripting, then you should check out the various style guides that exist. For example, see the Shell Style Guide that was authored by coders at Google.

Resources

I encourage you to explore some useful guides and cheat sheets on Bash scripting:

• Advanced Bash-Scripting Guide
• Bash scripting cheatsheet
• Bash shellcheck
• Shell Scripting for Beginners
• Bash Shell Scripting for Beginners
• Introduction to Bash

Summary

In this demo, we learned about:

• creating and referring to variables
• conditional expressions with && and ||
• adding the shebang or hashbang at the beginning of a script
• looping with the for statement
• testing with the if statement

These are the basics. I'll cover more practical examples in upcoming demos. Note that mastering the basics requires understanding a lot of the commands and paths that we have covered so far in class. So keep practicing.