The basics
diff [option] [files]
diff
outputs the differences between two files, line by line. For files that are identical it produces no output, for binary files only reports if they are different or not.
The set of differences produced by diff
is often called a diff or patch, and this output can be used later on by the patch
command to change other files.
Play by play
Before digging into the examples, let's define two files that will help us to illustrate some concepts.
To keep the conceptual overhead as low as possible, the files are with-text.js
and without-text.js
to represent if the text
variable is present or not.
File with-text.js
:
function example (text) {
console.log("====")
console.log(text)
}
File without-text.js
:
function something () {
console.log("====")
}
Default output
By default, diff
outputs a list of all the lines that are different between the files, one after the other, alongside with useful information about what changed. A change can be:
- An addition, represented by the letter
a
. - A deletion, represented by the letter
d
. - A change, represented by the letter
c
.
For every line in which diff
finds a change, it outputs:
- What type of change is (
a/c/d
) - What is the line number affected in each file
- The content of the lines affected
In a more concrete example, if you run:
$ diff with-text.js without-text.js
1c1
< function example (text) {
---
> function something () {
3d2
< console.log(text)
You get:
-
1c1
indicating that there's ac
hange in line1
of both files. -
< function example (text) {
indicating how the line looks inwith-text.js
. -
> function something () {
indicating how the line looks inwithout-text.js
.
Are you able to deduce what the rest of the lines are expressing in the output?
Side by side comparison
Apart from the default output format, you can specify the --side-by-side
(short -y
) to produce a side-by-side view of what changed.
This format is more visual and can be easier to understand at first sight, for example:
$ diff with-text.js without-text.js --side-by-side
function example (text) { | function something () {
console.log("====") console.log("====")
console.log(text) <
} }
Working with directories
A very powerful feature of diff
is the ability to work with directories. It's important to note that actual contents of directories are never compared as if they were a file, instead diff
uses the following rules:
- If one file is a directory and the other is not,
diff
compares the file in the directory whose name is that of the non-directory. - If two file names are given and both are directories,
diff
compares corresponding files in both directories, in alphabetical order.
Colorised output
Since version 3.4 (launched around August of 2016), diff
supports the --color
flag print colorised output in the terminal.
Reading standard input
If you provide -
as a file name, diff
will use it for text read from the standard input. As a special case, diff - -
compares a copy of standard input to itself.
Omitting differences
diff
also provides ways to suppress differences between files that may not be important to you, common examples of this are changes in the amount of white space between words or lines.
Flag | Description |
---|---|
-i --ignore-case | Ignore case differences in file contents. |
--ignore-file-name-case | Ignore case when comparing file names. |
--no-ignore-file-name-case | Consider case when comparing file names. |
-E --ignore-tab-expansion | Ignore changes due to tab expansion. |
-b --ignore-space-change | Ignore changes in the amount of white space. |
-w --ignore-all-space | Ignore all white space. |
-B --ignore-blank-lines | Ignore changes whose lines are all blank. |
-I RE --ignore-matching-lines=RE | Ignore changes whose lines all match RE. |
Using the right tool for the job
While diff
awesome, it's not always the right tool for the job, here are some better fits for different scenarios:
- The
diff3
command can be useful to show differences among three files. - For binary files,
cmp
reports the differences between two files byte by byte, instead of line by line. - If you are looking only for visual diffing without the patching,
vim -d
andgit diff
do a good job.
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