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

对上面的答案做一点扩展。后面的>将输入引导到文件中，覆盖现有内容。然而，一个特别方便的用法是双箭头>>追加，将您的新内容添加到文件的末尾，如:

cat <<EOF >> /etc/fstab
data_server:/var/sharedServer/authority/cert /var/sharedFolder/sometin/authority/cert nfs
data_server:/var/sharedServer/cert   /var/sharedFolder/sometin/vsdc/cert nfs
EOF

这将扩展fstab，而不必担心意外修改其中的任何内容。

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”

在你的例子中，“EOF”被称为“Here Tag”。基本上，<<Here告诉shell，您将输入一个多行字符串，直到“tag”在这里。你可以随意命名这个标签，通常是EOF或STOP。

关于Here标签的一些规则:

标签可以是任何字符串，大写或小写，尽管大多数人习惯使用大写。 如果该行中有其他单词，则该标记将不被视为Here标记。在这种情况下，它仅仅被认为是字符串的一部分。标签本身应该在单独的行中，被认为是一个标签。 标签在该行中不应该有前导或尾随空格，才能被视为标签。否则它将被视为字符串的一部分。

例子:

$ cat >> test <<HERE
> Hello world HERE <-- Not by itself on a separate line -> not considered end of string
> This is a test
>  HERE <-- Leading space, so not considered end of string
> and a new line
> HERE <-- Now we have the end of the string

在Bash中处理多行文本时，cat <<EOF语法非常有用。当将多行字符串分配给shell变量、文件或管道时。

在Bash中使用cat <<EOF语法的例子:

1. 为shell变量分配多行字符串

$ sql=$(cat <<EOF
SELECT foo, bar FROM db
WHERE foo='baz'
EOF
)

$sql变量现在也保存换行字符。你可以用echo -e "$sql"来验证。

2. 在Bash中传递多行字符串到文件

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

print.sh文件现在包含:

#!/bin/bash
echo $PWD
echo /home/user

3.在Bash中将多行字符串传递给管道

$ cat <<EOF | grep 'b' | tee b.txt
foo
bar
baz
EOF

b.txt文件包含bar和baz行。相同的输出被打印到stdout。

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.