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curl: why your POST works in bash but not in a script

📅 2026-09-15 📊 beginner ["curl" "bash" "http" "apis"]

The Problem

The same curl -X POST command works perfectly when you type it in your terminal, but returns HTTP 400 (or a JSON parse error) when you paste it into a .sh script. Or worse: it works locally but fails in CI.

The Mistake

Embedding JSON inside single quotes (-d '{...}'). The shell treats the single quotes as literal delimiters, but the moment your JSON contains an apostrophe (a name like O'Brien, a comment with it's, or a SQL WHERE name = 'foo') the quote-pair breaks and the shell starts parsing the rest of the command as code.

The Fix

Use --data-raw with a here-doc, or pipe the body via stdin:

curl -X POST https://api.example.com/things \
  -H "Content-Type: application/json" \
  --data-binary @- <<'EOF'
{"name": "O'Brien", "tag": "it's a test"}
EOF

The <<'EOF' (with quotes around the delimiter) disables variable expansion inside the body, so $ characters and backticks stay literal.

Why It Works

The shell only does its quote-matching at parse time. Inside '...' the shell treats every byte as literal, but the first unescaped ' ends the string. JSON often contains apostrophes, so single-quoting is fragile. Heredocs with a quoted delimiter bypass shell parsing entirely.

Copy-Paste

# Read JSON body from a file (cleanest)
curl -X POST https://api.example.com/things \
  -H "Content-Type: application/json" \
  --data-binary @body.json

Next Steps

• For interactive testing, use curl ... | jq so you can see parsed JSON.
• For repeatable calls, save the full request as a .http file and use the REST Client extension in VS Code.
• For production code, use a real HTTP client (Python requests, Node fetch) instead of shelling out.

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