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# Bash Style and Structure
## Shebang
Use `#!/usr/bin/env bash` for portability across Unix-like systems (not all have Bash at `/bin/bash`).
```bash
#!/usr/bin/env bash
```
## Strict Mode Header
Start every script with a strict-mode header. Combine `set -e`, `set -u`, and `set -o pipefail` so the script aborts on unexpected errors, unset variables, and pipeline failures.
```bash
set -euo pipefail
```
Some projects also add `set -f` (disable pathname expansion). Choose the combination appropriate for your script and apply it consistently.
If a script sources configuration files that may leave variables unset, initialize those variables to empty strings **before** enabling `set -u`:
```bash
test -z "$CONFIG_FILE_PATH" && CONFIG_FILE_PATH=''
test -z "$LOG_VERBOSE" && LOG_VERBOSE=''
set -euo pipefail
```
Alternatively, access potentially-unset optional variables with `${VAR:-}` or `${VAR:-default}`:
```bash
if [ -f "${HTML_JS_SCRIPT:-}" ]; then
cp "$HTML_JS_SCRIPT" "$dest"
fi
```
## Command Substitution
Always use `$(...)` instead of backticks. It nests cleanly, is easier to read, and avoids quoting issues.
```bash
# Good
date_stamp=$(date +%Y%m%d)
# Bad (backticks)
date_stamp=`date +%Y%m%d`
```
## Indentation and Line Length
- **Indentation**: Use soft-tabs (spaces), not tabs. Two or four spaces are both acceptable; pick one and be consistent within the project.
- **Line length**: Limit to 80 characters where practical. It encourages smaller functions and is friendlier on small screens.
## Breaking Long Pipelines
Break long pipelines with a backslash before the pipe and a leading pipe on continuation lines. The leading pipe is a visual eye-catcher.
```bash
# Good
command1 \
| command2 \
| command3 \
| command4
```
## Quoting Variables
- Quote variables when the value comes from external input, may contain whitespace, or is unknown.
- In small scripts with simple bare-word values, unquoted variables are acceptable for readability.
- In large or shared scripts, quote consistently to avoid accidents and keep ShellCheck happy.
- Use `${var}` braces only when required (adjacent text, arrays) or when they improve clarity.
```bash
# Unknown/external input: quote
echo "${greeting} ${name}!"
# Simple bare words: optional but be consistent
local -r greeting=Hello
local -r name=Paul
echo "$greeting $name!"
# Braces required
echo "foo${FOO}baz"
```
## Boolean Style
Bash has no native boolean. Use the string literals `yes` and `no`.
```bash
declare -r SUGAR_FREE=yes
declare -r I_NEED_THE_BUZZ=no
```
## Multi-line Comments
Use a here-doc redirected to the null command for multi-line comments.
```bash
: <<COMMENT
This is a multi-line comment.
COMMENT
```
## `declare` Modifiers
Use `declare` modifiers for clarity and safety:
- `-r` for read-only (constants)
- `-i` for integers
- `-a` for indexed arrays
- `-A` for associative arrays (Bash 4+)
```bash
declare -r MAX_RETRIES=3
declare -i counter=0
declare -a fruits=(apple banana cherry)
```
## `local -i` for Integer Variables
Declare integer locals with `local -i` so arithmetic is cleaner and safer.
```bash
some_function () {
local -i num_files=0
num_files=$(( num_files + 1 ))
# Alternatively:
(( num_files++ ))
}
```
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