Prerequisites:

sudo apt install base58
sudo apt install xxd
sudo apt install qrencode

Usage:

Make the script executable for the current user: chmod u+x brainwallet.sh

Run it: ./brainwallet.sh

Execution results:

1. A file data.txt which includes the sensitive data (keys etc.) of the wallet.
2. A file legacy_address.png which displays a QR code for the wallet's legacy (P2PKH) address.
3. A file segwit_address.png which displays a QR code for the wallet's segwit (P2WPKH-P2SH) address.

data.txt file format:

Entropy: apogio created a brainwallet generator using bash
PK: 913fc1abf77ae447c662cbd14a0803e519df65f8c40b3bcb20a911f0f31091dc
WIF: L264Cp6WU73fzmQCvJ8Te2EazXTr3A17yAC13NQDQBwQvyUAaiG3
Public Key: 04582ed090da2d4e4fda943923910a0720391a9903fa5259aa9d50cf3710ed40bbc6ce378a86ab86f2b2d6635e8797e9c4fa2021eff4f57942c22395d7ad1afe83
Compressed Public Key: 03582ed090da2d4e4fda943923910a0720391a9903fa5259aa9d50cf3710ed40bb
HASH160: 8ef81d4f19a7f284e68b32dd58931c6817ceb275
Legacy Address: 1E2xBY8kVhGgNZuRK8RwbvimpeW1E6DPat
Segwit Address: 3MpZWJr5ct3Y4zEeSmbA1R17vj2RrRhfNw

Some notes:

1. Make sure to use single quotes to include the phrase. Otherwise Bash will think that each word is a separate command line argument and the results will be totally unexpected.
2. Make sure to remember that in brain wallets, every character matters. Thus, 'I am the best' is different from 'i am the best', or from 'I am the best '.