Shell 无处不在，异常强大，是每个开发人员必备技能。

写脚本很简单，但是写一个运行正常、稳定可靠的脚本并非易事，需要扎实的基础知识。

本文仅包含 Bourne shell 语法。

内建命令

# 别名，用短名表示复杂命令
alias listsource="ls *.c *.h"
# 别名只在当前 shell 会话期间有效，
# 若要使其永久生效，可将其保存至登录脚本。

登录脚本

对于 bash, zsh 等 Bourne shell 家族，以下文件可以保存设置：

  • ~/.profile 所有登录脚本自动执行
  • ~/.bash_profile 与 .profile 类似，但仅在 bash 中执行
  • ~/.bashrc 和 ~/.zshrc 对非登陆 bash 或 zsh shell 脚本自动运行。

可在 .bashrc 中读取 .profile 内容：

. $HOME/.profile

输入特殊字符

输入特殊字符时，可以在其之前输入 Ctrl-v，这会禁止之后特殊字符的功能：

echo "control-V control-G"

将在屏幕输出：

echo "^G"

Shell 脚本基础

shell 脚本分两派： Bourne shell 派和 C shell 派。Bourne shell 派更灵活，故应用范围更广。

Bourne 家族包括： sh, bash, zsh, ksh。

Shell 变量及输出

#!/bin/sh

echo "Hello, world!"

#!/bin/sh 名为 interpreter line，用来指定当前脚本文件的解析器。

以下是变量的用法：

#!/bin/sh

FIRST_ARGUMENT="$1"
echo "Hello, world $FIRST_ARGUMENT!"

变量 $1 包含传入 shell 脚本的首个实参。

If you dereference a variable, you precede it with a dollor sign $.

Using arguments And Variables That Contain Spaces

Using quotation marks is particularly important when working with variables that contain filenames or paths. For example, type the following commands:

mkdir "/tmp/My Folder"
FILENAME="/tmp/My Folder"
ls "$FILENAME"
ls $FILENAME

Notice that the shell misinterprets the second ls command as being an attempt to list the files in /tmp/My and the files in Folder

Handling Quotation Marks in Strings

MYSTRING="The word of the day is \"sedentary\"."

Shell scripts also allow the use of single quote marks. Variables between single quotes are not replaced by their contents.

name='monica'
echo "$name" # monica
echo '$name' # $name

Exporting Shell Variables

The exported variables are commonly known as environment variables. The most famous environment variable is PATH variable.

Using the export Buitlin

Generally speaking, the first time you assign a value to an environment variable, the Bourne shell creates a new, local copy of this shell variable that is specific to your script. Any tool executed from your script is passed the original value inherited from whatever script, tool, or shell that launched it.

To guarantee that your modifications to a shell variable are passed to any script or tool that your shell script calls, you must use the export builtin.

export PATH="/usr/local/bin:$PATH"

# or
PATH="/usr/local/bin:$PATH"
export PATH

You can use printenv command to obtain a complete list of defined variables and the grep to see if it is in the list. For example:

printenv | grep -c '^USER'

Overriding Environment Variables for Child Processes

#!/bin/sh

# printmyvar.sh
echo $MYVAR

MYVAR=7 ./printmyvar.sh

Deleting Shell Variables

To delete the variable entirely, use the unset builtin. For example:

MYVAR="this is a test"
unset MYVAR
echo "MYVAR is \"$MYVAR\""

Shell Input and Output

The Bourne shell provides a number of ways to read and write files, display text, and get information from the user, including echo, printf, read, cat, pipes, and redirection.

Shell Script Input and Output Using printf and read

#!/bin/sh

printf "What is your name? -> "
read NAME
echo "Hello, $NAME. Nice to meet you."

The printf command does not automatically add a newline to the end of the line, handy for prompts.

The read command takes a line of input and separates it into a series of arguments.

You can modify the behavior of the read command by modifying the shell variable IFS (short for internal field separators). For example:

#!/bin/sh

printf "Type three numbers separated by 'q'. -> "
IFS="q"
read NUMBER1 NUMBER2 NUMBER3
echo "You said: $NUMBER1, $NUMBER2, $NUMBER3"

You run this script and enter 1q3q57q65, the script replies with You said: 1, 3, 57q65.

Bulk I/O Using the cat Command

By combining cat with <<, you can send a large quantity of text to a file. For example:

cat > myprogram.c << EOF
#include <stdio.h>
int main(int argc, char * argv[]) {
    // ...
}
EOF

Pipes and Redirection

The true power of shell scripting lies in the ability to read and write files and chain multiple program together in interesting ways.

Each program in a UNIX-based or UNIX-like system has three basic file descriptors: standard input (stdin), standard output (stdout), and standard error (stderr).

Basic File Redirection

# create a new file
echo "a single line of text" > MyFile

# append another line of text
echo "another line of text" >> MyFile

文件和目录操作

查找文件 find

find [路径] [选项] [操作]

# 查找当前目录下 t 开头，权限是 744 的文件
find . -name 't*' -perm 744 -print

# 查找
find /etc -type f -name "rc*" -exec ls -l {} \;

正则表达式

POSIX 标准将正则表达式分为两类：基本的正则表达式和扩展的正则表达式。

基本的正则表达式包括：*, ., ^, $, [], \, \<\>, \{n\}, \{n,\}, \{n,m\}

扩展的正则表达式元字符包括：?, +, (), |

grep 和正则是绝配：

grep [a-c]ello data.txt

# 使用扩展正则表达式元字符
grep -E "re(a|e|o)d" data.txt

通配

bash shell 本身不支持正则表达式，支持通配。常用的通配符有：?, *, [], {}, ^ 等。

通配符和正则表达式元字符意义不甚相同，比如 ? 表示一个任意字符，* 表示任意位字符，^ 代表取反。

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