Wrap long lines using fold

To line wrap a text file at 80 columns, and only break at spaces, use the command:

cat filename.txt | fold -80 -s

fold can also be told to wrap at bytes instead of columns, but I've never been quite sure how that would be useful.

Comparing two files using comm

comm compares contents of two files. It has 3 columns available in its output — the lines only in file 1, the lines only in file 2, and the lines in both. You'll need to sort both files first.

sort test1.txt > test1-sorted.txt
sort test2.txt > test2-sorted.txt

This will show you lines only in test1.txt:

comm -23 test1-sorted.txt test2-sorted.txt

This will show you lines only in test2.txt:

comm -13 test1-sorted.txt test2-sorted.txt

This will show you lines only common to both files:

comm -12 test1-sorted.txt test2-sorted.txt

We can also do some neat tricks with uniq/sort -u:

cat test1.txt | sort > test1-sorted.txt
cat test1.txt | sort -u > test1-sorted-u.txt

This will show you lines only in test1-sorted-u.txt, which means those are the lines that appear multiple times in your original test1.txt file:

comm -13 test1-sorted.txt test1-sorted-u.txt

Neat, huh?

Setting a lot of user passwords at once

Say you have a whole class of users who have lost their passwords. Or perhaps you've just created a set of users with some automated method that doesn't allow you to easily set their passwords (such as a scriptfile full of useradd commands).

You'll want to use a file containing the following information:

user1:passwd1
user2:passwd2

Then run the command:

cat passwords.txt | chpasswd

Don't forget to delete your passwords.txt file just after you're done. Seriously bad idea to leave this file hanging around.

If you're looking for a way to automatically generate a whole bunch of passwords that your users won't balk at, the pwgen command may be installed on your system already!

If not, and you don't want to go hunting for one, try this: http://www.multicians.org/thvv/gpw.html

Deleting a large file that’s causing problems

When you have a process that's going haywire and filling up a filesystem, you have to remember to kill the process that's accessing/writing the file before the disk space will be freed up.

If you're not sure what process that is, use the fuser command:

fuser filename

Sure, you can use options in the fuser command to automatically kill off all those processes, but that would make me really nervous if I wasn't absolutely sure what each of them was... killing one of those off could be worse than having a full filesystem.