Bash error: unary operator expected

The "[: -eq: unary operator expected" error appears when one side of a `[ ]` test is empty. Quote the variable or default it.

Error String

bash: [: -eq: unary operator expected

Tldr

When `[ $x -eq 1 ]` and `$x` is empty, Bash sees `[ -eq 1 ]` — a unary operator with no operand. Quote the variable so the empty case becomes `[ "" -eq 1 ]`, which is a clean error you can handle.

Cause

Unquoted empty variables disappear from the argument list. `[` then thinks the next token is the only operand, which is the wrong shape.

Repro

if [ $x -eq 1 ]; then ...        # x empty -> [: -eq: unary operator expected

Fix

if [ "${x:-0}" -eq 1 ]; then ...

# Or use (( )) which treats empty as 0:
if (( x == 1 )); then ...

Explanation

Same root cause as "integer expression expected" and "too many arguments" — always quote variables in `[ ]` tests, or use `[[ ]]` / `(( ))` which are Bash-only but parse cleanly.

Related Shellcheck

SC2086

Faq

Q

What is the x-prefix trick?

A

`[ "x$var" = "xvalue" ]` guarantees a non-empty operand in ancient shells. Quoting is sufficient in any modern shell.

Q

Why did an unquoted file test return true instead of erroring?

A

`[ -f ]` degenerates to a non-empty-string test on the literal `-f`, which is always true.

Deep Dive

Heading

An empty expansion removes the operand

Body

The mirror image of the binary-operator error: `[ $var = value ]` with an empty `$var` becomes `[ = value ]`, and the test finds an operator with nothing on the left. Quoting fixes it — `[ "$var" = value ]` becomes `[ "" = value ]`, a perfectly valid comparison that simply returns false. The same happens for a missing positional parameter, so guard `$#` before touching `$1`.

Heading

File tests on empty paths

Body

`[ -f $path ]` with an empty `$path` collapses to `[ -f ]`, which tests whether the string `-f` is non-empty and quietly returns true — an even worse outcome than an error. Always quote path expansions and reject empty values explicitly. In Bash, `[[ -f $path ]]` avoids the splitting problem entirely, which is the main reason to prefer it in scripts that never need POSIX `sh`.

Checklist

Quote every expansion inside `[ ]`.

Check `$#` before comparing positional parameters.

Reject empty values with `[ -z "$var" ]` and a clear message.

Prefer `[[ ]]` in Bash-only scripts.