第 19 章. Here Documents
此时此地，孩子们。 --阿道司·赫胥黎，《岛》 |
here document 是一种特殊用途的代码块。它使用一种 I/O 重定向 的形式，将命令列表馈送到交互式程序或命令，例如 ftp、cat 或 ex 文本编辑器。
COMMAND <<InputComesFromHERE ... ... ... InputComesFromHERE |
界定字符串 划定（框定）命令列表。特殊符号 << 先于界定字符串。这具有将命令块的输出重定向到stdin程序或命令的输入。它类似于interactive-program < command-file，其中command-file包含
command #1 command #2 ... |
here document 等效形式如下所示
interactive-program <<LimitString command #1 command #2 ... LimitString |
选择一个足够不寻常的 界定字符串，使其不会出现在命令列表中的任何位置并造成混淆。
请注意，here document 有时可以有效地用于非交互式实用程序和命令，例如 wall。
示例 19-1. broadcast：向所有登录用户发送消息
#!/bin/bash
wall <<zzz23EndOfMessagezzz23
E-mail your noontime orders for pizza to the system administrator.
(Add an extra dollar for anchovy or mushroom topping.)
# Additional message text goes here.
# Note: 'wall' prints comment lines.
zzz23EndOfMessagezzz23
# Could have been done more efficiently by
# wall <message-file
# However, embedding the message template in a script
#+ is a quick-and-dirty one-off solution.
exit |
即使像 vi 文本编辑器这样不太可能的对象也适用于 here document。
示例 19-2. dummyfile：创建一个 2 行的虚拟文件
#!/bin/bash # Noninteractive use of 'vi' to edit a file. # Emulates 'sed'. E_BADARGS=85 if [ -z "$1" ] then echo "Usage: `basename $0` filename" exit $E_BADARGS fi TARGETFILE=$1 # Insert 2 lines in file, then save. #--------Begin here document-----------# vi $TARGETFILE <<x23LimitStringx23 i This is line 1 of the example file. This is line 2 of the example file. ^[ ZZ x23LimitStringx23 #----------End here document-----------# # Note that ^[ above is a literal escape #+ typed by Control-V <Esc>. # Bram Moolenaar points out that this may not work with 'vim' #+ because of possible problems with terminal interaction. exit |
上述脚本同样可以使用 ex 而不是 vi 来实现。包含 ex 命令列表的 Here documents 非常常见，足以形成自己的类别，称为 ex scripts。
#!/bin/bash # Replace all instances of "Smith" with "Jones" #+ in files with a ".txt" filename suffix. ORIGINAL=Smith REPLACEMENT=Jones for word in $(fgrep -l $ORIGINAL *.txt) do # ------------------------------------- ex $word <<EOF :%s/$ORIGINAL/$REPLACEMENT/g :wq EOF # :%s is the "ex" substitution command. # :wq is write-and-quit. # ------------------------------------- done |
类似于 "ex scripts" 的是 cat scripts。
示例 19-3. 使用 cat 的多行消息
#!/bin/bash # 'echo' is fine for printing single line messages, #+ but somewhat problematic for for message blocks. # A 'cat' here document overcomes this limitation. cat <<End-of-message ------------------------------------- This is line 1 of the message. This is line 2 of the message. This is line 3 of the message. This is line 4 of the message. This is the last line of the message. ------------------------------------- End-of-message # Replacing line 7, above, with #+ cat > $Newfile <<End-of-message #+ ^^^^^^^^^^ #+ writes the output to the file $Newfile, rather than to stdout. exit 0 #-------------------------------------------- # Code below disabled, due to "exit 0" above. # S.C. points out that the following also works. echo "------------------------------------- This is line 1 of the message. This is line 2 of the message. This is line 3 of the message. This is line 4 of the message. This is the last line of the message. -------------------------------------" # However, text may not include double quotes unless they are escaped. |
标记 here document 界定字符串的-选项（<<-LimitString）会抑制输出中前导制表符（但不包括空格）。这可能有助于使脚本更具可读性。
示例 19-4. 抑制制表符的多行消息
#!/bin/bash # Same as previous example, but... # The - option to a here document <<- #+ suppresses leading tabs in the body of the document, #+ but *not* spaces. cat <<-ENDOFMESSAGE This is line 1 of the message. This is line 2 of the message. This is line 3 of the message. This is line 4 of the message. This is the last line of the message. ENDOFMESSAGE # The output of the script will be flush left. # Leading tab in each line will not show. # Above 5 lines of "message" prefaced by a tab, not spaces. # Spaces not affected by <<- . # Note that this option has no effect on *embedded* tabs. exit 0 |
Here document 支持参数和命令替换。因此，可以向 here document 的主体传递不同的参数，从而相应地更改其输出。
示例 19-5. 带有可替换参数的 Here document
#!/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 |
这是一个有用的脚本，包含带有参数替换的 here document。
示例 19-6. 将文件对上传到 Sunsite incoming 目录
#!/bin/bash
# upload.sh
# Upload file pair (Filename.lsm, Filename.tar.gz)
#+ to incoming directory at Sunsite/UNC (ibiblio.org).
# Filename.tar.gz is the tarball itself.
# Filename.lsm is the descriptor file.
# Sunsite requires "lsm" file, otherwise will bounce contributions.
E_ARGERROR=85
if [ -z "$1" ]
then
echo "Usage: `basename $0` Filename-to-upload"
exit $E_ARGERROR
fi
Filename=`basename $1` # Strips pathname out of file name.
Server="ibiblio.org"
Directory="/incoming/Linux"
# These need not be hard-coded into script,
#+ but may instead be changed to command-line argument.
Password="your.e-mail.address" # Change above to suit.
ftp -n $Server <<End-Of-Session
# -n option disables auto-logon
user anonymous "$Password" # If this doesn't work, then try:
# quote user anonymous "$Password"
binary
bell # Ring 'bell' after each file transfer.
cd $Directory
put "$Filename.lsm"
put "$Filename.tar.gz"
bye
End-Of-Session
exit 0 |
在 here document 的头部引用或转义 "界定字符串" 会禁用其主体内的参数替换。原因是 引用/转义界定字符串 实际上 转义 了 $、` 和 \ 特殊字符，并使它们被字面解释。（感谢 Allen Halsey 指出这一点。）
示例 19-7. 参数替换已关闭
#!/bin/bash # A 'cat' here-document, but with parameter substitution disabled. NAME="John Doe" RESPONDENT="the author of this fine script" cat <<'Endofmessage' Hello, there, $NAME. Greetings to you, $NAME, from $RESPONDENT. Endofmessage # No parameter substitution when the "limit string" is quoted or escaped. # Either of the following at the head of the here document would have #+ the same effect. # cat <<"Endofmessage" # cat <<\Endofmessage # And, likewise: cat <<"SpecialCharTest" Directory listing would follow if limit string were not quoted. `ls -l` Arithmetic expansion would take place if limit string were not quoted. $((5 + 3)) A a single backslash would echo if limit string were not quoted. \\ SpecialCharTest exit |
禁用参数替换允许输出文字文本。生成脚本甚至程序代码是其用途之一。
示例 19-8. 生成另一个脚本的脚本
#!/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 also works for generating #+ C programs, Perl programs, Python programs, Makefiles, #+ and the like. exit 0 |
可以从 here document 的输出中设置变量。这实际上是 命令替换 的一种隐蔽形式。
variable=$(cat <<SETVAR This variable runs over multiple lines. SETVAR ) echo "$variable" |
Here document 可以为同一脚本中的函数提供输入。
示例 19-9. Here documents 和函数
#!/bin/bash
# here-function.sh
GetPersonalData ()
{
read firstname
read lastname
read address
read city
read state
read zipcode
} # This certainly appears to be an interactive function, but . . .
# Supply input to the above function.
GetPersonalData <<RECORD001
Bozo
Bozeman
2726 Nondescript Dr.
Bozeman
MT
21226
RECORD001
echo
echo "$firstname $lastname"
echo "$address"
echo "$city, $state $zipcode"
echo
exit 0 |
可以使用 : 作为虚拟命令，接受来自 here document 的输出。实际上，这创建了一个 "匿名" here document。
示例 19-10. "匿名" Here Document
#!/bin/bash
: <<TESTVARIABLES
${HOSTNAME?}${USER?}${MAIL?} # Print error message if one of the variables not set.
TESTVARIABLES
exit $? |
 | 上述技术的变体允许 "注释掉" 代码块。 |
示例 19-11. 注释掉代码块
#!/bin/bash
# commentblock.sh
: <<COMMENTBLOCK
echo "This line will not echo."
This is a comment line missing the "#" prefix.
This is another comment line missing the "#" prefix.
&*@!!++=
The above line will cause no error message,
because the Bash interpreter will ignore it.
COMMENTBLOCK
echo "Exit value of above \"COMMENTBLOCK\" is $?." # 0
# No error shown.
echo
# The above technique also comes in useful for commenting out
#+ a block of working code for debugging purposes.
# This saves having to put a "#" at the beginning of each line,
#+ then having to go back and delete each "#" later.
# Note that the use of of colon, above, is optional.
echo "Just before commented-out code block."
# The lines of code between the double-dashed lines will not execute.
# ===================================================================
: <<DEBUGXXX
for file in *
do
cat "$file"
done
DEBUGXXX
# ===================================================================
echo "Just after commented-out code block."
exit 0
######################################################################
# Note, however, that if a bracketed variable is contained within
#+ the commented-out code block,
#+ then this could cause problems.
# for example:
#/!/bin/bash
: <<COMMENTBLOCK
echo "This line will not echo."
&*@!!++=
${foo_bar_bazz?}
$(rm -rf /tmp/foobar/)
$(touch my_build_directory/cups/Makefile)
COMMENTBLOCK
$ sh commented-bad.sh
commented-bad.sh: line 3: foo_bar_bazz: parameter null or not set
# The remedy for this is to strong-quote the 'COMMENTBLOCK' in line 49, above.
: <<'COMMENTBLOCK'
# Thank you, Kurt Pfeifle, for pointing this out. |
| 这种巧妙技巧的又一个转折使 "自文档化" 脚本成为可能。 |
示例 19-12. 自文档化脚本
#!/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 |
使用 cat script 是完成此操作的另一种方法。
DOC_REQUEST=70 if [ "$1" = "-h" -o "$1" = "--help" ] # Request help. then # Use a "cat script" . . . cat <<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 exit $DOC_REQUEST fi |
另请参阅 示例 A-28、示例 A-40、示例 A-41 和 示例 A-42，以获取更多自文档化脚本的示例。
 | Here documents 创建临时文件，但这些文件在打开后会被删除，并且任何其他进程都无法访问。
|
 | 某些实用程序在 here document 内无法工作。 |
 | 在 here document 的最后一行，结束 界定字符串 必须从第一个字符位置开始。不能有前导空白字符。界定字符串之后的尾随空白字符同样会导致意外行为。空白字符会阻止界定字符串被识别。[1]
|
对于那些对于 here document 来说太复杂的任务，请考虑使用expect脚本语言，它是专门为向交互式程序馈送输入而设计的。