如何使用diff命令提取file2中相对于file1的新增行？

Hey there! I totally get why you might have missed this in the man page—some of diff's more niche options are easy to overlook. But there are actually a couple of straightforward ways to get exactly what you want: only the lines that are unique to file2.

Method 1: Use diff's Custom Format Options #

The most direct way is to leverage diff's formatting flags to suppress lines from file1 and unchanged lines, while keeping only the new lines from file2:

diff --new-line-format='%L' --old-line-format='' --unchanged-line-format='' file1 file2

Let's break down what each part does:

• --new-line-format='%L': Tells diff to print any line that's only present in file2 exactly as it appears (%L represents the line content).
• --old-line-format='': Suppresses any lines that are only in file1 (we don't care about those here).
• --unchanged-line-format='': Suppresses lines that exist in both files (since we only want the new ones).

Running this command with your files will output:

/root/new
/root/new_new

Method 2: Unified Diff + Grep (Simpler to Remember) #

If the format flags feel too verbose, you can use the unified diff format and filter the output with grep:

diff -u file1 file2 | grep '^+' | grep -v '^+++'

Here's the breakdown:

• diff -u: Generates a unified diff where lines starting with + are new additions in file2.
• grep '^+': Filters to keep only lines that start with + (the new lines).
• grep -v '^+++': Excludes the header line that starts with +++ (which just lists the filename of file2).

This will also give you the exact output you're expecting.

Note for Unsorted Files #

If your files aren't sorted, diff might not correctly identify matching lines (since it compares line order). To fix this, sort both files on the fly using process substitution:

For Method 1:

diff --new-line-format='%L' --old-line-format='' --unchanged-line-format='' <(sort file1) <(sort file2)

For Method 2:

diff -u <(sort file1) <(sort file2) | grep '^+' | grep -v '^+++'

Either approach will work perfectly for your use case!

内容的提问来源于stack exchange，提问作者Dsujan