# Shell shell-control-flow

## Conditionals

**Conditionals** run commands based on conditions. Use `if` to test exit codes or use `[[ ... ]]` for test expressions. Common operators: `-f` (file exists), `-z` (string empty), `-n` (string non-empty), `=` or `==` (string equal), `-eq` (number equal), `-lt` (less than), `-gt` (greater than).

```bash
if [[ -f "$file" ]]; then
  echo "File exists"
elif [[ $count -gt 10 ]]; then
  echo "Count is greater than 10"
else
  echo "Neither condition true"
fi
```

Use `[[ ]]` instead of `[ ]` in Bash—it's more robust and handles quoting better. Use `&&` (AND) and `||` (OR) for one-liners: `cmd1 && cmd2` (run cmd2 if cmd1 succeeds), `cmd1 || cmd2` (run cmd2 if cmd1 fails).

## Loops

**Loops** repeat commands. Use `for` to iterate, `while` to repeat while a condition is true, `until` to repeat until a condition is true.

```bash
for i in 1 2 3; do
  echo "Number: $i"
done

for file in *.txt; do
  wc -l "$file"
done

while [[ $count -lt 10 ]]; do
  echo $count
  count=$((count + 1))
done
```

Loop control: `break` exits the loop, `continue` skips to the next iteration.

## Exit codes and error handling

**Exit codes** (0-255) indicate success or failure. Zero means success, non-zero means failure. Access the last exit code with `$?`. Use `set -e` to exit immediately on error, `set -u` to error on undefined variables, `set -o pipefail` to fail if any command in a pipe fails.

```bash
$ command
$ echo $?                       # prints exit code
$ command || echo "Failed"      # run second command if first fails
$ set -e                        # exit on error
$ grep "pattern" file || true   # don't exit even if grep fails
```

Always check exit codes and handle errors in scripts. Use `set -e -u -o pipefail` at the top of production scripts.