Shell 脚本进阶：循环、函数与健壮性

for i in 1 2 3; do echo "round $i"; done      # 遍历字面列表
for f in *.log; do gzip "$f"; done            # 遍历通配符匹配的文件
for i in $(seq 1 5); do ...; done             # 遍历命令输出
for ((i=0; i<5; i++)); do ...; done           # C 风格（bash 专属）

mkdir -p reports
for name in jan feb mar apr; do
  echo "sales data of $name" > reports/$name.csv
done
ls reports
 
echo ''
echo '$ 批量重命名：.csv 改成 .txt（basename 去掉路径和后缀）'
for f in reports/*.csv; do
  BASE=$(basename "$f" .csv)
  mv "$f" "reports/$BASE.txt"
done
ls reports
 
echo ''
echo '$ seq 生成数字序列 + 格式化：'
for i in $(seq 1 3); do
  echo "worker-$i starting"
done

while [ 条件 ]; do ...; done                  # 条件循环
while read -r line; do ...; done < file      # 逐行读文件的标准姿势

cat > hosts.txt << 'EOF'
web-01 10.0.0.11
web-02 10.0.0.12
db-01 10.0.0.21
EOF
 
echo '$ 逐行读并拆字段（read 可以一次读进多个变量）：'
while read -r NAME IP; do
  echo "checking $NAME at $IP ... ok"
done < hosts.txt
 
echo ''
echo '$ 计数循环：'
COUNT=0
while [ $COUNT -lt 3 ]; do
  COUNT=$((COUNT + 1))
  echo "retry #$COUNT"
done
 
echo ''
echo '$ 经典重试模式：失败就等待重试，最多 3 次'
ATTEMPT=1
until [ -f /tmp/ready.flag ] || [ $ATTEMPT -gt 3 ]; do
  echo "waiting... attempt $ATTEMPT"
  ATTEMPT=$((ATTEMPT + 1))
  test $ATTEMPT -eq 3 && touch /tmp/ready.flag
done
echo 'ready!'

log() {                        # 定义（也可写 function log）
  echo "[$(date +%T)] $1"      # $1 是函数的第一个参数（不是脚本的）
}
log "server started"           # 调用：像命令一样，不写括号

#!/bin/bash
log() {
  local LEVEL=$1
  local MSG=$2
  echo "[$(date +%T)] [$LEVEL] $MSG"
}
 
# 用退出码表达成败，用 echo 传回数据
disk_usage() {
  local DIR=$1
  if [ ! -d "$DIR" ]; then
    return 1
  fi
  du -sk "$DIR" | awk '{print $1}'
}
 
log INFO 'script started'
 
mkdir -p data && truncate -s 300K data/blob.bin
USAGE=$(disk_usage data)
if [ $? -eq 0 ]; then
  log INFO "data 目录占用 $USAGE KB"
else
  log ERROR 'data 目录不存在'
fi
 
disk_usage /nope || log WARN '/nope 检查失败（演示 return 1）'
log INFO 'script finished'

#!/bin/bash
# 常用默认值语法（参数展开）：
NAME=${1:-guest}        # $1 为空时用 guest
DIR=${DEPLOY_DIR:?未设置 DEPLOY_DIR}    # 为空直接报错退出
echo "hello $NAME"

cat > args.sh << 'EOF'
#!/bin/bash
echo "脚本名: $0, 参数个数: $#"
echo "第一个参数: $1"
for ARG in "$@"; do
  echo "  got: $ARG"
done
FIRST=$1
shift
echo "shift 后剩余 $# 个: $*"
echo "first 已存为: $FIRST"
EOF
chmod +x args.sh
 
echo '$ ./args.sh apple "big banana" cherry'
./args.sh apple 'big banana' cherry

case $1 in
  start)   echo 'starting...' ;;
  stop)    echo 'stopping...' ;;
  restart) echo 'restarting...' ;;
  status)  echo 'running' ;;
  *)       echo "usage: $0 {start|stop|restart|status}"; exit 1 ;;
esac

cat > unsafe.sh << 'EOF'
#!/bin/bash
TARGET_DIR=/data/output
rm -rf "$TAGRET_DIR/cache"   # 变量名拼错了！$TAGRET_DIR 是空串
echo "unsafe.sh 跑完了，rm 实际执行的是: rm -rf /cache（好险）"
EOF
 
cat > safe.sh << 'EOF'
#!/bin/bash
set -euo pipefail
TARGET_DIR=/data/output
rm -rf "$TAGRET_DIR/cache"
echo '这行永远不会执行'
EOF
chmod +x unsafe.sh safe.sh
 
echo '$ ./unsafe.sh   # 拼错变量静默通过，险些酿祸：'
./unsafe.sh
 
echo ''
echo '$ ./safe.sh     # set -u 当场抓获：'
./safe.sh
echo "exit code = $?"

#!/bin/bash
set -euo pipefail
TMP=$(mktemp -d)
cleanup() { rm -rf "$TMP"; echo 'cleaned up'; }
trap cleanup EXIT        # 无论正常结束、报错还是被信号打断，都执行 cleanup
 
# ...使用 $TMP 干活...