cat Skill

Use caution when running cat or heredocs (<<EOF) in automated environments or agentic runtimes, as missing or truncated EOF delimiters can cause persistent shell hangs.

When to Use

• Using cat to write files.
• Using heredoc (<<EOF) to pass multiline strings.
• Reading or streaming file contents.

When Not to Use

• For modifying or appending to files when specific, robust file-writing tools (like the Edit or Write tools) are available.
• When printing massive, multi-gigabyte log files to standard out, which will completely crash the terminal or context window.
• In automated scripts that write complex YAML or JSON where a missing quote or delimiter can silently break the entire file structure.

Common Pitfalls

• Heredoc Hangs: Using cat <<EOF inside a script without quoting the EOF delimiter or failing to print the delimiter exactly, causing the shell to hang indefinitely waiting for input.
• Overwriting Files: Accidentally using > instead of >> and instantly destroying the contents of a critical configuration file.
• Binary Garbling: Running cat on a compiled binary or PDF, dumping control characters into the terminal that corrupt the session's display state.

Avoid Heredocs for Long Strings

Never use heredocs (cat <<EOF > file.md or command --body "$(cat <<EOF)") for long strings or generated code. If the output gets truncated before the EOF delimiter is printed, the cat command will hang forever waiting for input, which forces the runtime to cancel the job.

Bad Pattern (Hangs easily)

# DO NOT DO THIS
gh issue comment 123 --body "$(cat <<'EOF'
Very long text...
EOF
)"

# DO NOT DO THIS
cat <<'EOF' > /tmp/file.md
Very long text...
EOF

Good Pattern (Safe alternatives)

Alternative 1: Use native file flags

Use your agentic file-writing tools (like the Write or Edit tool) to create a temporary file first, and then pass that file using commands' native file flags (like --body-file).

# First, use your Write tool to save content to /tmp/comment.md
gh issue comment 123 --body-file /tmp/comment.md

Alternative 2: mktemp and quoted writes

COMMENT_FILE=$(mktemp)
# Use your Write tool to write to $COMMENT_FILE, or use safe quoted echoing if short
echo "Short text" > "$COMMENT_FILE"
gh issue comment 123 --body-file "$COMMENT_FILE"
rm "$COMMENT_FILE" # Always clean up temporary files after use

Always Use Timeouts with Stdin and Heredocs

When you absolutely must execute cat reading from stdin or a heredoc in a shell command, enforce a timeout so that if it hangs, it will fail fast instead of locking up the workflow. Note that simple file reads like cat somefile.txt do not typically require a timeout.

timeout 10s cat <<'EOF'
Long text...
EOF

Related Skills

• robust-commands: You MUST load this skill when executing commands requiring resilient error recovery or fallbacks.