Shell - What does ":" do?

In shell (especially POSIX shells like sh, bash, dash, zsh), : is a builtin command called the null command.

What it does

• Does nothing

• Always succeeds (returns exit status 0)

:
echo $?
# 0

Why it’s useful

Even though it does nothing, : is surprisingly handy.

---

1. Placeholder in scripts

When syntax requires a command but you don’t want to do anything yet:

if condition; then
    :
fi

---

2. Infinite loops

Common idiom:

while :; do
    echo "Running..."
    sleep 1
done

(: always succeeds, so the loop never ends.)

---

3. Default action in conditionals

When you want to explicitly do nothing:

case "$var" in
    yes) echo "Yes" ;;
    no)  echo "No" ;;
    *)   : ;;
esac

---

4. Variable expansion with side effects

Often used to safely trigger parameter expansion:

: "${VAR:=default}"

• Sets VAR to default if it’s unset or empty

• No output, no side effects besides assignment

---

5. Redirecting without running a command

Since : does nothing, it’s useful with redirections:

: > file.txt        # truncate or create file
: >> logfile.log    # append (touch-like)

---

6. Alternative to true

: is faster and more portable than calling /bin/true:

while :; do ...; done

---

Summary

Feature	:
Type	Shell builtin
Effect	Does nothing
Exit status	Always 0
Common uses	Loops, placeholders, parameter expansion, redirection