a b
[root@jfht ~]#
输出命令执行结果
格式1：command line
就是直接执行命令，当然可以
格式2：echo `command line`
格式3：echo $(command line)
这两种格式的执行效果是一样的，都会把命令输出的前后空格去掉、中间的空格换行压缩为一个空格。
格式2：echo "$(command line)"
格式3：echo "`command line`"
这两种格式的执行效果也是一样的，并且会保持命令输出的空格和换行。一般与直接执行命令的输出一致。
[root@jfht tmp]# ls
ct08 ct08.min.tar.gz ls0.txt ls1.txt ls2.txt
[root@jfht tmp]# echo `ls`
ct08 ct08.min.tar.gz ls0.txt ls1.txt ls2.txt
[root@jfht tmp]# echo $(ls)
ct08 ct08.min.tar.gz ls0.txt ls1.txt ls2.txt
[root@jfht tmp]# echo "`ls`"
ct08
ct08.min.tar.gz
ls0.txt
ls1.txt
ls2.txt
[root@jfht tmp]# echo "$(ls)"
ct08
ct08.min.tar.gz
ls0.txt
ls1.txt
ls2.txt
[root@jfht tmp]#
问题来了：为什么 echo "`ls`" 和 echo "$(ls)" 的输出结果与 直接执行 ls 不一致呢？可以看看下面的解释：
如果标准输出是终端，那么是显示为多列形式的；否则就是一行一列。而对于$(ls)和`ls`，实际上标准输出已经不是终端了。
info ls 写道
By default, the output is sorted alphabetically, according to the
locale settings in effect.(1) If standard output is a terminal, the
output is in columns (sorted vertically) and control characters are
output as question marks; otherwise, the output is listed one per line
and control characters are output as-is.
输出到文件（标准输出重定向）
覆盖原来的文件
格式：echo "$S" >output.txt
追加到文件
格式：echo "$S" >>output.txt
[root@jfht tmp]# S=123456789
[root@jfht tmp]# echo "$S" >output.txt
[root@jfht tmp]# cat output.txt
123456789
[root@jfht tmp]# echo "$S" >output.txt
[root@jfht tmp]# cat output.txt
123456789
[root@jfht tmp]# echo "$S" >>output.txt
[root@jfht tmp]# cat output.txt
123456789
123456789
[root@jfht tmp]#
输出到标准错误设备
比如用来打印程序的出错信息
echo "$S" >&2
>&2表示把标准输出重定向到文件描述符2，而这正是标准错误输出的文件描述符。
[root@jfht tmp]# if ! ls "*"; then echo "error ls" >&2; fi
ls: *: 没有那个文件或目录
error ls
[root@jfht tmp]#
上面命令行的意思是说列出文件名为*的文件，如果出错，就打印错误信息。
输出到别的进程
管道线(|)
示例：echo "$S" | wc -c
Advanced Bash-Scripting Guide: Chapter 3. Special Characters 写道
|
pipe. Passes the output (stdout of a previous command to the input (stdin) of the next one, or to the shell. This is a method of chaining commands together.
A pipe, as a classic method of interprocess communication, sends the stdout of one process to the stdin of another. In a typical case, a command, such as cat or echo, pipes a stream of data to a filter, a command that transforms its input for processing.
The output of a command or commands may be piped to a script.
Here Document方式
示例：wc -c <
EOF
Advanced Bash-Scripting Guide: Chapter 19. Here Documents 写道
A here document is a special-purpose code block. It uses a form of I/O redirection to feed a command list to an interactive program or a command, such as ftp, cat, or the ex text editor.
COMMAND <
...
...
InputComesFromHERE
A here document supports parameter and command substitution. It is therefore possible to pass different parameters to the body of the here document, changing its output accordingly.
Here String方式
示例：wc -c <<<"$S"
Advanced Bash-Scripting Guide: 19.1. Here Strings 写道
A here string can be considered as a stripped-down form of a here document.
It consists of nothing more than COMMAND <<< $WORD,
where $WORD is expanded and fed to the stdin of COMMAND.
[root@jfht tmp]# S=123456789
[root@jfht tmp]# echo "$S" | wc -c
10
[root@jfht tmp]# wc -c <<<"$S"
10
[root@jfht tmp]# wc -c <
> EOF
10
[root@jfht tmp]#
格式化输出（printf）
printf "%8s" "$S"
类似C语言的格式化输出，此处不深入探讨。
作者“Bash @ Linux”