# File Operators

File operators are most often used with merge when needing to merge specific files together. Note that when doing this, you will need to use `eval-all` to ensure all yaml documents are loaded into memory before performing the merge (as opposed to `eval` which runs the expression once per document).

Note that the `fileIndex` operator has a short alias of `fi`.

## Merging files

Note the use of eval-all to ensure all documents are loaded into memory.

```bash
yq eval-all 'select(fi == 0) * select(filename == "file2.yaml")' file1.yaml file2.yaml
```

## Get filename

Given a sample.yml file of:

```yaml
a: cat
```

then

```bash
yq 'filename' sample.yml
```

will output

```yaml
sample.yml
```

## Get file index

Given a sample.yml file of:

```yaml
a: cat
```

then

```bash
yq 'file_index' sample.yml
```

will output

```yaml
0
```

## Get file indices of multiple documents

Given a sample.yml file of:

```yaml
a: cat
```

And another sample another.yml file of:

```yaml
a: cat
```

then

```bash
yq eval-all 'file_index' sample.yml another.yml
```

will output

```yaml
0
1
```

## Get file index alias

Given a sample.yml file of:

```yaml
a: cat
```

then

```bash
yq 'fi' sample.yml
```

will output

```yaml
0
```