Arguments, Input and Exit Codes

Arguments: $1, $2, $@

A script can take arguments, the words you type after its name. Inside the script, $1 is the first, $2 the second, and so on. $0 is the script's own name, $# how many arguments there were, and $@ all of them:

cat <<'EOF' > greet.sh
#!/bin/bash
echo "Running $0"
echo "Hello, $1 and $2"
echo "You gave $# arguments: $@"
EOF
chmod +x greet.sh
./greet.sh Ada Bo

In the terminal

student@vulntech:~$ cat <<'EOF' > greet.sh
> #!/bin/bash
> echo "Running $0"
> echo "Hello, $1 and $2"
> echo "You gave $# arguments: $@"
> EOF
student@vulntech:~$ chmod +x greet.sh
student@vulntech:~$ ./greet.sh Ada Bo
Running ./greet.sh
Hello, Ada and Bo
You gave 2 arguments: Ada Bo

Run it with different words and the output changes. This is how real commands take their arguments: when you type cp a b, cp sees $1 as a and $2 as b.

Asking the user: read

read pauses the script and reads a line the user types into a variable. -p shows a prompt first:

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

In the terminal (the name is piped in, as if typed)

student@vulntech:~$ cat <<'EOF' > ask.sh
> #!/bin/bash
> read -p "What is your name? " who
> echo "Welcome aboard, $who!"
> EOF
student@vulntech:~$ chmod +x ask.sh
student@vulntech:~$ echo Grace | ./ask.sh
Welcome aboard, Grace!

Exit codes: did the script succeed?

Every command ends with an exit code: 0 for success, anything from 1 to 255 for a problem. Your script sets its own with exit. Other scripts (and &&, ||) rely on it:

cat <<'EOF' > check.sh
#!/bin/bash
if [ -f "$1" ]; then
  echo "$1 exists"
  exit 0
else
  echo "$1 is missing"
  exit 1
fi
EOF
chmod +x check.sh
./check.sh /etc/hostname
echo $?
./check.sh /etc/nope
echo $?

In the terminal

student@vulntech:~$ cat <<'EOF' > check.sh
> #!/bin/bash
> if [ -f "$1" ]; then
>   echo "$1 exists"
>   exit 0
> else
>   echo "$1 is missing"
>   exit 1
> fi
> EOF
student@vulntech:~$ chmod +x check.sh
student@vulntech:~$ ./check.sh /etc/hostname
/etc/hostname exists
student@vulntech:~$ echo $?
0
student@vulntech:~$ ./check.sh /etc/nope
/etc/nope is missing
student@vulntech:~$ echo $?
1

Tip: exit on its own passes on the exit code of the last command that ran. A script with no exit does the same.

Common mistake: $1 is empty if the user gave no arguments. Check with if [ -z "$1" ]; then echo "Usage: $0 FILE"; exit 1; fi at the top of a script.

Remember: $1, $2 are the arguments, $@ all of them, $# the count. read asks the user. exit 0 = success, non-zero = failure.