Newbie question - Recover associated hex string of SHA1 hash

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That's exactly what hashcat is for.

root@et:~/oclHashcat-1.36# ./oclHashcat64.bin -m 100 054b71ba03578aa5b476b3d3af663b3180cecc59 -1 0123456789ABCDEF ?1?1?1?1?1?1?1?1 -w 3 -a 3
oclHashcat v1.36 starting...

Device #1: Tahiti, 3022MB, 1000Mhz, 32MCU
Device #2: Tahiti, 3022MB, 1000Mhz, 32MCU
Device #3: Tahiti, 3022MB, 1000Mhz, 32MCU

Hashes: 1 hashes; 1 unique digests, 1 unique salts
Bitmaps: 16 bits, 65536 entries, 0x0000ffff mask, 262144 bytes, 5/13 rotates
Applicable Optimizers:
* Zero-Byte
* Precompute-Init
* Precompute-Merkle-Demgard
* Early-Skip
* Not-Salted
* Not-Iterated
* Single-Hash
* Single-Salt
* Brute-Force
* Scalar-Mode
* Raw-Hash
Watchdog: Temperature abort trigger set to 90c
Watchdog: Temperature retain trigger set to 80c
Device #1: Kernel ./kernels/4098/m00100_a3.Tahiti_1573.4_1573.4 (VM)_1429260956.kernel (227716 bytes)
Device #1: Kernel ./kernels/4098/markov_be_v1.Tahiti_1573.4_1573.4 (VM)_1429260956.kernel (35108 bytes)
Device #2: Kernel ./kernels/4098/m00100_a3.Tahiti_1573.4_1573.4 (VM)_1429260956.kernel (227716 bytes)
Device #2: Kernel ./kernels/4098/markov_be_v1.Tahiti_1573.4_1573.4 (VM)_1429260956.kernel (35108 bytes)
Device #3: Kernel ./kernels/4098/m00100_a3.Tahiti_1573.4_1573.4 (VM)_1429260956.kernel (227716 bytes)
Device #3: Kernel ./kernels/4098/markov_be_v1.Tahiti_1573.4_1573.4 (VM)_1429260956.kernel (35108 bytes)


ATTENTION!
The wordlist or mask you are using is too small.
Therefore, oclHashcat is unable to utilize the full parallelization power of your GPU(s).
The cracking speed will drop.
Workaround: https://hashcat.net/forum/thread-4161.html


INFO: approaching final keyspace, workload adjusted

054b71ba03578aa5b476b3d3af663b3180cecc59:AB0EFFB8

Session.Name...: oclHashcat
Status.........: Cracked
Input.Mode.....: Mask (?1?1?1?1?1?1?1?1) [8]
Hash.Target....: 054b71ba03578aa5b476b3d3af663b3180cecc59
Hash.Type......: SHA1
Time.Started...: 0 secs
Speed.GPU.#1...: 2960.3 MH/s
Speed.GPU.#2...: 2949.9 MH/s
Speed.GPU.#3...: 2990.1 MH/s
Speed.GPU.#*...: 8900.3 MH/s
Recovered......: 1/1 (100.00%) Digests, 1/1 (100.00%) Salts
Progress.......: 0/4294967296 (0.00%)
Rejected.......: 0/0 (0.00%)
Restore.Point..: 0/1048576 (0.00%)
HWMon.GPU.#1...: 62% Util, 46c Temp, 41% Fan
HWMon.GPU.#2...: 44% Util, 47c Temp, 43% Fan
HWMon.GPU.#3...: 61% Util, 47c Temp, 43% Fan

Started: Fri Apr 17 11:04:05 2015
Stopped: Fri Apr 17 11:04:06 2015

This is for upper case. If you want lower case, change A-F to a-f in the -1 Parameter