Shell Script Notes for Beginners

• 2018/10/06

Getting started with shell scripts

I expect to write more shell soon, so I decided to learn the basics even as a beginner.

Build the environment

• CentOS 7
• Vagrant
• VirtualBox

Steps:

1. Install Vagrant and VirtualBox.
2. Grab the CentOS 7 box script from here.
3. Run the commands:

vagrant init centos/7
vagrant up
vagrant ssh

4. Confirm you can SSH into CentOS (if prompted for credentials, use vagrant / vagrant).

Write and run a shell script

1. Create a file:

touch hello.sh

2. Edit it:

vi ./hello.sh

#!/bin/sh
echo "helloworld"

3. Run it. Three options:

• sh hello.sh
• bash hello.sh
• Make it executable:

sudo chmod 755 ./hello.sh
./hello.sh

Basic operations (omitting #!/bin/sh)

• Output:

echo text-to-print

• Read input:

read

• Arithmetic:

expr 2 \+ 2
expr 2 \- 2
expr 2 \* 2
expr 2 \/ 2

• Assign variables:

a=1
b=2

• Capture command results with backticks (note the spaces):

r=`expr $a \+ $b`

• Print variables:

echo $r

• if statements:

if [ $res -eq $zero ]; then
  echo "Even"
else
  echo "Odd"
fi
# String comparison uses =
if [ "ABC" = "ABC" ]; then
  echo "equal"
else
  echo "not equal"
fi

-eq compares numbers.

• case (switch):

var="red"
case "$var" in
  "blue" ) echo "blue" ;;
  "green" ) echo "green" ;;
  "red" ) echo "red" ;;
esac

• while loop (-lt == <):

int=0
while [ $int -lt 5 ]; do
  echo $int
  int=`expr $int \+ 1`
done

• until loop (-gt == >):

until [ ! \( $int -gt 0 \) ]; do
  echo $int
  int=`expr $int \- 1`
done

• for loop:

for var in 10 11 12 ; do
  echo $var
done

• Function:

MyFunction () {
  echo "Function executed"
}
MyFunction

• Function with arguments:

add () {
  echo `expr $1 \+ $2`
}
add 1 3

Closing thoughts

Shell scripting is unavoidable, so I am studying it early in my career. I stumbled over missing spaces and the CRLF line endings that Windows editors insert—Linux expects LF, so watch out!