# comm

_Peios / Using Peios / Peiosutils / Viewing and joining text_

> Compare two sorted files line by line and report which lines they share.

`comm` compares two sorted files and tells you which lines are **unique to each** and which are **common to both**.

```
comm [options] file1 file2
```

By default it prints three columns:

```
$ comm list-a.txt list-b.txt
            apple
banana
                        cherry
date
```

- **Column 1** — lines only in `file1`.
- **Column 2** — lines only in `file2`.
- **Column 3** — lines in both.

Each column is indented past the one before, so a line's column position tells you where it was found.

## The input must be sorted

`comm` steps through both files together, so **both must be sorted**. On unsorted input the comparison is meaningless. Sort the files first with [`sort`](/peios/using-peios/peiosutils/transforming-text/sort.md).

`comm` checks the ordering and reports a file that is out of order. `--check-order` forces the check; `--nocheck-order` disables it.

## Showing only the columns you want

Each column can be switched off. The remaining columns close up to fill the gap.

| Option | Effect |
|---|---|
| `-1` | Suppress column 1 — hide lines unique to `file1`. |
| `-2` | Suppress column 2 — hide lines unique to `file2`. |
| `-3` | Suppress column 3 — hide lines common to both. |

These combine to answer specific questions. `comm -12` shows *only* the common lines — the intersection of the two files. `comm -23` shows lines in `file1` that are not in `file2` — the difference.

## Other options

| Option | Effect |
|---|---|
| `--output-delimiter=STR` | Separate the columns with `STR` instead of spaces. |
| `--total` | Print a final summary line with the count in each column. |
| `-z`, `--zero-terminated` | Treat the NUL character as the line delimiter. |

## Exit status

| Code | Meaning |
|---|---|
| `0` | Success. |
| `1` | A file could not be read, or was not sorted. |

Related content:

- [join](/peios/using-peios/peiosutils/viewing-and-joining-text/join.md)
- [sort](/peios/using-peios/peiosutils/transforming-text/sort.md)
- [uniq](/peios/using-peios/peiosutils/transforming-text/uniq.md)
- [Viewing and joining text](/peios/using-peios/peiosutils/viewing-and-joining-text/overview.md)