Bash – Elegant way of diff’ing two variables

bashdiff()here-stringio-redirection

I have $a and $b. I want to run diff on those.

The best I have come up with is:

diff <(cat <<<"$a") <(cat <<<"$b")

But I have the district feeling that I am missing a clever Bash syntax to do that (as in "Why don't you just use foo?").

Best Answer

echo. Clearly less weird.

#!/bin/bash

a="`seq 10`"
b="`seq 0 11`"

diff <(echo "$a") <(echo "$b")

It's called "paging output" or (somewhat erroneously) "pagination" …

… and man does it by invoking your preferred pager shell command, named by your PAGER environment variable, upon the output of whatever pipeline was used to generate the output form of the manual page. It falls back to a default if you haven't specified a pager command. On early Unices the default pager program, that man invokes as the default pager command with known hardwired options, was pg. On your system it is probably more or less. Some man commands look at other environment variables for pager commands, as well.

You do it by doing what man is doing: pipe the output of what you want to see into the standard input of a pager program.

You should get yourself a good book on Unix. There are a fair few that explain this, which is a very basic feature of the system. A simple Google Books search for pager unix more by me today turned up over a hundred books that discuss this. (I stopped counting at a hundred.)

Result of diff two files with switched lines says missing the same line twice

To understand the report, remember that diff is prescriptive, describing what changes need to be made to the first file (file1) to make it the same as the second file (file2).

Specifically, the d in 1d0 means delete and the a in 2a2 means add.

Thus:

1d0 means line 1 must be deleted in file1 (apples). 0 in 1d0 means line 0 is where they would have appeared in the second file (file2) had they not been deleted. That means when changing file2 to file1 (backwards) append line 1 of file1 after line 0 of file2.
2a2 means append the second line (oranges) from file2 to the now second line of file1 (after deleting the first line in file1, oranges switched to line 1)

Best Answer

Related Solutions

On Unix how to get the output from the diff command to look like a man page? Where I can press ‘b’ or ‘f’ to read further or back

It's called "paging output" or (somewhat erroneously) "pagination" …

Result of diff two files with switched lines says missing the same line twice

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