Shell Output Filtering for Token Efficiency

Rule

Always filter shell output with grep/head/tail when expecting large results (>1000 lines).

Context

When running shell commands that produce extensive output like logs, file listings, or data dumps.

Detection

Observable signals that filtering is needed:

• Command expected to produce 1000+ lines
• Reading entire log files without filtering
• Listing all files in large directories
• Dumping full JSON/XML without jq/xmllint
• Previous command dumped 10k+ tokens into context

Pattern

Filter output before it enters context:

# Wrong: Dump entire log (could be 10k+ lines)
cat large-log.log

# Correct: Filter to relevant sections
grep "ERROR" large-log.log | tail -50

# Wrong: List all files
ls -la /var/log/

# Correct: Filter by pattern or limit count
ls -la /var/log/*.error | head -20

# Wrong: Full JSON dump
cat huge-config.json

# Correct: Extract specific fields
cat huge-config.json | jq '.relevant.section'

Outcome

Following this pattern results in:

• Token efficiency: 1k tokens instead of 10k+
• Faster responses: Less content to process
• Focused analysis: Only relevant information
• Cost savings: Fewer input/output tokens

Examples:

• Full log: 50k lines, ~40k tokens → Filtered: 50 lines, ~400 tokens (99% reduction)
• Directory listing: 10k files, ~15k tokens → Filtered: 20 files, ~300 tokens (98% reduction)

Related

• Shell Command Chaining - Combining commands
• Shell Path Quoting - Proper path handling