POSIX 7

kennytm引用了man bash，但其中大部分也是POSIX 7: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_07_04:

The redirection operators "<<" and "<<-" both allow redirection of lines contained in a shell input file, known as a "here-document", to the input of a command. The here-document shall be treated as a single word that begins after the next <newline> and continues until there is a line containing only the delimiter and a <newline>, with no <blank> characters in between. Then the next here-document starts, if there is one. The format is as follows: [n]<<word here-document delimiter where the optional n represents the file descriptor number. If the number is omitted, the here-document refers to standard input (file descriptor 0). If any character in word is quoted, the delimiter shall be formed by performing quote removal on word, and the here-document lines shall not be expanded. Otherwise, the delimiter shall be the word itself. If no characters in word are quoted, all lines of the here-document shall be expanded for parameter expansion, command substitution, and arithmetic expansion. In this case, the <backslash> in the input behaves as the <backslash> inside double-quotes (see Double-Quotes). However, the double-quote character ( '"' ) shall not be treated specially within a here-document, except when the double-quote appears within "$()", "``", or "${}". If the redirection symbol is "<<-", all leading <tab> characters shall be stripped from input lines and the line containing the trailing delimiter. If more than one "<<" or "<<-" operator is specified on a line, the here-document associated with the first operator shall be supplied first by the application and shall be read first by the shell. When a here-document is read from a terminal device and the shell is interactive, it shall write the contents of the variable PS2, processed as described in Shell Variables, to standard error before reading each line of input until the delimiter has been recognized.

例子

有些例子还没有给出。

引号阻止参数扩展

没有引用:

a=0
cat <<EOF
$a
EOF

输出:

0

报价:

a=0
cat <<'EOF'
$a
EOF

或者(丑陋但有效):

a=0
cat <<E"O"F
$a
EOF

输出:

$a

连字符删除开头制表符

没有连字符:

cat <<EOF
<tab>a
EOF

其中<tab>是一个文字制表符，可以用Ctrl + V <tab>

输出:

<tab>a

用连字符:

cat <<-EOF
<tab>a
<tab>EOF

输出:

a

当然，这是为了让您可以像周围的代码一样缩进您的cat，这更容易阅读和维护。例如:

if true; then
    cat <<-EOF
    a
    EOF
fi

不幸的是，这并不适用于空格字符:POSIX偏爱制表符缩进。呵。

值得注意的是，这里的文档也在bash循环中工作。 下面的例子展示了如何获取table的列列表:

export postgres_db_name='my_db'
export table_name='my_table_name'

# start copy 
while read -r c; do test -z "$c" || echo $table_name.$c , ; done < <(cat << EOF | psql -t -q -d $postgres_db_name -v table_name="${table_name:-}"
SELECT column_name
FROM information_schema.columns
WHERE 1=1
AND table_schema = 'public'
AND table_name   =:'table_name'  ;
EOF
)
# stop copy , now paste straight into the bash shell ...

output: 
my_table_name.guid ,
my_table_name.id ,
my_table_name.level ,
my_table_name.seq ,

或者甚至没有新的线条

while read -r c; do test -z "$c" || echo $table_name.$c , | perl -ne 
's/\n//gm;print' ; done < <(cat << EOF | psql -t -q -d $postgres_db_name -v table_name="${table_name:-}"
 SELECT column_name
 FROM information_schema.columns
 WHERE 1=1
 AND table_schema = 'public'
 AND table_name   =:'table_name'  ;
 EOF
 )

 # output: daily_issues.guid ,daily_issues.id ,daily_issues.level ,daily_issues.seq ,daily_issues.prio ,daily_issues.weight ,daily_issues.status ,daily_issues.category ,daily_issues.name ,daily_issues.description ,daily_issues.type ,daily_issues.owner

需要注意的是，cat << \EOT(参见反斜杠)不会展开内部的任何变量，而cat << EOT则可以。

例子:

FOO="bar"

cat << \EOT > foobar.txt
echo "$FOO"
EOT

将输出: echo $ FOO

而:

FOO="bar"

cat << EOT > foobar.txt
echo "$FOO"
EOT

将输出: 回声“酒吧”

这被称为heredoc格式，用于向stdin提供字符串。详情见https://en.wikipedia.org/wiki/Here_document#Unix_shells。

---

来自man bash:

Here Documents This type of redirection instructs the shell to read input from the current source until a line containing only word (with no trailing blanks) is seen. All of the lines read up to that point are then used as the standard input for a command. The format of here-documents is: <<[-]word here-document delimiter No parameter expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word. If any characters in word are quoted, the delimiter is the result of quote removal on word, and the lines in the here-document are not expanded. If word is unquoted, all lines of the here-document are subjected to parameter expansion, command substitution, and arithmetic expansion. In the latter case, the character sequence \<newline> is ignored, and \ must be used to quote the characters \, $, and `. If the redirection operator is <<-, then all leading tab characters are stripped from input lines and the line containing delimiter. This allows here-documents within shell scripts to be indented in a natural fashion.

这并不一定是对最初问题的回答，而是分享我自己测试的一些结果。这样的:

<<test > print.sh
#!/bin/bash
echo \$PWD
echo $PWD
test

将生成与以下相同的文件:

cat <<test > print.sh
#!/bin/bash
echo \$PWD
echo $PWD
test

所以，我不明白使用cat命令的意义。

长话短说，EOF标记(但也可以使用不同的文字)是一种heredoc格式，允许您以多行方式提供输入。 很多困惑来自于猫实际上是如何工作的。 你可以使用cat >>或>，如下所示:

$ cat >> temp.txt
line 1
line 2

虽然在手动写入控制台时可以这样使用cat，但如果我想以一种更声明性的方式提供输入，以便工具可以重用它，并保留缩进、空格等，则不方便。 Heredoc允许定义您的整个输入，就好像您不是在使用stdin，而是在单独的文本编辑器中输入一样。这是维基百科文章的意思:

它是源代码文件的一个部分，它被当作 单独的文件中。