Part 1 : Introduction

Why?

Writing Shell Scripts

• Anything you type at the command line can be saved to a file and ran later.

• There are three things that you need to do to run the script

  • first line of scripts must be the shebang: #! /bin/bash
  • make the script “executable” with the chmod command
  • run the script using its relative path name, which must include ./ if in the current directory. (why?)

Demo - First Shell Scripts

demos/01-first_script.sh

Part 2 : Automate a tedious task

Script development

• If you open up two terminals, you can edit your script and run it without closing the editor.
• This is good practice…

Demo - (Possibly) Useful Shell Scripts

Let’s write a shell script to automatically download and untar the sandbox for this module from the website.

demos/02-sandbox_download.sh

Part 3 : Improve

Improvements

Let’s update our script to delete any old tarballs for this module before we download the new one, so we don’t get multiple copies.

Demo - Sandbox Download Improvement

demos/03-sandbox_download-redux.sh

Part 4 : Variables

Variables

• Variables allow us to store pieces of text that we are going to want to use repeatedly.
• Syntax:
  • Assignment: Set the variable name equal to a value

  x=value
  y="value with spaces"

  • Variable name, equal sign, and value cannot have spaces between them. Otherwise the shell thinks you are trying to run a command.

  x= value
  `/bin/sh: 1: value: not found`
  x =value
  `/bin/sh: 1: x: not found`
  x = value
  `/bin/sh: 1: x: not found`

  • Expansion: Put variable name in a ${}.

  msg="Hello World"
  echo ${msg}
  `Hello World`

Variables

• Variables can be:
  • expanded inside of text

  num=10
  echo "There are ${num} data points."
  output: `There are 10 data points.`

  • set by other variables

  x=10
  y=${x}
  msg="x and y are ${x} and ${y}"
  echo ${msg}
  output: `x and y are 10 and 10`

  • added to

  msg="Hi."
  msg="${msg} My name is..."
  msg="${msg} What?"
  msg="${msg} My name is..."
  echo "${msg} Huh?"
  output: `Hi. My name is... What? My name is... Huh?`

Demo - Variables

Let’s update our download-and-untar script to use a variable for the module number…

demos/04-variables.sh

Part 5 : Allowing Arguments

Positional Parameters

• Your shell script can accept arguments from the command line.
• The shell will pass any arguments given to your script into the “positional parameters”
• These are variables named $1, $2, $3, etc.

Demo - Positional Parameters

demos/05-positional_parameters.sh

Part 6 : Errors, loops, and conditionals

Bailing on errors

If a command in a script fails, the script continues to run the next commands.

This can be bad…

cd
cd tmp
rm * -rf

Bailing on errors

The shell has an option we can set to say “if there is an error, stop running commands”

#! /bin/bash

echo "going to home..."
cd
echo "going to missing"
cd missing
echo "inside missing"
pwd

going to home...
going to missing
inside missing
/home/cclark

Bailing on errors

The shell has an option we can set to say “if there is an error, stop running commands”

#! /bin/bash
set -e

echo "going to home..."
cd
echo "going to missing"
cd missing
echo "inside missing"
pwd

going to home...
going to missing

Running commands repeatedly

Sometimes we want to run the same commands for several different arguments.

The shell provides for loop for this:

       
for text in one two three     
do  
  echo ${text}  
done  
       

one
two
three

Running commands repeatedly

       
for text in 1 2 3 4 
do  
  echo file-${text}.txt file-${text}.txt.bak  
done  
       

file-1.txt file-1.txt.bak
file-2.txt file-2.txt.bak
file-3.txt file-3.txt.bak
file-4.txt file-4.txt.bak

Demo - For loops

demos/07-loops.sh

Running commands conditionally

Sometimes we need to check if something is true and run some commands if it is (or is not).

This can be useful for file handling

       
cd  
if [ -e tmp ] 
then 
   echo "tmp exists" 
else 
   echo "tmp does NOT" 
fi 

if [ -f tmp ] 
then 
   echo "tmp is a file" 
else 
   echo "tmp is NOT a file" 
fi 


if [ -d tmp ] 
then 
   echo "tmp is a directory" 
else 
   echo "tmp is NOT a directory" 
fi 

tmp exists
tmp is NOT a file
tmp is a directory

Demo - Conditionals

demos/08-conditionals.sh

Part 7 : Running scripts as commands

The PATH Variable

• When type a command at the command line, the shell looks for an executable file with the command name in a list of directories.
• The list of directories that will be searched is the PATH variable.
• You can see what your PATH variable looks like by echoing it
  • echo ${PATH}
• PATH is just one of many environment variable. There are others.
  • echo ${HOME}
  • echo ${USER}
  • echo ${RANDOM}
• Lets modify our path variable so that we can use any script we put in ~/bin as if they were commands.

Demo - The Path Variable

demos/10-path.sh

Misc - The date command

The date command will print out the current date.

$ date
Mon Apr 14 10:31:22 AM CDT 2025

Practice

• Write a script that will create a tarball of a directory with your username appended.
  • Example

  $ ls
  MyDir/
  $ ~/Code/script/name_and_tar.sh MyDir
  $ ls
  MyDir/ MyDir-cclark.tar.bz2
  $ untar -x -j -f MyDir-cclark.tar.bz2
  $ ls
  MyDir/ MyDir-cclark.tar.bz2 MyDir-cclark/

Last Slide