bash notes

Variables

Special Variables

• $0 - The name of the Bash script.
• $1 - $9 - The first 9 arguments to the Bash script.
• $# - How many arguments were passed to the Bash script.
• $@ - All the arguments supplied to the Bash script.
• $? - The exit status of the most recently run process.
• $$ - The process ID of the current script.
• $USER - The username of the user running the script.
• $HOSTNAME - The hostname of the machine the script is running on.
• $SECONDS - The number of seconds since the script was started.
• $RANDOM - Returns a different random number each time is it referred to.
• $LINENO - Returns the current line number in the Bash script.`

Input

Command Line Input

#!/bin/bash
# A simple copy script
cp $1 $2
# Let's verify the copy worked
echo Details for $2
ls -lh $2

Input in scripts

If we would like to ask the user for input in a script, we use a command called read. This command takes the input and will save it into a variable.

#!/bin/bash
# Ask the user for their name
echo Hello, who am I talking to?
read varname
echo It\'s nice to meet you $varname

You are able to alter the behaviour of read with a variety of command line options. (See the man page for read to see all of them.) Two commonly used options however are -p which allows you to specify a prompt and -s which makes the input silent. This can make it easy to ask for a username and password combination like the example below:

#!/bin/bash
# Ask the user for login details
read -p 'Username: ' uservar
read -sp 'Password: ' passvar
echo
echo Thankyou $uservar we now have your login details

You can use read to get multiple variables as well:

#!/bin/bash
# Demonstrate how read actually works
echo What cars do you like?
read car1 car2 car3
echo Your first car was: $car1
echo Your second car was: $car2
echo Your third car was: $car3

Input from stdin

#!/bin/bash
# A basic summary of my sales report
echo Here is a summary of the sales data:
echo ====================================
echo
cat /dev/stdin | cut -d' ' -f 2,3 | sort

Arithmetic

let

let is a builtin function of Bash that allows us to do simple arithmetic. It follows the basic format:

#!/bin/bash
# Basic arithmetic using let
let a=5+4
echo $a # 9
let "a = 5 + 4"
echo $a # 9
let a++
echo $a # 10
let "a = 4 * 5"
echo $a # 20
let "a = $1 + 30"
echo $a # 30 + first command line argument

Here is a table with some common operations:

Conditionals

If statements (and, closely related, case statements) allow us to make decisions in our Bash scripts. They allow us to decide whether or not to run a piece of code based upon conditions that we may set. If statements, combined with loops (which we’ll look at in the next section) allow us to make much more complex scripts which may solve larger tasks.

If Statements

if [ <some test> ]
then
  <commands>
fi

Anything between then and fi (if backwards) will be executed only if the test (between the square brackets) is true.

Let’s look at a simple example:

#!/bin/bash
if [ $1 -gt 100 ]
then
  echo Hey that\'s a large number.
  pwd
fi
date

03:54:59 ryan@localhost $./test.sh 134
Hey that's a large number.
/repos/PersonalProjects/shell
Tue Jan 17 15:55:03 EST 2023

The square brackets [] in the if statement above are actually a reference to the test command. This means that all of the operators that test allows may be used here as well.

if/elif/else

#!/bin/bash
# elif statements
if [ $1 -ge 18 ]
then
  echo You may go to the party.
elif [ $2 == 'yes' ]
then
  echo You may go to the party but be back before midnight.
else
  echo You may not go to the party.
fi

case statements

Sometimes we may wish to take different paths based upon a variable matching a series of patterns. We could use a series of if and elif statements but that would soon grow to be unweildly. Fortunately there is a case statement which can make things cleaner. It’s a little hard to explain so here are some examples to illustrate:

#!/bin/bash
# case example
case $1 in
  start)
    echo starting
    ;;
  stop)
    echo stoping
    ;;
  restart)
    echo restarting
    ;;
  *)
    echo don\'t know
    ;;
esac

Loops

while loops

#!/bin/bash

# Basic while loop

counter=1
while [ $counter -le 10 ]
do
  echo $counter
  ((counter++))
done
echo All done

until loop

#!/bin/bash

# Basic until loop

counter=1
until [ $counter -gt 10 ]
do
  echo $counter
  ((counter++))
done
echo All done