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# Advanced Bash Patterns
## Avoid `eval`
Avoid `eval`. Prefer sourcing files or process substitution to generate and source code dynamically.
```bash
# Good: source a file of declarations
source vars.source.sh
# Good: source generated code from a command
source <(./vars.sh)
```
## Namerefs (`declare -n`)
Use namerefs (Bash 4.3+) for cleaner indirection instead of `eval`.
```bash
set_value() {
local -n ref="$1"
ref="$2"
}
set_value my_var hello
```
You can also construct the target name dynamically:
```bash
make_var() {
local idx=$1; shift
local name="slot_$idx"
printf -v "$name" '%s' "$*"
}
get_var() {
local idx=$1
local -n ref="slot_$idx"
printf '%s\n' "$ref"
}
```
## Background-Job Throttling
When spawning parallel background jobs, cap them to the number of CPU cores and use `wait -n` to pause only until any slot frees up. This avoids process explosions.
```bash
local -r max_jobs=$(( $(nproc 2>/dev/null || echo 4) ))
for item in ...; do
while (( $(jobs -rp | wc -l) >= max_jobs )); do
wait -n
done
do_work "$item" &
done
wait
```
## Build Commands Dynamically with Arrays
When constructing commands conditionally, build an array to avoid word-splitting and empty-argument bugs.
```bash
local -a cmd=("$SOURCE_HIGHLIGHT" "--src-lang=$lang")
if [ -n "$SOURCE_HIGHLIGHT_CSS" ]; then
cmd+=("--style-css-file=$SOURCE_HIGHLIGHT_CSS")
fi
"${cmd[@]}" <<< "$text"
```
## Atomic / Safe File Overwrite
Write to a temporary file, compare with `diff -q`, and `mv` only if content actually changed. This preserves mtime (helpful for downstream skip-logic) and avoids leaving partial files on interrupt.
```bash
safe_overwrite () {
local -r tmp="$1"; shift
local -r dest="$1"; shift
if [[ -f "$dest" ]] && diff -q "$tmp" "$dest" >/dev/null 2>&1; then
rm "$tmp"
else
mv "$tmp" "$dest"
fi
}
# Usage:
echo 'new content' > "$dest.tmp"
safe_overwrite "$dest.tmp" "$dest"
```
## Self-Testing and ShellCheck
Include an `assert` module with helpers like `assert::equals`, `assert::contains`, `assert::not_empty`, and `assert::matches`. Run them as part of a `--test` target.
Also run `shellcheck` against your scripts as part of the test suite:
```bash
assert::shellcheck () {
shellcheck \
--norc \
--external-sources \
--check-sourced \
--exclude=SC2155,SC2010,SC2154,SC1090,SC2012,SC2016,SC1091 \
./"$0"
}
```
If ShellCheck flags are unavoidable, document the specific `--exclude` reasons in comments. Common gemtexter exclusions:
| Code | Meaning | Why it's excluded in gemtexter |
|------|---------|------------------------------|
| SC2155 | Declare and assign separately to avoid masking return values | `local -r var=$(cmd)` is idiomatic and the return value is rarely needed |
| SC2010 | Don't use `ls \| grep`; use `find` or globs | Interactive/listing tasks where `ls \| grep` is intentional and readable |
| SC2154 | Variable referenced but not assigned | Variables come from sourced config files or the environment |
| SC1090 | Can't follow non-constant source (e.g. `source "$var"`) | Template/config sourcing with dynamic paths is by design |
| SC2012 | Use `find` instead of `ls` to parse outputs | Legacy listing patterns where line splitting is controlled |
| SC2016 | Expressions don't expand in single quotes | Here-doc strings or template blocks intentionally contain literal `$vars` |
| SC1091 | Not following sourced file (file not on disk or outside project) | External configs or generated files not available at lint time |
## Random Numbers
Use the special `$RANDOM` variable for quick pseudo-random integers.
```bash
declare -i delay=$(( RANDOM % 60 ))
sleep $delay
```
## Environment Variables for Arguments
Pass required arguments via environment variables with `${VAR:?message}` for mandatory checks.
```bash
#!/usr/bin/env bash
declare -r USER=${USER:?Missing the username}
declare -r PASS=${PASS:?Missing the secret password for $USER}
```
## Atomic Locking with `mkdir`
Portable advisory locks can be emulated with `mkdir` because it is atomic:
```bash
lockdir=/tmp/myjob.lock
if mkdir "$lockdir" 2>/dev/null; then
trap 'rmdir "$lockdir"' EXIT INT TERM
# critical section
do_work
else
echo "Another instance is running" >&2
exit 1
fi
```
## Smarter globs and faster find-exec
- Enable extended globs when useful: `shopt -s extglob`; then patterns like `!(tmp|cache)` work.
- Use `-exec ... {} +` to batch many paths in fewer process invocations:
```bash
find . -name '*.log' -exec gzip -9 {} +
```
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