Written 29 Aug 2026. This is a how-to, not a product card.

See whether two files are the same

After SSH works, compare two files with diff so you can tell if the spare still matches. You don't need a file viewer for this.

Copy a file without leaving the computer makes a spare. Read a file without opening the editor dumps one short file. After you edit the spare, diff tells you whether the two files still match, without opening both. You are still only looking. It does not need a desktop, a compare app, or a NAS for this one job.

Skip until SSH already works

SSH from the couch should already get you a prompt. If you only need a spare, that is copy a file without leaving the computer. If you want to dump one short file, that is read a file without opening the editor. If you want to change a line, that is edit a file from the couch. Skip this whole note if you only need a house share other computers browse. That is a folder the house can open.

What this is

diff compares two files line by line. You run it on the computer, after SSH. The files stay on disk. You are only looking. It does not make a spare. That is cp. It does not dump one file. That is cat. It does not open an editor. That is nano. It does not ask what kind of file this is. That is file. Two paths, same computer, a match or the lines that differ.

Compare two files

SSH in. Make a throwaway text file, then a spare:

printf 'apple\\nbanana\\ncherry\\n' > ~/fruit.txt

cp ~/fruit.txt ~/fruit-spare.txt

Then ask whether they match:

diff ~/fruit.txt ~/fruit-spare.txt

A blank window means they match. That is the whole first-night trick. No news is the good answer.

Now change the spare and ask again:

printf 'apple\\nblueberry\\ncherry\\n' > ~/fruit-spare.txt

diff ~/fruit.txt ~/fruit-spare.txt

You should see the banana line and the blueberry line. Lines with < are in the first file. Lines with > are in the second. You don't need to memorize the rest of the marks tonight.

Useful extras, still looking only

Need a spare first? That is copy a file without leaving the computer. Need to know if the path is even text? That is see what kind of file this is. Need a short dump of one file? That is read a file without opening the editor.

If you see command not found, install the usual tool, then try again:

sudo apt install diffutils

Debian and Ubuntu Server usually already have it. That install is not a reason to buy a compare app.

When you are done practicing:

rm -i ~/fruit.txt ~/fruit-spare.txt

What this is not

This is not a copy. That is copy a file without leaving the computer. It is not a dump of one file. That is read a file without opening the editor. It is not an editor. That is edit a file from the couch. It is not a line count. That is see how long a file is. It is not asking what kind of file this is. That is see what kind of file this is. It is not a search for a word. That is find a word in a file. It is not a hunt for a file by name. That is find a file by name. It is not an empty file. That is make an empty file. It is not a house share other computers browse. That is a folder the house can open.

What next

Need an empty file first? Make an empty file. Need a spare to compare against? Copy a file without leaving the computer. Need to change the spare? Edit a file from the couch. Need a short dump of one file? Read a file without opening the editor. Need to know if the path is text before you compare it? See what kind of file this is. Need the first lines of a long file? See the start of a file. Need the last lines? See the end of a file. Need only the lines with one word? Find a word in a file. Need the throwaway gone for real? Delete a file carefully. Need a file from the laptop instead? Copy a file to the computer from your laptop. When a house job broke, look at the computer's own diary: when something broke, look at the log. When key login is boring on every computer you use, turn off password login once the key works. Keep SSH on the house network: keep the computer on the house network. Don't forward port 22.

Skip

Skip if SSH still fails. Finish SSH from the couch first.
Skip diff on photos, zip files, or programs. Those print junk. Two text files are the job. Ask first with see what kind of file this is.
Skip a compare app, Git, or a NAS to see whether one spare still matches.
Skip extra flags tonight. A blank window or the differing lines is enough. You don't need a side-by-side view.
Skip SSH settings, /etc/shadow, and anything under /etc/ssh. Looking at trusted laptop keys is see which laptop keys the computer already trusts.
Skip opening SSH to the internet so you can compare files from a coffee shop. That is keep the computer on the house network.

Sources

Debian: diff. Ubuntu: diff. Official pages only. For the login, see SSH from the couch. For a spare, see copy a file without leaving the computer. For a short dump, see read a file without opening the editor. For a kind-of-file check, see see what kind of file this is.

Also on this topic

SSH from the couchType commands on the computer from your laptop, on the house network. Password login first. Don't open this to the internet. Copy a file without leaving the computerAfter SSH works, copy a file with cp so you can edit a spare without wrecking the original. You don't need a NAS for this. Read a file without opening the editorAfter SSH works, print a file with cat so you can read it without opening nano. Short files only. You don't need a file viewer for this. Edit a file from the couchAfter SSH works, open a file with nano, change a line, and save. The keys at the bottom of the screen are the whole trick. You don't need a new editor. See what kind of file this isAfter SSH works, ask what kind of file this is with file before you dump it. You don't need a file viewer for this. See the start of a fileAfter SSH works, print the first lines of a file with head so a long file does not flood the window. You don't need a file viewer for this. See how long a file isAfter SSH works, count lines with wc so you know if a dump will flood the window. You don't need a file manager for this. See what's in a folderAfter SSH works, list the folder you are sitting in with ls. You don't need a file manager for this. Delete a file carefullyAfter SSH works, delete one file with rm -i. Ask first. Skip rm -rf. You don't need a NAS for this. Make an empty fileAfter SSH works, make an empty file with touch so nano and copies have a file to land on. You don't need a file manager for this. Turn off password login once the key worksAfter the laptop key login is boring, tell SSH to stop accepting the password. The key is how you get in. A keyboard on the computer is the backup now.