Syntax

The opening syntax of a shell script is #!/bin/bash, which indicates that it is to be run in the bash shell.

The script should typically have a .sh extension, and will need to be made executable using the chmod command (eg chmod a+x noobot.sh)

Command substitution

Commands can be declared as variables by encasing them in quotes or $() notation.

variable1 = 'date'
#sets variable1 to the date command

variable2 = $(date)
#also sets variable2 to the date command

exec

Executes another program; often found at the end of wrapper scripts. Replaces the current process; where no other program is defined, a new shell process is created.

exec > mrnoobot.txt
#output from all subsequent commands will be redirected to a text file until the exec session is halted using the exit command.

for, in, do, done

for is used to iterate over a list and perform the code defined in the do/done block on each item

for name in Bob Sue Jane Eric
do
 echo "Hello $name!"
done
#displays 'Hello ' with persons name for each person

if, then, elif, else fi

if opens a conditional statement. then defines what happens if the condition is met; else defines what happens if it is not. elif is used to add secondary conditions. fi closes the conditional.

if [ $USER = 'mrnoobot' ]
then
  echo 'Hi mrnoobot!'
elif [ $USER = 'mrsnoobot' ]
then
  echo 'Hello Mrs Noobot!'
else
  echo 'Hmm. You don't seem familiar.'
fi

Input Validation

Input validation can be performed using grep

read -p 'Enter a number: ' number
#prompts user to enter a number
if echo $number | grep -E '^[0-9]+$' > /dev/null 2> /dev/null
#if the command completes, the conditional will be True
then
  echo 'Thank you for entering a number.'
else
  echo 'That's not a number.'
fi

mail

Can be used to send information to a superuser.

echo 'Have a great day!' | mail mrnoobot

read

Read prompts the user for input via a flashing cursor and stores the result to a variable.

read noobot
#stores user input to the variable noobot

read -p 'What's your name? ' noobot
#the -p option provides a text prompt

seq

Used to generate integer lists, similar to range in Python.

#Syntax
seq start increment stop
#increment is optional and can be negative (decrement)
seq 0 2 20
#display all even numbers from 0 to 20 INCLUSIVE

test

Checks for a condition, returning True or False. Can be called using [] square brackets but must be padded with spaces. Commonly found in IF statements.

#Syntax
test option soloargument
test arg1 option arg2
#Examples
test -n string
test 0 -eq 4

Options
-z #True if length of string is zero
-n #true if length of string is NOT zero
= #True if strings are equal
!= #True if strings are NOT equal
-eq #True if integers are equal
-ne #True if integers are NOT equal
-gt #True if int1 > int2
-ge #True if int1 >= int2
-lt #True if int1 < int2
-le #True if int1 <= int2
-d #True if file is directory
-f #True if file is simple file
-e #True if file exists
-r #True if file has read permission for current user
-w #True if file has write permission for current user
-x #True if file has execute permission for current user
-a #AND to combine two conditions
-o #OR to combine two conditions
! #checks if a condition is False

if test $var -eq 999
#equates to
if [ $var -eq 999 ]
#note the whitespace padding!

while, do, done

while can be used to repeatedly check the state of a conditional and execute code accordingly, via the do/done code block

read -p 'Enter a number: ' number
#prompts user to enter a number
while echo $number | grep -v - E '^[0-9]+$' > /dev/null 2> /dev/null
#the -v grep option inverts the search, so this while statement will only be True if $number is NOT a number
do
  echo 'That's not a number. Please try again.'
  read -p 'Enter a number: ' number
done
echo 'Thank you for correctly entering a number.'

Wrapper

A wrapper script is used to configure and launch another program.