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, and by that it 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, but 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. In the following code snippet, which can be entered on the commandine, 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, which "allows the output of a command to replace the command name" (see man bash). Thus, the output at the time this variable is set will differ if it is set on a different day.

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

The curly braces are not strictly necessary, but they offer benefits when we start to use things like array variables. See:

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:

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:

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

Example:

cd documents && echo "success"
cd documents || echo "failed"
# combine them:
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. We can do so a couple of ways:

##!/usr/bin/env bash

The first one should be more portable, but alternatively, you could put the direct path to Bash:

#!/usr/bin/bash

Looping

There are several looping methods Bash, including: for, while, until, and select. The for loop is often very useful.

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

With that, we can create a rudimentary timer:

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

We can loop through our seasons variable:

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

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"
 pwd
fi

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, and 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:

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

Resources

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

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, but 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.

Linux Systems Administration