Shell fundamentals

This article primarily uses Bash-style syntax. Fish differs in its syntax for variables, command substitution, history, and configuration.

The previous three articles covered "which commands to use"; this one covers "how to join them together". Pipes and redirection put the Linux philosophy into practice: small tools combine to do powerful things.

Pipes#

| — Feed one command’s output into the next command

command1 | command2

The basic idea: standard output from command1 becomes standard input to command2 directly.

ps aux | grep python        # ps 的输出交给 grep 筛选
ls -l | wc -l               # 统计显示行数，含 total 行；不是可靠文件数
cat file.txt | grep error   # 等价于 grep error file.txt
du -sh * | sort -hr         # 列出文件大小，从大到小排

You can keep chaining pipes:

cat access.log | grep "404" | cut -d' ' -f1 | sort | uniq -c | sort -rn
# 解释：日志 → 筛404 → 取IP列 → 排序 → 去重计数 → 按次数倒排
# 结果：谁在疯狂 404 一目了然

Redirection#

> — Write output to a file (overwrite)

echo "hello" > file.txt       # 写内容到文件，覆盖原有内容
ls -la > listing.txt          # ls 的结果存到文件，终端不显示

>> — Write output to a file (append)

echo "another line" >> file.txt    # 追加到末尾

< — Read input from a file

wc -l < file.txt         # 把文件内容喂给 wc（等价于 wc -l file.txt）

2>&1 — Merge standard error into standard output

./script.sh > output.log 2>&1    # 正常输出和报错都进同一个文件
./script.sh > /dev/null 2>&1     # 静默运行，什么都不显示

A model for understanding it:

Number	Name	Default source or destination
0	stdin (standard input)	Keyboard
1	stdout (standard output)	Terminal
2	stderr (standard error)	Terminal

2>&1 means: redirect stderr (2) to wherever stdout (1) is currently going.

Wildcards#

*          # 匹配任意多个字符
?          # 匹配恰好一个字符
[abc]      # 匹配 a、b、c 中的任意一个
[0-9]      # 匹配 0-9 任意数字

ls *.md           # 所有 .md 文件
ls report_??.txt  # 先预览任意两个字符后缀（不限定数字）
rm -- report_??.txt  # 核对列表后才删除；不匹配时的行为取决于 Shell
ls [A-Z]*.md      # 大写字母开头的 .md 文件

Variables#

Definition and use

name="hello"          # 定义变量，等号两边不能有空格
echo "$name"            # 使用变量，加 $ 前缀
echo "${name}"          # 也可以用花括号包起来

Environment variables

echo $PATH            # 可执行文件的搜索路径
echo $HOME            # 你的家目录
echo $USER            # 当前用户名
env                   # 列出所有环境变量
export MY_VAR="val"   # 设为环境变量（子进程可见）

Command substitution

now=$(date)           # 把 date 命令的输出赋给变量
echo "现在时间：$now"
files=$(find . -maxdepth 1 -type f -printf 'x\n' | wc -l)  # GNU find：当前目录普通文件数

Aliases#

alias — Give a long command a short name

alias ll='ls -lah'              # ll 代替 ls -lah
alias gs='git status'           # git 常用操作缩到两字母
alias ..='cd ..'                # 快速回上级

alias                          # 查看当前所有别名

To make it persistent, put it in ~/.bashrc (bash) or ~/.config/fish/config.fish (fish).

History and shortcuts#

history — View command history

history                     # 全部历史
history | grep git          # 在历史中搜 git 相关命令

Reuse commands from history

!!          # 重复上一条命令
!git        # 重复最近一条以 git 开头的命令
!$          # 上一条命令的最后一个参数

Terminal keyboard shortcuts

Shortcut	Action
`Ctrl+C`	Terminate the current foreground program
`Ctrl+Z`	Suspend the current foreground program
`Ctrl+D`	Send EOF (exit the terminal or close the input stream)
`Ctrl+L`	Clear the screen (equivalent to `clear`)
`Ctrl+R`	Search command history backward
`Ctrl+A`	Move the cursor to the beginning of the line
`Ctrl+E`	Move the cursor to the end of the line
`Ctrl+W`	Delete the word before the cursor
`Ctrl+U`	Delete everything before the cursor

Ctrl+R is particularly useful: search a long history for a command you have used, then press it again to continue searching backward.

Combination examples#

Solve practical problems with a one-line command

# 当前目录项目（含目录）的磁盘占用前 5 名
du -sh * | sort -hr | head -5

# 统计名称匹配的进程数（不是线程数）
pgrep -c nginx
# 查看已知 PID 的线程数与线程列表
ps -o nlwp= -p PID
ps -T -p PID

# 查看 8080 端口被哪个程序占了
ss -tlnp | grep 8080

# 递归找所有 .md 文件中有多少行包含 "TODO"
grep -rn --include="*.md" "TODO" . | wc -l

This is the Unix philosophy: have each command do one thing well, and connect them with pipes.

The core mental model#

输入(stdin) → [命令] → 输出(stdout)
                  ↓
              错误(stderr)

管道 | ：把 A 的 stdout 给 B 的 stdin
重定向 > ：把 stdout 存到文件
2>&1   ：把 stderr 合并到 stdout 同一去处

Once you understand this, the ways of connecting commands fall naturally into place.

Redirections are processed from left to right: > output.log 2>&1 and 2>&1 > output.log are not equivalent. History expansions such as !! are available in interactive Bash; check the expanded command before executing it.