---
title: Avoid double negation in string test
description: Datadog, the leading service for cloud-scale monitoring.
breadcrumbs: >-
  Docs > Datadog Security > Code Security > Static Code Analysis (SAST) > SAST
  Rules > Avoid double negation in string test
---

# Avoid double negation in string test

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## Metadata{% #metadata %}

**ID:** `bash-code-quality/double-negation-in-string-test`

**Language:** Bash

**Severity:** Notice

**Category:** Code Style

## Description{% #description %}

Using `!` with `[ -z … ]`, `[ -n … ]`, or the same inside `[[ … ]]` is a double negative: `[ ! -z x ]` means the same as `[ -n x ]`, and `[ ! -n x ]` the same as `[ -z x ]`. Prefer the direct `-n` or `-z` form for clarity. The same applies when `!` is placed before the whole test, as in `! [ -z x ]`, which is equivalent to `[ -n x ]`.

## Non-Compliant Code Examples{% #non-compliant-code-examples %}

```bash
#!/bin/bash
[ ! -z "$x" ]
[ ! -n "$x" ]
[[ ! -z "$x" ]]
! [ -z "$x" ]
! [[ -z "$x" ]]
! [ -n "$x" ]
```

## Compliant Code Examples{% #compliant-code-examples %}

```bash
#!/bin/bash
[ -z "$x" ]
[ -n "$x" ]
[[ -n "$x" ]]
[[ -z "$x" ]]
[ "$x" = y ]
```
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