How can I convert a SHA256 hash into a Bitcoin base58 private key? How can I take the output of sha256sum

sudofox@ubuntu:~$ echo -n 'Hi guys!' | sha256sum
7542fb6685f9fd8f37d56faf62f0bb4563684a51539e4b26f0840db361e0027c  -


and turn it into a Base58Check encoded private key?

I need to be able to do this with a list of SHA256 keys. Is there a script I can download, or must I spend a few weeks making it just so I can make my temporary addresses?

In response to first comment: 
No, I must input a string, like "fluttershy", for example, into sha256sum and then I must encode the output of sha256sum into a base58check private key.

string -->sha256sum --> some automagic process, which is the purpose of this question --> bitcoin private key.

Update: For anyone wanting to use Grondilu's Bitcoin Bash Tools: You need to source the bitcoin.sh file after extracting it into your directory to use the functions.

source ./bitcoin.sh

Update 2 (Aug 2017): Rewrote script to go from step 1 to WIF in one go. You can just add the declaration of base58 and the encodeBase58 functions from bitcoin.sh if you want to skip the rest of the lib.

#!/bin/bash
#Tool to convert bitcoin privkeys into WIF keys
# by sudofox

source ./bitcoin.sh

KEY=$1 # first arg

# add 0x80 to beginning
EXTENDEDKEY=$(echo 80$KEY)
FIRSTHASH=$(echo -n "$EXTENDEDKEY" |xxd -r -p |sha256sum -b|awk '{print $1}')
SECONDHASH=$(echo -n "$FIRSTHASH" |xxd -r -p |sha256sum -b|awk '{print $1}')
CHECKSUM=$(echo $SECONDHASH|cut -c1-8)
FINAL=$(encodeBase58 $EXTENDEDKEY$CHECKSUM)
echo $FINAL


The usage would be 

./sha256_to_privkey.sh key

where in my example (see Stephen Gornick's answer), key would be

807542FB6685F9FD8F37D56FAF62F0BB4563684A51539E4B26F0840DB361E0027CCD5C4A8E


Do the same thing for the various functions, modifying as needed. Hope this helps anyone struggling to use the tools.
What you are requesting is described as computing the Wallet Import Format for that private key:


http://en.bitcoin.it/wiki/Wallet_import_format


Using your example:

1.) Take a private key (Below is the HEX representation of binary value)


  7542FB6685F9FD8F37D56FAF62F0BB4563684A51539E4B26F0840DB361E0027C


2.) Add a 0x80 byte in front of it


  807542FB6685F9FD8F37D56FAF62F0BB4563684A51539E4B26F0840DB361E0027C


3.) Perform SHA-256 hash on the extended key

$ echo -n '807542FB6685F9FD8F37D56FAF62F0BB4563684A51539E4B26F0840DB361E0027C' | xxd -r -p | sha256sum -b



  7DE4708EB23AB611371BB778FC0C8BDE80394AB2D8704D7129FB5771E2F1730D


4.) Perform SHA-256 hash on result of SHA-256 hash

$ echo -n '7DE4708EB23AB611371BB778FC0C8BDE80394AB2D8704D7129FB5771E2F1730D' | xxd -r -p | sha256sum -b



  CD5C4A8E03DFBB0E3AA021C2D74A9EAA43CE4C9CB1B20FC88729A7A5834141CA


5.) Take the first 4 bytes of the second SHA-256 hash, this is the checksum


  CD5C4A8E


6.) Add the 4 checksum bytes from point 5 at the end of the extended key from point 2


  807542FB6685F9FD8F37D56FAF62F0BB4563684A51539E4B26F0840DB361E0027CCD5C4A8E


7.) Convert the result from a byte string into Base58 to get it into the Base58Check format. This is also known as the Wallet Import Format

(Converted from point 6 to base58 using https://bitcointools.appspot.com, or using encodeBase58 from https://github.com/grondilu/bitcoin-bash-tools)


  5JhvsapkHeHjy2FiUQYwXh1d74evuMd3rGcKGnifCdFR5G8e6nH