【bash学习011】Here document是什么

eg 1 here document：立即文档，可以用于输出提示信息，或者自动输入某些内容，其标准书写格式基本与下面脚本中的一个模样；

1 #!/bin/bash 2  3 # 'echo'对于打印单行消息来说是非常好用的, 4 #+ 但是在打印消息块时可能就有点问题了. 5 # 'cat' here document可以解决这个限制. 6  7 cat <<End-of-message 8 ------------------------------------- 9 This is line 1 of the message. 10 This is line 2 of the message. 11 This is line 3 of the message. 12 This is line 4 of the message. 13 This is the last line of the message. 14 ------------------------------------- 15 End-of-message 16  17 # 用下边这行代替上边的第7行, 18 #+ cat > $Newfile <<End-of-message 19 #+ ^^^^^^^^^^ 20 #+ 那么就会把输出写到文件$Newfile中, 而不是stdout. 21  22 exit 0

eg 2 变量名也能被立即文档识别

　　

#!/bin/bash

# Another 'cat' here document, using parameter substitution.



# Try it with no command-line parameters,   ./scriptname

# Try it with one command-line parameter,   ./scriptname Mortimer

# Try it with one two-word quoted command-line parameter,

#                           ./scriptname "Mortimer Jones"



CMDLINEPARAM=1     #  Expect at least command-line parameter.



if [ $# -ge $CMDLINEPARAM ]

then

  NAME=$1          #  If more than one command-line param,

                   #+ then just take the first.

else

  NAME="John Doe"  #  Default, if no command-line parameter.

fi  



RESPONDENT="the author of this fine script"  

  



cat <<Endofmessage



Hello, there, $NAME.

Greetings to you, $NAME, from $RESPONDENT.



# This comment shows up in the output (why?).



Endofmessage



# Note that the blank lines show up in the output.

# So does the comment.



exit


eg 3 在当前脚本中创建另一个脚本（或c语言程序或者其他）

#!/bin/bash # generate-script.sh # Based on an idea by Albert Reiner. OUTFILE=generated.sh # Name of the file to generate. # ----------------------------------------------------------- # 'Here document containing the body of the generated script. ( cat <<'EOF' #!/bin/bash echo "This is a generated shell script." # Note that since we are inside a subshell, #+ we can't access variables in the "outside" script. echo "Generated file will be named: $OUTFILE" # Above line will not work as normally expected #+ because parameter expansion has been disabled. # Instead, the result is literal output. a=7 b=3 let "c = $a * $b" echo "c = $c" exit 0 EOF ) > $OUTFILE # ----------------------------------------------------------- # Quoting the 'limit string' prevents variable expansion #+ within the body of the above 'here document.' # This permits outputting literal strings in the output file. if [ -f "$OUTFILE" ] then chmod 755 $OUTFILE # Make the generated file executable. else echo "Problem in creating file: \"$OUTFILE\"" fi # This method can also be used for generating #+ C programs, Perl programs, Python programs, Makefiles, #+ and the like. exit 0 variable=$(cat <<SETVAR This variable runs over multiple lines. SETVAR) echo "$variable"

eg 4 隐蔽的错误（“错误”是一个裸男，他正念念有词地说：“你看不见我，你看不见我，你就是看不见我。”虽然它偶尔可以隐藏地非常好，但只要眼尖心细，你还是能发现它不小心漏出的马脚。）

#!/bin/bash



echo "----------------------------------------------------------------------"



cat <<LimitString

echo "This is line 1 of the message inside the here document."

echo "This is line 2 of the message inside the here document."

echo "This is the final line of the message inside the here document."

     LimitString（错误在这里，LimitString前面不能有空格）

#^^^^Indented limit string. Error! This script will not behave as expected.



echo "----------------------------------------------------------------------"



#  These comments are outside the 'here document',

#+ and should not echo.



echo "Outside the here document."



exit 0



echo "This line had better not echo."  # Follows an 'exit' command.

eg 5  我也看不懂，留在这里，慢慢地看

#!/bin/bash # self-document.sh: self-documenting script # Modification of "colm.sh". DOC_REQUEST=70 if [ "$1" = "-h" -o "$1" = "--help" ] # Request help. then echo; echo "Usage: $0 [directory-name]"; echo sed --silent -e '/DOCUMENTATIONXX$/,/^DOCUMENTATIONXX$/p' "$0" | sed -e '/DOCUMENTATIONXX$/d'; exit $DOC_REQUEST; fi : <<DOCUMENTATIONXX List the statistics of a specified directory in tabular format. --------------------------------------------------------------- The command-line parameter gives the directory to be listed. If no directory specified or directory specified cannot be read, then list the current working directory. DOCUMENTATIONXX if [ -z "$1" -o ! -r "$1" ] then directory=. else directory="$1" fi echo "Listing of "$directory":"; echo (printf "PERMISSIONS LINKS OWNER GROUP SIZE MONTH DAY HH:MM PROG-NAME\n" \ ; ls -l "$directory" | sed 1d) | column -t exit 0

　　所有用例均来自http://tldp.org/LDP/abs/html/here-docs.html（ABS高级bash脚本指南)。