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# Bash Newer Than
The `-nt` (newer than) operator is a binary file test operator in Bash used to compare the modification timestamps (`mtime`) of two files. It evaluates to true (exit status `0`) if the first file was modified more recently than the second file, or if the first file exists and the second file does not.
## Syntax and Variable Quoting
The operator is evaluated using either the `test` command (`[`), or the Bash extended test construct (`[[`).
```bash theme={"dark"}
[ "$file1" -nt "$file2" ]
```
```bash theme={"dark"}
[[ $file1 -nt $file2 ]]
```
When using the `[` command, variables **must be quoted**. If an unquoted variable evaluates to an empty string or contains spaces, word splitting alters the number of arguments passed to `[`, resulting in a syntax error (e.g., `[: -nt: unary operator expected`). The Bash extended test construct (`[[`) safely handles unquoted variables because it suppresses word splitting and pathname expansion during evaluation.
## Portability
While the `[` command is a standard POSIX utility, the `-nt` operator is historically a Bash and KornShell (ksh) extension. Although `-nt` was recently formalized in the POSIX.1-2024 standard, using `[ "$file1" -nt "$file2" ]` is not universally portable. Executing this operator in older or strictly POSIX-compliant shells (such as `yash` or older `dash`/`ash` environments) may result in an evaluation failure or syntax error.
## Evaluation Logic
The operator returns a boolean exit status based on the following matrix:
| State of `file1` | State of `file2` | Condition | Exit Status |
| :--------------- | :--------------- | :----------------------------- | :---------- |
| Exists | Exists | `mtime(file1) > mtime(file2)` | `0` (True) |
| Exists | Exists | `mtime(file1) <= mtime(file2)` | `1` (False) |
| Exists | Does not exist | N/A | `0` (True) |
| Does not exist | Exists | N/A | `1` (False) |
| Does not exist | Does not exist | N/A | `1` (False) |
## Technical Characteristics
* **Timestamp Resolution:** The precision of the `-nt` comparison is strictly bound by the underlying filesystem. Modern filesystems (like `ext4`, `xfs`, or `zfs`) support nanosecond precision, allowing `-nt` to differentiate between files created fractions of a second apart. Older filesystems (like `FAT32`) may only support 2-second resolution, which can result in a false negative (`1`) if two files are modified within the same resolution window.
* **Symlink Dereferencing:** The `-nt` operator automatically dereferences symbolic links. It compares the `mtime` of the ultimate target file, not the inode of the symlink itself. If a symlink is broken (points to a non-existent target), Bash evaluates it as a non-existent file according to the logic matrix above.
* **Directory Handling:** Directories are treated as standard files. The operator evaluates the `mtime` of the directory's inode, which is updated only when the directory's direct contents are modified (e.g., a file is created, deleted, or renamed within it), not when the contents of files within the directory are modified.
* **Strict Inequality:** The operator performs a strictly greater-than comparison. If `file1` and `file2` have the exact same `mtime`, `-nt` evaluates to false (`1`).
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