By

The read Command

read variables

shell从standard input中读取一行，然后将其依次赋值给变量列表。 如果读取的单词个数大于变量个数，则将剩余的单词全部赋值给最后一个变量

a program to copy file

if [ "$#" -ne 2]; then
  echo "Usage: mycp from to"
  exit 1
fi

from="$1"
to="$2"

if [ -e "$to" ]; then
  echo "$to already exists; overwrite [yes/no]?"
  read answer

  if [ "$answer" != yes ]; then
    echo "Copy not performed"
    exit 0
  fi
fi

cp $from $to

special echo escape characters

echo命令运行后会自动在结尾添加换行符，所以如果想让用户在同一行输入可以添加\c符号

echo "$to already exists; overwrite [yes/no]? \c"

注意，\c是被echo命令进行转义的，而不是shell，所以必须添加在引号类。
其它转义符

Character Prints
\b backspace
\c the line without a terminating newline
\f formfeed
\n newline
\r carriage return
\t tab character
\ backslash
\0nnn character whose ASCII value is nnn, where nnn is a one- to three-digit octal number

an improved version of mycp

当目标路径只是文件夹时，默认将原文件以相同文件名复制到目标文件夹中

if [ "$#" -ne 2 ]; then
  echo "Usage: mycp from to"
  exit 1
fi

from="$1"
to="$2"

if [[ -d "$to" ]]; then
  to="$to/$(basename $from)"
fi

if [[ -e "$to" ]]; then
  echo "$to already exists; overwrite [yes/no]? \c"
  read answer

  if [[ "$answer" != yes ]]; then
    echo "Copy not performed"
    exit 0
  fi
fi

cp $from $to

basename命令可以将包含路径的字符串中提取除单纯的文件名

a final version of mycp

将多个文件复制到同一目录下

numargs=$#
filelist=
copylist=

while [[ "$#" -gt 1 ]]; do
  filelist="$filelist $1"
  shift
done

to="$1"

if [[ "$numargs" -lt 2 -o "$numargs" -gt 2 -a ! -d "$to" ]]; then
  echo "Usage: mycp file1 file2"
  echo "       mycp file(s) dir"
  exit 1
fi

for from in $filelist ; do
  if [[ -d "$to" ]]; then
    tofile="$to/$(basename $from)"
  else
    tofile="$to"
  fi

  if [[ -e "$tofile" ]]; then
    echo "$tofile already exists; overwrite [yes/no]? \c"
    read answer

    if [[ "$answer" = yes ]]; then
      copylist="$copylist $from"
    fi
  else
    copylist="$copylist $from"
  fi
done

if [[ -n "$copylist" ]]; then
  cp $copylist $to
fi

a menu-driven phone program

if [[ "$#" -ne 0 ]]; then
  lu "$@"
  exit
fi

validchoice=""

until [[ -n "$validchoice" ]]; do
  echo '
  would you like to :
    1. look someone up
    2. add someone to the phone book
    3. remove someone from the phone book
  please select one of the above (1-3): \c'

  read choice
  echo

  case "$choice" in
    1) echo "Enter name to look up: \c"
      read name
      lu "$name"
      validchoice=TRUE;;
    2) echo "Enter name to be added: \c"
      read name
      echo "Enter number: \c"
      read number
      add "$name" "$number"
      validchoice=TRUE;;
    3) echo "Enter name to be removed: \c"
      read name
      rem "$name"
      validchoice=TRUE
    *) echo "Bad choice";;
  esac
done

$$

$$表示当前登录shell的进程ID

the exit status from read

通常read的exit status都为0，除非遇到了end-of-file标识。 如果数据来源于控制台，则表示用户按了ctrl + d；如果数据源为文件，则表示到达了文件尾部。
利用这个特性，可以很方便地按行遍历文件

lineno=1

cat $* |
while [[ read -r line ]]; do
  echo "$lineno: $line"
  lineno=$((lineno + 1))
done

-r参数选项表示忽略”\“的转义功能

the printf command

printf "format" arg1 arg2 ...

功能和用法类似于c/c++中的printf

character Use for Printing
%d integer
%u unsigned integer
%o octal integer
%x hexadecimal integer, using a-f
%X hexadecimal integer, using A-F
%c single character
%s literal string
%b string containing backslash escape character
%% percent sign

其中format的一般形式

%[flags][width][.precision]type

其中type就是上表中所列出的字符(d,u,o,x,X,c,s,b)

modifier meaning
[flags]  
- left justify value
+ precede integer with + or -
(space) precede positive integer with space character
# precede octal integer with 0, hexadecimal integer with 0x or 0X
[width] minimum width of field; * means use next argument as width
[.precision] minimum number of digits for integer; maximum number of characters to display for strings; * means use next argument as precision