Encoding and Decoding Utilities

Base64 Encode/Decode

String

# Encode
echo -n 'hello world' | base64

# Decode (macOS)
echo 'aGVsbG8gd29ybGQ=' | base64 -D

# Decode (Linux)
echo 'aGVsbG8gd29ybGQ=' | base64 -d

File

# Encode a file
base64 < input.bin > output.b64

# Decode to file (macOS)
base64 -D < output.b64 > restored.bin

# Decode to file (Linux)
base64 -d < output.b64 > restored.bin

Stdin pipe

curl -s https://example.com/image.png | base64

macOS vs Linux

Portable: base64 --decode 2>/dev/null || base64 -D

URL Encoding/Decoding

# Python 3
python3 -c "import urllib.parse; print(urllib.parse.quote('hello world&foo=bar'))"
python3 -c "import urllib.parse; print(urllib.parse.unquote('hello%20world%26foo%3Dbar'))"

# Node.js
node -e "console.log(encodeURIComponent('hello world&foo=bar'))"
node -e "console.log(decodeURIComponent('hello%20world%26foo%3Dbar'))"

Hex Encode/Decode

# String to hex
echo -n 'hello' | xxd -p
# Output: 68656c6c6f

# Hex to string
echo '68656c6c6f' | xxd -r -p
# Output: hello

# File to hex dump
xxd input.bin

# Compact hex (no formatting)
xxd -p input.bin

# Using od
echo -n 'hello' | od -A n -t x1 | tr -d ' \n'

# Hex to bytes with printf
printf '\x68\x65\x6c\x6c\x6f'

Hashing

String hashing

# MD5
echo -n 'hello' | md5sum          # Linux
echo -n 'hello' | md5             # macOS

# SHA-1
echo -n 'hello' | sha1sum         # Linux
echo -n 'hello' | shasum -a 1     # macOS

# SHA-256
echo -n 'hello' | sha256sum       # Linux
echo -n 'hello' | shasum -a 256   # macOS

# SHA-512
echo -n 'hello' | sha512sum       # Linux
echo -n 'hello' | shasum -a 512   # macOS

Use echo -n to avoid hashing the trailing newline.

File hashing

sha256sum myfile.tar.gz           # Linux
shasum -a 256 myfile.tar.gz       # macOS

OpenSSL (cross-platform)

openssl dgst -sha256 myfile.tar.gz
echo -n 'hello' | openssl dgst -sha256

Verify File Integrity with Checksums

# Generate checksum file
sha256sum release.tar.gz > release.sha256

# Verify (Linux)
sha256sum -c release.sha256

# Verify (macOS)
shasum -a 256 -c release.sha256

# Quick inline comparison
EXPECTED="e3b0c44298fc1c149afbf4c8996fb924..."
ACTUAL=$(sha256sum downloaded.tar.gz | awk '{print $1}')
[ "$EXPECTED" = "$ACTUAL" ] && echo "OK" || echo "MISMATCH"

JWT Decode Without a Library

A JWT has three dot-separated base64url segments: header, payload, signature.

JWT="eyJhbGciOiJIUzI1NiJ9.eyJzdWIiOiIxMjM0NTY3ODkwIn0.dozjgNryP4J3jVmNHl0w5N_XgL0n3I9PlFUP0THsR8U"

# Decode header
echo "$JWT" | cut -d. -f1 | tr '_-' '/+' | base64 -d 2>/dev/null || \
echo "$JWT" | cut -d. -f1 | tr '_-' '/+' | base64 -D

# Decode payload
echo "$JWT" | cut -d. -f2 | tr '_-' '/+' | base64 -d 2>/dev/null || \
echo "$JWT" | cut -d. -f2 | tr '_-' '/+' | base64 -D

Base64url uses - and _ instead of + and /. The tr call converts back. Add padding if needed:

decode_b64url() {
  local d="$1" pad=$(( 4 - ${#1} % 4 ))
  [ "$pad" -lt 4 ] && d="${d}$(printf '%0.s=' $(seq 1 $pad))"
  echo "$d" | tr '_-' '/+' | base64 -d 2>/dev/null || echo "$d" | tr '_-' '/+' | base64 -D
}
decode_b64url "$(echo "$JWT" | cut -d. -f2)"

Generate Random Strings

# Alphanumeric, 32 chars
cat /dev/urandom | LC_ALL=C tr -dc 'a-zA-Z0-9' | head -c 32

# Hex string, 32 chars
openssl rand -hex 16

# Base64 string
openssl rand -base64 24

# URL-safe random string
openssl rand -base64 32 | tr '+/' '-_' | tr -d '='

# UUID-like
cat /proc/sys/kernel/random/uuid 2>/dev/null || uuidgen

HTML Entity Encode/Decode

# Encode (Python)
python3 -c "import html; print(html.escape('<div class=\"x\">&</div>'))"
# Output: &lt;div class=&quot;x&quot;&gt;&amp;&lt;/div&gt;

# Decode (Python)
python3 -c "import html; print(html.unescape('&lt;div&gt;hello&lt;/div&gt;'))"

# Decode (Perl)
perl -MHTML::Entities -e 'print decode_entities("&lt;p&gt;hello&lt;/p&gt;")'

Unicode Escape/Unescape

# Unescape \uXXXX sequences (Python)
python3 -c "print('\\u0048\\u0065\\u006c\\u006c\\u006f')"

# Escape string to \uXXXX
python3 -c "print(''.join(f'\\\\u{ord(c):04x}' for c in 'Hello'))"

# Unescape JSON unicode (jq)
echo '"\\u0048\\u0065\\u006c\\u006c\\u006f"' | jq -r .

# UTF-8 hex bytes to character
printf '\xc3\xa9'    # prints: e with accent (é)

# Character to UTF-8 hex
echo -n 'é' | xxd -p

Common Patterns

Encode a file for JSON embedding

BASE64=$(base64 < image.png | tr -d '\n')
echo "{\"image\": \"data:image/png;base64,${BASE64}\"}"

Decode a base64 field from an API response

curl -s https://api.example.com/data | jq -r '.content' | base64 -d

Verify a download checksum

curl -LO https://example.com/release.tar.gz
curl -sL https://example.com/release.sha256 | sha256sum -c -

Generate an API key

openssl rand -base64 32 | tr -d '\n'
openssl rand -hex 32

Encode credentials for Basic Auth

echo -n 'user:password' | base64
# Use in header: Authorization: Basic dXNlcjpwYXNzd29yZA==

Quick Reference