Merge pull request #6853 from lujun9972/add-MjAxNzExMTYgSG93IHRvIHVzZSBhIGhlcmUgZG9jdW1lbnRzIHRvIHdyaXRlIGRhdGEgdG8gYSBmaWxlIGluIGJhc2ggc2NyaXB0Lm1kCg==

选题: How to use a here documents to write data to a file in bash script

sources/tech/20171116 How to use a here documents to write data to a file in bash script.md

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+How to use a here documents to write data to a file in bash script
+======
+
+A here document is nothing but I/O redirection that tells the bash shell to read input from the current source until a line containing only delimiter is seen.
+[![redirect output of here document to a text file][1]][1]
+This is useful for providing commands to ftp, cat, echo, ssh and many other useful Linux/Unix commands. This feature should work with bash or Bourne/Korn/POSIX shell too.
+
+## heredoc syntax
+
+How do I use a heredoc redirection feature (here documents) to write data to a file in my bash shell scripts? [A here document][2] is nothing but I/O redirection that tells the bash shell to read input from the current source until a line containing only delimiter is seen.This is useful for providing commands to ftp, cat, echo, ssh and many other useful Linux/Unix commands. This feature should work with bash or Bourne/Korn/POSIX shell too.
+
+The syntax is:
+```
+command <<EOF
+cmd1
+cmd2 arg1
+EOF
+```
+
+OR allow here-documents within shell scripts to be indented in a natural fashion using **EOF <**
+```
+command <<-EOF
+ msg1
+ msg2
+ $var on line
+EOF
+```
+
+OR
+```
+command <<'EOF'
+ cmd1
+ cmd2 arg1
+ $var won't expand as parameter substitution turned off
+ by single quoting
+EOF
+```
+
+OR **redirect and overwrite it** to a file named my_output_file.txt:
+```
+command << EOF > my_output_file.txt
+ mesg1
+ msg2
+ msg3
+ $var on $foo
+EOF
+```
+
+OR **redirect and append it** to a file named my_output_file.txt:
+```
+command << EOF >> my_output_file.txt
+ mesg1
+ msg2
+ msg3
+ $var on $foo
+EOF
+```
+
+## Examples
+
+The following script will write the needed contents to a file named /tmp/output.txt:
+```
+#!/bin/bash
+OUT=/tmp/output.txt
+ 
+echo "Starting my script..."
+echo "Doing something..."
+ 
+cat <<EOF >$OUT
+  Status of backup as on $(date)
+  Backing up files $HOME and /etc/
+EOF
+ 
+echo "Starting backup using rsync..."
+```
+
+
+You can view /tmp/output.txt with the [cat command][3]:
+`$ cat /tmp/output.txt`
+Sample outputs:
+```
+ Status of backup as on Thu Nov 16 17:00:21 IST 2017
+ Backing up files /home/vivek and /etc/
+
+```
+
+### Disabling pathname/parameter/variable expansion, command substitution, arithmetic expansion
+
+Variable such as $HOME and command such as $(date) were interpreted substitution in script. To disable it use single quotes with 'EOF' as follows:
+```
+#!/bin/bash
+OUT=/tmp/output.txt
+ 
+echo "Starting my script..."
+echo "Doing something..."
+# No parameter and variable expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word.  
+# If any part of word is quoted, the delimiter  is  the  result  of  quote removal  on word, and the lines in the here-document 
+# are not expanded. So EOF is quoted as follows
+cat <<'EOF' >$OUT
+  Status of backup as on $(date)
+  Backing up files $HOME and /etc/
+EOF
+ 
+echo "Starting backup using rsync..."
+```
+
+#!/bin/bash OUT=/tmp/output.txtecho "Starting my script..." echo "Doing something..." # No parameter and variable expansion, command substitution, arithmetic expansion, or pathname expansion is performed on word. # If any part of word is quoted, the delimiter is the result of quote removal on word, and the lines in the here-document # are not expanded. So EOF is quoted as follows cat <<'EOF' >$OUT Status of backup as on $(date) Backing up files $HOME and /etc/ EOFecho "Starting backup using rsync..."
+
+You can view /tmp/output.txt with the [cat command][3]:
+`$ cat /tmp/output.txt`
+Sample outputs:
+```
+ Status of backup as on $(date)
+ Backing up files $HOME and /etc/
+
+```
+
+## A note about using tee command
+
+The syntax is:
+```
+tee /tmp/filename <<EOF >/dev/null
+line 1
+line 2
+line 3
+$(cmd)
+$var on $foo
+EOF
+```
+
+tee /tmp/filename <<EOF >/dev/null line 1 line 2 line 3 $(cmd) $var on $foo EOF
+
+Or disable variable substitution/command substitution by quoting EOF in a single quote:
+```
+tee /tmp/filename <<'EOF' >/dev/null
+line 1
+line 2
+line 3
+$(cmd)
+$var on $foo
+EOF
+```
+
+tee /tmp/filename <<'EOF' >/dev/null line 1 line 2 line 3 $(cmd) $var on $foo EOF
+
+Here is my updated script:
+```
+#!/bin/bash
+OUT=/tmp/output.txt
+ 
+echo "Starting my script..."
+echo "Doing something..."
+ 
+tee $OUT <<EOF >/dev/null
+  Status of backup as on $(date)
+  Backing up files $HOME and /etc/
+EOF
+ 
+echo "Starting backup using rsync..."
+```
+
+#!/bin/bash OUT=/tmp/output.txtecho "Starting my script..." echo "Doing something..."tee $OUT <<EOF >/dev/null Status of backup as on $(date) Backing up files $HOME and /etc/ EOFecho "Starting backup using rsync..."
+
+## A note about using in-memory here-docs
+
+Here is my updated script:
+```
+#!/bin/bash
+OUT=/tmp/output.txt
+ 
+## in memory here docs 
+## thanks https://twitter.com/freebsdfrau
+exec 9<<EOF
+  Status of backup as on $(date)
+  Backing up files $HOME and /etc/
+EOF
+ 
+## continue
+echo "Starting my script..."
+echo "Doing something..."
+ 
+## do it
+cat <&9 >$OUT
+ 
+echo "Starting backup using rsync..."
+```
+
+
+
+--------------------------------------------------------------------------------
+
+via: https://www.cyberciti.biz/faq/using-heredoc-rediection-in-bash-shell-script-to-write-to-file/
+
+作者：[Vivek Gite][a]
+译者：[译者ID](https://github.com/译者ID)
+校对：[校对者ID](https://github.com/校对者ID)
+
+本文由 [LCTT](https://github.com/LCTT/TranslateProject) 原创编译，[Linux中国](https://linux.cn/) 荣誉推出
+
+[a]:https://www.cyberciti.biz
+[1]:https://www.cyberciti.biz/media/new/faq/2017/11/redirect-output-of-here-document-to-a-text-file.jpg
+[2]:https://bash.cyberciti.biz/guide/Here_documents
+[3]:https//www.cyberciti.biz/faq/linux-unix-appleosx-bsd-cat-command-examples/ (See Linux/Unix cat command examples for more info)