What text comparison software do you use?

I use VSCode for pretty much everything text related.

Open both files, open the Explorer in the sidebar, select both files in the 'Open Editors' sections, right click a file -> 'Compare Selected'

I've used so many of these and I'll say there is still nothing as good as Beyond Compare. That said, as datavoid said, VS Code is pretty decent.

I just use diff

If it's a lot, I just pipe the results to a file first.

If I don't have diff available I just use an online diff checker.

I've used Meld before, it's graphical.

On macOS I use FileMerge. It comes with Xcode and as such is located in a weird place ("Applications > Xcode.app > Contents > Applications > FileMerge.app"), but you can access it via Spotlight. A simple tool but it does the job.

When using IntelliJ I use its built in diff tools. Especially when doing VCS diffs or merge resolution, their VCS UI is really good. Similarly for VSCode or any other IDE.

Vim and Emacs unfortunately don't seem to have good built-in diff tools, although Neovim has diffview.nvim. On the command line I use diff often, but I'll admit it's not nearly as good as the GUI tools, so it only works well for simple diffs.

Lastly, when I need to generate a report for a diff, e.g. to share with others, I use diff2html

If the lines of the CSV are not sorted you could try sorting on a column then use something like kdiff3 for diffing.

xsv sort will sort lexicographically

It sounds like your CSVs are not too different but in the case that your CSVs are a real mess without much similarity you could try sorting with a tool I wrote which will sort the lines based on the term similarity of all the columns:

pip install xklb
lb clustersort 1.csv 1_sorted.csv

As for merging:

If you just need to concatenate the CSVs use xsv cat

Otherwise, if you can convert your CSVs to SQLITE with sqlite-utils...

ie.

pip install sqlite-utils
sqlite-utils insert --alter --csv --detect-types 1.db tablename 1.csv

then you can merge the SQLITE files together based on a specific column:

lb mergedbs --pk id --upsert combined.db 1.db 2.db 3.db

lb mergedbs -h
usage: library merge-dbs DEST_DB SOURCE_DB ... [--only-target-columns] [--only-new-rows] [--upsert] [--pk PK ...] [--table TABLE ...]

    Merge-DBs will insert new rows from source dbs to target db, table by table. If primary key(s) are provided,
    and there is an existing row with the same PK, the default action is to delete the existing row and insert the new row
    replacing all existing fields.

    Upsert mode will update matching PK rows such that if a source row has a NULL field and
    the destination row has a value then the value will be preserved instead of changed to the source row's NULL value.

    Ignore mode (--only-new-rows) will insert only rows which don't already exist in the destination db

    Test first by using temp databases as the destination db.
    Try out different modes / flags until you are satisfied with the behavior of the program

        library merge-dbs --pk path (mktemp --suffix .db) tv.db movies.db

    Merge database data and tables

        library merge-dbs --upsert --pk path video.db tv.db movies.db
        library merge-dbs --only-target-columns --only-new-rows --table media,playlists --pk path --skip-column id audio-fts.db audio.db

        library merge-dbs --pk id --only-tables subreddits reddit/81_New_Music.db audio.db
        library merge-dbs --only-new-rows --pk subreddit,path --only-tables reddit_posts reddit/81_New_Music.db audio.db -v

positional arguments:
  database
  source_dbs

options:
  -h, --help            show this help message and exit
  --primary-keys PRIMARY_KEYS, --pk PRIMARY_KEYS
                        Comma separated primary keys
  --business-keys BUSINESS_KEYS, --bk BUSINESS_KEYS
                        Comma separated business keys
  --upsert
  --ignore, --only-new-rows
  --only-tables ONLY_TABLES, -t ONLY_TABLES
                        Comma separated specific table(s)
  --only-target-columns
  --skip-columns SKIP_COLUMNS

I've previously used 010 Editors's compare functionality, before I used Beyond.

Most use cases Beyond will do just fine.

If you want to see why two tars are differently sized but their opened content is equal, 010 might be more comfortable imo.

I've been a fan of kdiff3 for years and I'm not even a Linux user! There are binaries available for Windows and Mac.

It's got a usable UI, smart whitespace display options, merge support (including auto-merge), 3-way diff/merge, recursive directory diffing, and it's FOSS. That ticks all my boxes. It's not the prettiest app but it's extremely capable. I configured git to use it for diffs and merges and it works like a champ. In a previous career I used the directory diff to compare a production site's web root with my local dev environment to detect PHP malware after the server was compromised. Nowadays I'm mostly using it to review staged files before committing them, or resolve merge conflicts.