把一个shell命令的输出作为另一个shell命令的输入；特殊变量$0, $#, $*, $@, $?, $$，以及"$*", "$@"代表的含义

本地有两个文件： arg.txt 和 about.sh。

目标是把arg.txt里的内容输出，并作为about.sh的输入。

先看arg.txt里存的是什么：

[liusiyi@localhost ~]$ cat arg.txt
Bob
Ella
Justin
Christin
Steven
Olivia

再看 about.sh里存的是什么（其实是一段对Shell特殊变量的解释，包括 $0, $#, $*, $@, $?, $$）：

  [liusiyi@localhost ~]$ cat about.sh

#!/bin/bash
echo "File name, \$0: $0"
echo "First argument, \$1: $1"
echo "Second argument, \$2: $2"
echo "Quated values, \$*: $*"
echo "Quated values, \$@: $@"
echo "Shell Id, \$$: $$"
echo "Return value (success 0, fail 1), \$?: $?"
echo "Total # of arguments, \$#: $#"
echo -e "\nSee differences about \$*, \$@, \"\$*\", and \"\$@\" as below:"
echo "---- print each argument from \$* ----"
count=0
for var in $*
do
count=$((count+1))
echo "$var"
done
echo "(totally $count)"

echo "---- print each argument from \$@ ----"
count=0
for var in $@
do
count=$((count+1))
echo "$var"
done
echo "(totally $count)"

echo "---- print each argument from \"\$*\" ----"
count=0
for var in "$*"
do
count=$((count+1))
echo "$var"
done
echo "(totally $count)"

echo "---- print each argument from \"\$@\" ----"
count=0
for var in "$@"
do
count=$((count+1))
echo "$var"
done
echo "(totally $count)"

echo "----------------end--------------"

步骤一，把arg.txt的内容传给变量a：

[liusiyi@localhost ~]$ a=$(cat arg.txt)

步骤二，查看变量a的值：

[liusiyi@localhost ~]$ echo $a
Bob Ella Justin Christin Steven Olivia

步骤三，变量a的值被about.sh调用，显示了$0, $#, $*, $@, $?, $$，以及"$*", "$@"在这个命令中的真实值：

[liusiyi@localhost ~]$ about.sh $a
File name, $0: ./about.sh
First argument, $1: Bob
Second argument, $2: Ella
Quated values, $*: Bob Ella Justin Christin Steven Olivia
Quated values, $@: Bob Ella Justin Christin Steven Olivia
Shell Id, $$: 25985
Return value (success 0, fail 1), $?: 0
Total # of arguments, $#: 6

See differences about $*, $@, "$*", and "$@" as below:
---- print each argument from $* ----
Bob
Ella
Justin
Christin
Steven
Olivia
(totally 6)
---- print each argument from $@ ----
Bob
Ella
Justin
Christin
Steven
Olivia
(totally 6)
---- print each argument from "$*" ----
Bob Ella Justin Christin Steven Olivia
(totally 1)
---- print each argument from "$@" ----
Bob
Ella
Justin
Christin
Steven
Olivia
(totally 6)
----------------end--------------

注意，被双引号包含时，"$*" 的参数被当做一个整体，所以遍历里面的元素之后 totally count=1；而 "$@" 依旧是遍历每一个参数，所以count=6

以上步骤也可以简写为：

[liusiyi@localhost ~]$ about.sh $(cat arg.txt)