How to set default value for a variable in Shell

This line is a Bash parameter-expansion idiom used to set a default value for a variable while being safe under strict shell modes.

: "${WORKSPACE:=/workspace}"

Let’s break it down piece by piece.

1. `:` — the null (no-op) command

: is a built-in command that does nothing and always succeeds (exit status 0).
It is often used to force evaluation of parameter expansions without running a real command.

So here, : is just a wrapper to make sure the expansion happens.

2. `"${WORKSPACE:=/workspace}"` — default assignment

This is parameter expansion with default assignment.

Meaning

If WORKSPACE is unset or empty, assign it the value:
```
/workspace
```
If WORKSPACE already has a value, leave it unchanged.

Equivalent logic in plain English

“If WORKSPACE is not set, set it to /workspace.”

3. Why the quotes matter

"${WORKSPACE:=/workspace}"

Protects against word splitting and glob expansion
Safe even if the value contains spaces

This is best practice.

4. Why not just write `WORKSPACE=/workspace`?

Because that would always overwrite the variable.

This line is conditional.

5. Why is `:` needed at all?

Because ${VAR:=value} returns the value, but on its own it’s not a valid shell command.

This would be invalid:

"${WORKSPACE:=/workspace}"   # ❌ not a command

So we use : to consume the result safely.

6. Why this is common in `set -u` scripts

With set -u (treat unset variables as errors):

echo "$WORKSPACE"   # ❌ error if unset

But:

: "${WORKSPACE:=/workspace}"
echo "$WORKSPACE"   # ✅ always safe

This makes the pattern strict-mode friendly.

7. Equivalent forms

Using `if`

if [ -z "${WORKSPACE:-}" ]; then
  WORKSPACE=/workspace
fi

Using `export`

: "${WORKSPACE:=/workspace}"
export WORKSPACE

8. Common real-world usage

You’ll often see this in:

Docker entrypoint scripts
CI/CD scripts
Kubernetes init scripts
Strict Bash scripts (set -euo pipefail)

Example:

: "${HOME:=/root}"
: "${PORT:=8080}"
: "${LOG_LEVEL:=info}"

One-line summary

This line ensures WORKSPACE is set to /workspace if it wasn’t already, without overwriting an existing value, and works safely under set -u.

1. : — the null (no-op) command

2. "${WORKSPACE:=/workspace}" — default assignment