How to generate a random string?

To generate a random password you can use pwgen:

pwgen generates random, meaningless but pronounceable passwords. These passwords contain either only lowercase letters, or upper and lower case mixed, or digits thrown in. Uppercase letters and digits are placed in a way that eases remembering their position when memorizing only the word.

Generate 7 passwords of length 13:

geek@liv-inspiron:~$ pwgen 13 7
Eu7Teadiphaec giepahl3Oyaiy iecoo9Aetaib4 phaiChae6Eivi athoo3igee8Co
Iphu4ufeDeelo aesoYi2lie9he 

As mentioned in the comments, you can avoid reducing entropy by using the -s argument (i.e. generate more secure, completely random but hard to remember passwords):

geek@liv-inspiron:~$ pwgen -s 13 7
eAfycrPlM4cYv 4MRXmZmyIVNBp D8y71iqjG7Zq7 FQRHcserl4R8O yRCUtPtV3dsqV
0vJpp2h0OrgF1 QTp7MKtJyTrjz 

To generate random user names you can use gpw:

This package generates pronounceable passwords. It uses the statistics of three-letter combinations (trigraphs) taken from whatever dictionaries you feed it.

Generate 7 passwords (user names) of length 13:

geek@liv-inspiron:~$ gpw 7 13
sreepoidahsas
risadiestinge
ntodynesssine
deodstestress
natinglumperm
riasigentspir
enderiferback

My favorite way to do it is by using /dev/urandom together with tr to delete unwanted characters for instance to get only numbers and letters:

head /dev/urandom | tr -dc A-Za-z0-9 | head -c 13

Here is how, I do it. It generates 10 characters random string. You can optimize it by replacing the "fold", with other string processing tools.

cat /dev/urandom | tr -dc 'a-zA-Z0-9' | fold -w 10 | head -n 1

@Brandin explained in a comment to another answer how to get at most 100 bytes from /dev/urandom using head -c 100. Another way to do this is with dd:

tr -dc A-Za-z0-9 < /dev/urandom | dd bs=100 count=1 2>/dev/null

The 2>/dev/null at the end of the dd command is to suppress the "... records in / ... records out" output.

I'm not aware of any substantial advantages/disadvantages between these two methods.

I had an issue with both methods of tr complaining about the input. I thought this was because it didn't like receiving binary input, and hence suggested first filtering /dev/random with iconv -c -t US. However, Gilles suggested a different diagnosis and solution, which works for me:

LC_ALL=C tr -dc A-Za-z0-9 </dev/urandom | dd bs=100 count=1 2>/dev/null

Depending on the level of randomness you want, you could simply go with bash's (also zsh and ksh, possibly others) builtin $RANDOM variable:

$ echo $RANDOM | tr '[0-9]' '[a-zA-Z]'
bfeci
$ echo $RANDOM | tr '[0-9]' '[a-zA-Z]'
cijjj

The methods reading directly from /dev/urandom are far simpler, but for the sake of completion, you could also use $RANDOM:

echo $(for((i=1;i<=13;i++)); do printf '%s' "${RANDOM:0:1}"; done) | tr '[0-9]' '[a-zA-Z]'

A super easy and simple (probably more simple than you're looking for) way to achieve this would be to generate a random number in a given range, convert that number to its equivalent ASCII character, and append it to the end of a string.

Here's a basic example in Python:

import random # import random library  
passFile = open("passwords.txt", 'w') # create file passwords.txt
passNum = int(input("Number of passwords: ")) # get number of  passwords
passLength = int(input("Password length: ")) # get length of passwords  
for i in range(passNum):
    password = "" # reset password
    for i in range(passLength):
        num = random.randint(65, 122) # get random number
        while num in range(91, 97): # Don't include punctuation
            num = random.randint(65, 122)
        password += chr(num) # add character to current password 
    passFile.write(password + "\n") # add current password to file  
passFile.close() # close file

EDIT: added comments, and added code to generate multiple passwords and write them to a file