HERE document的几个用法

shopt -s -o nounset: 开启变量检查，如果一个使用的变量没有值，则会报错。unbound variable
<<'HERE' : 这样的用法，则所有的内容都会被认为是纯文本。
<<-EOF   : 则默认会在每行前面加上indent.
但是很奇怪的是，在here document里面的echo -e good\t也无法正常的输出了。输出是goodt， \t没有被解释为tab.

shopt -s -o nounset

Script="$( cat <<'HERE'
hostname;
cat /etc/issue;
id root
HERE
)"

cat <<-HERE>command.hist
#cat <<-'HERE'>command.hist
echo $SHELL
`hostname`;
`cat /etc/issue`;
id root
`echo -e good\t`
HERE

root=`id`
echo $root

#ssh -t root@192.168.0.102 ${Script}

我本来想用here document 生成文件里面含有tab，或者空格，但是感觉还是不方便。
还是只能用echo 或者printf 实现。

echo -e "email\tlast\tfirst
[e-mail]\tkobe\the
[e-mail]\x09jason\011kid" >tab_delim.txt
printf "email\tlast\tfirst
[e-mail]\tkobe\the
[e-mail]\x09jason\011kid" > tab_delim.txt
#\x09 或者\011 \t 都是tab

查看文件的不可打印字符：

cat -A tab_delim.txt
vim: set list
vim: set nolist
vim: digraph  #查看可以输入的特殊字符

最后介绍下shopt的几个用法：shopt 是bash的builtin.

man builtin     #可以寻找shopt
shopt -p   #查看当前设置
shopt -o
shopt -s #enable a set
shopt -u #disable a set

       shopt [-pqsu] [-o] [optname ...]
              Toggle  the values of settings controlling optional shell behavior. The settings can be either those listed below, or, if the -o option is used, those available with the -o option to the set builtin com‐
              mand. With no options, or with the -p option, a list of all settable options is displayed, with an indication of whether or not each is set. The -p option causes output to be displayed in a  form  that
              may be reused as input. Other options have the following meanings:
              -s     Enable (set) each optname.
              -u     Disable (unset) each optname.
              -q     Suppresses  normal  output (quiet mode); the return status indicates whether the optname is set or unset. If multiple optname arguments are given with -q, the return status is zero if all optnames
                     are enabled; non-zero otherwise.
              -o     Restricts the values of optname to be those defined for the -o option to the set builtin.