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+# Bash I/O, Pipelines, and Data Processing
+
+## Built-ins vs External Commands
+
+- Prefer Bash built-ins for light text processing and arithmetic.
+- Use external commands (`sed`, `awk`, `grep`, `cut`, `tr`, `bc`) for heavy or complex text processing.
+
+```bash
+# Prefer built-in
+addition=$(( X + Y ))
+
+# Prefer external for sophisticated transforms
+substitution="$(echo "$string" | sed -e 's/^foo/bar/')"
+```
+
+## Process Substitution
+
+Use `<(command)` and `>(command)` for treating command output as a file.
+
+```bash
+diff -u <(sort file1) <(sort file2)
+tar cjf >(bzip2 -c > file.tar.bz2) foo
+```
+
+## `while read` with Process Substitution
+
+When iterating over command output and modifying variables in the parent shell, use process substitution as the input source rather than piping into `while`. A pipe creates a subshell, so variable changes are lost.
+
+```bash
+# Good: changes to $count survive
+local -i count=0
+while IFS='' read -r line; do
+    (( count++ ))
+done < <(command)
+
+# Bad: $count is lost because the while runs in a subshell
+local -i count=0
+command | while IFS='' read -r line; do
+    (( count++ ))
+done
+```
+
+### `IFS='' read -r line` for exact line preservation
+
+Use `IFS='' read -r line` when reading lines you intend to preserve exactly, including leading and trailing whitespace. Without `IFS=''`, leading/trailing whitespace is stripped; without `-r`, backslashes are interpreted.
+
+```bash
+while IFS='' read -r line; do
+    echo "$line"
+done < file.txt
+```
+
+## Here-Documents and Here-Strings
+
+Use here-docs and here-strings for multi-line or inline input.
+
+```bash
+# Here-document with variable interpolation
+cat <<EOF
+Hello $USER
+EOF
+
+# Literal here-document (no interpolation)
+cat <<'EOF'
+$USER is not expanded
+EOF
+
+# Here-string
+if grep -q foo <<< "$VAR"; then
+    echo match
+fi
+```
+
+Use `<<-EOF` to strip leading tabs from the body.
+
+### Prefer here-strings over `echo | command`
+
+For single-line input, prefer `command <<< "$var"` instead of `echo "$var" | command`. It avoids an extra pipe, subshell, and process spawn.
+
+```bash
+# Good
+tr '[:upper:]' '[:lower:]' <<< "$text"
+
+# Avoid
+echo "$text" | tr '[:upper:]' '[:lower:]'
+```
+
+## Input Placeholders and Redirection
+
+- `-` as stdin/stdout placeholder for commands like `tar` and `cat`.
+- Redirect via file descriptors explicitly (`2>/dev/null`, `1>&2`).
+- Remember redirection order matters.
+
+```bash
+echo Foo 2>/dev/null 1>&2   # suppresses everything
+```
+
+## `/dev/tcp` Networking
+
+Bash supports TCP via pseudo-files:
+
+```bash
+cat < /dev/tcp/time.nist.gov/13
+exec 5<>/dev/tcp/google.de/80
+```
+
+## List Processing: Pipes over Arrays
+
+For simple list processing, prefer pipelines over arrays. Pass data through stdout to the next stage and use stderr for logging.
+
+```bash
+main () {
+    filter_lines |
+        process_lines |
+        postprocess_lines |
+        generate_report
+}
+```
+
+## Reading Files and Arrays
+
+- **Read a whole file into a variable** without spawning `cat`: `cfg=$(<config.ini)`
+- **Read lines into an array** safely with `mapfile` (aka `readarray`): `mapfile -t lines < file`
+- **Assign formatted strings without a subshell** using `printf -v`: `printf -v msg 'Hello %s' "$USER"`
+
+## Safe xargs with NULs
+
+Avoid breaking on spaces/newlines by pairing `find -print0` with `xargs -0`:
+
+```bash
+find . -type f -name '*.log' -print0 | xargs -0 rm -f
+```