# Shell Compatibility Issue in Nexus Repository 3.78.2-04 Startup Script

## Environment
- Nexus Repository: 3.78.2-04 (Community Edition)
- OS: Debian 12.10

## Issue
During service startup, the following error appears:

```bash
/opt/nexus-3.78.2-04/bin/nexus: 155: [[: not found
```

## Root Cause
The startup script (`bin/nexus`) uses bash-specific syntax with `/bin/sh` shebang:

```sh
#!/bin/sh
# ...
if [[ "${data_dir}" != /* ]]; then  # Line 155
  data_dir="${PARENT}/${data_dir}"
fi
```

On systems where `/bin/sh` links to a POSIX-compliant shell like dash, the `[[` syntax is unsupported.

## Reproduction
Standard startup via systemd produces the error:

```bash
Mar 20 21:33:46 host systemd[1]: Starting nexus.service - nexus service...
Mar 20 21:33:46 host nexus[92263]: /opt/nexus-3.78.2-04/bin/nexus: 155: [[: not found
Mar 20 21:33:46 host nexus[92263]: Starting nexus
Mar 20 21:33:46 host systemd[1]: Started nexus.service - nexus service.
```

## Proposed Solutions
1. Change shebang from `#!/bin/sh` to `#!/bin/bash`

OR

2. Replace bash-specific syntax with POSIX-compliant case statement:

```sh
case "${data_dir}" in
     /*)
       ;;
     *)
       data_dir="${PARENT}/${data_dir}"
       ;;
esac
```

## Impact
The issue does not prevent service operation but generates unnecessary warnings during startup.