# Bash 快速参考

*命令、脚本、管道、重定向、作业控制*

> Source: GNU Bash Manual (gnu.org/software/bash/manual) · MIT

## 基础

### echo 与导航

```
echo "Hello, World!"     # print text
pwd                      # print working directory
cd /path/to/dir          # change directory
cd ..                    # go up one level
cd ~                     # go to home directory
cd -                     # go to previous directory
```

### 列出与创建

```
ls                # list files
ls -la            # long format, show hidden
ls -lh            # human-readable sizes
mkdir mydir       # create directory
mkdir -p a/b/c    # create nested directories
```

### 复制、移动与删除

```
cp file.txt copy.txt      # copy file
cp -r dir/ backup/        # copy directory recursively
mv old.txt new.txt        # rename / move
rm file.txt               # delete file
rm -r dir/                # delete directory recursively
rm -rf dir/               # force delete (no prompt)
```

## 变量与展开

### 变量

```
name="Alice"              # assign (no spaces!)
echo "$name"              # variable expansion
echo "${name}_file"       # braces for clarity
readonly PI=3.14          # constant
unset name                # delete variable
```

### 特殊变量

| Command | Description |
|---------|-------------|
| `$0` | 脚本名称 |
| `$1 $2 ...` | 位置参数 |
| `$#` | 参数个数 |
| `$@` | 所有参数（各自独立） |
| `$*` | 所有参数（单个字符串） |
| `$?` | 上条命令的退出状态 |
| `$$` | 当前进程 ID |
| `$!` | 最后一个后台进程的 PID |

### 命令替换与算术

```
files=$(ls)               # capture output
today=$(date +%Y-%m-%d)   # command substitution
count=$((5 + 3))          # arithmetic: 8
echo $((10 / 3))          # integer division: 3
echo $((10 % 3))          # modulo: 1
```

### 字符串操作

| Command | Description |
|---------|-------------|
| `${#str}` | 字符串长度 |
| `${str:0:5}` | 子串（偏移量：长度） |
| `${str/old/new}` | 替换第一个匹配 |
| `${str//old/new}` | 替换所有匹配 |
| `${str^^}` | 转大写 |
| `${str,,}` | 转小写 |

## 条件判断

### if / elif / else

```
if [[ "$name" == "Alice" ]]; then
    echo "Hi Alice"
elif [[ "$name" == "Bob" ]]; then
    echo "Hi Bob"
else
    echo "Who are you?"
fi
```

### 测试运算符

| Command | Description |
|---------|-------------|
| `-eq  -ne` | 整数等于 / 不等于 |
| `-lt  -gt` | 整数小于 / 大于 |
| `-le  -ge` | 整数小于等于 / 大于等于 |
| `==  !=` | 字符串等于 / 不等于 |
| `-z "$str"` | 字符串为空 |
| `-n "$str"` | 字符串非空 |
| `-f file` | 文件存在且为普通文件 |
| `-d dir` | 目录存在 |
| `-e path` | 路径存在（任意类型） |
| `-r  -w  -x` | 可读 / 可写 / 可执行 |
| `&&  \|\|` | 逻辑与 / 或 |

## 循环

### for 循环

```
for fruit in apple banana cherry; do
    echo "$fruit"
done

for f in *.txt; do
    echo "File: $f"
done
```

### C 风格 for 循环

```
for ((i=0; i<5; i++)); do
    echo "$i"
done
```

### while 循环

```
count=0
while [[ $count -lt 5 ]]; do
    echo "$count"
    ((count++))
done
```

### 循环控制

| Command | Description |
|---------|-------------|
| `break` | 退出循环 |
| `continue` | 跳到下一次迭代 |

## 函数

### 定义与调用

```
greet() {
    echo "Hello, $1!"   # $1 = first arg
    return 0             # exit status
}
greet "Alice"            # Hello, Alice!
```

### 局部变量与返回值

```
add() {
    local sum=$(($1 + $2))
    echo "$sum"          # "return" via stdout
}
result=$(add 3 5)        # capture: 8
```

## 管道与重定向

### 管道

```
ls -l | grep ".txt"      # pipe output
cat log | sort | uniq    # chain commands
cmd1 | tee out.txt       # pipe + save to file
```

### 重定向

| Command | Description |
|---------|-------------|
| `cmd > file` | 重定向 stdout（覆盖） |
| `cmd >> file` | 重定向 stdout（追加） |
| `cmd < file` | 从文件重定向 stdin |
| `cmd 2> file` | 重定向 stderr |
| `cmd 2>&1` | 将 stderr 重定向到 stdout |
| `cmd &> file` | 重定向 stdout + stderr |
| `cmd << EOF` | Here document（内联输入） |
| `/dev/null` | 丢弃输出：`cmd > /dev/null` |

## 文件操作

### 查看文件

```
cat file.txt             # print entire file
head -n 10 file.txt      # first 10 lines
tail -n 10 file.txt      # last 10 lines
tail -f log.txt          # follow (live updates)
less file.txt            # paginated viewer
```

### 统计与查找

```
wc -l file.txt           # count lines
wc -w file.txt           # count words
wc -c file.txt           # count bytes
find . -name "*.txt"     # find by name
find . -type d           # find directories
find . -mtime -7         # modified in last 7 days
```

### 其他文件命令

| Command | Description |
|---------|-------------|
| `touch file` | 创建文件 / 更新时间戳 |
| `stat file` | 文件元数据（大小、日期） |
| `file img.png` | 检测文件类型 |
| `diff a.txt b.txt` | 对比两个文件 |
| `sort file.txt` | 按行排序 |
| `uniq` | 去除相邻重复行 |
| `cut -d: -f1` | 按分隔符提取字段 |
| `tr 'a-z' 'A-Z'` | 字符转换 / 替换 |

## 文本处理

### grep

```
grep "error" log.txt       # search for pattern
grep -i "error" log.txt    # case-insensitive
grep -r "TODO" src/        # recursive search
grep -n "func" file.go     # show line numbers
grep -c "error" log.txt    # count matches
grep -v "debug" log.txt    # invert match
```

### sed

```
sed 's/old/new/' file      # replace first per line
sed 's/old/new/g' file     # replace all
sed -i 's/old/new/g' file  # edit in place
sed -n '5,10p' file        # print lines 5-10
sed '/pattern/d' file      # delete matching lines
```

### awk

```
awk '{print $1}' file      # print first field
awk -F: '{print $1}' file  # custom delimiter
awk '$3 > 100' file        # filter by field value
awk '{sum+=$1} END{print sum}' file  # sum column
```

## 权限

### chmod

```
chmod 755 script.sh    # rwxr-xr-x
chmod +x script.sh     # add execute
chmod -w file.txt      # remove write
chmod u+x,g-w file     # user +exec, group -write
```

### 权限参考

| Command | Description |
|---------|-------------|
| `r (4)` | 读 |
| `w (2)` | 写 |
| `x (1)` | 执行 |
| `u / g / o` | 所有者 / 组 / 其他 |
| `755` | 所有者：rwx，组/其他：r-x |
| `644` | 所有者：rw-，组/其他：r-- |

### 所有权

```
chown user file.txt         # change owner
chown user:group file.txt   # change owner + group
chown -R user:group dir/    # recursive
```