Bash Scripting Tutorial: Write Your First Shell Script

A script is a text file of commands. Anything you repeat, you can put in a script, and then run it with one word.

Write a script#

Create a file called hello.sh. You can use nano hello.sh. In this course, we use cat with a "here document", which writes the lines up to EOF into a file, so that every example can be copied in one go:

cat > hello.sh <<'EOF'
#!/bin/bash
# Greets the user and shows the time.
echo "Hello, $USER"
echo "Today is $(date +%A)"
EOF

• #!/bin/bash is the shebang. It says which program should run the file, and it must be the first line.
• Lines starting with # are comments.
• Everything else is what you would type in the terminal.

Run it#

Make it executable, then run it with ./:

chmod +x hello.sh
./hello.sh | sed 's/Today is .*/Today is .../'

Hello, asha
Today is ...

The ./ means "in this folder". The current folder is not in PATH, so plain hello.sh gives "command not found".

You can also run it with bash hello.sh, which needs no execute permission.

Input with read#

read waits for a line of text and stores it in a variable. -p prints a prompt:

cat > greet.sh <<'EOF'
#!/bin/bash
read -p "What is your name? " name
echo "Welcome, $name!"
EOF
chmod +x greet.sh
echo "Ravi" | ./greet.sh

Welcome, Ravi!

We piped in the answer here. In a terminal, you would type it. read -s hides typing, which suits passwords.

A useful example#

This script reports the state of a machine:

cat > status.sh <<'EOF'
#!/bin/bash
echo "Host:   $(hostname)"
echo "User:   $(whoami)"
echo "Disk:   $(df -h / | awk 'NR == 2 {print $5}') used"
echo "Files:  $(ls -A ~ | wc -l) in home"
EOF
chmod +x status.sh
./status.sh

Host:   laviyx
User:   asha
Disk:   83% used
Files:  6 in home

Printing with printf#

echo is fine for simple text. printf gives exact control over the format:

printf "%-8s %5s\n" "Item" "Price"
printf "%-8s %5d\n" "Tea" 20
printf "%.2f\n" 3.14159

Item     Price
Tea         20
3.14

Here documents#

A here document feeds several lines to a command. With unquoted EOF, variables are expanded; with quoted 'EOF', the text is kept exactly:

name=Asha
cat <<EOF
Dear $name,
Your report is ready.
EOF
cat <<'EOF'
Literal: $name
EOF

Dear Asha,
Your report is ready.
Literal: $name

Bash or Python?#

Debugging#

bash -x prints each command before it runs, which shows you what a script is doing:

bash -x greet.sh < /dev/null

+ read -p 'What is your name? ' name
+ echo 'Welcome, !'
Welcome, !

Common mistakes#

Practice#

cat > sysinfo.sh <<'EOF'
#!/bin/bash
echo "Kernel: $(uname -s)"
echo "Cores:  $(nproc)"
echo "User:   $(whoami)"
EOF
chmod +x sysinfo.sh
./sysinfo.sh | sed -n '1p;$p'

Kernel: Linux
User:   asha

Key takeaway#

Next: Variables and Quoting.