diff --git a/sources/tech/20171116 How to use a here documents to write data to a file in bash script.md b/sources/tech/20171116 How to use a here documents to write data to a file in bash script.md new file mode 100644 index 0000000000..12d15af78f --- /dev/null +++ b/sources/tech/20171116 How to use a here documents to write data to a file in bash script.md @@ -0,0 +1,203 @@ +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 < 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 $foo +EOF +``` + +## Examples + +The following script will write the needed contents to a file named /tmp/output.txt: +``` +#!/bin/bash +OUT=/tmp/output.txt + +echo "Starting my script..." +echo "Doing something..." + +cat <$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 $HOME and /etc/ + +``` + +## A note about using tee command + +The syntax is: +``` +tee /tmp/filename </dev/null +line 1 +line 2 +line 3 +$(cmd) +$var on $foo +EOF +``` + +tee /tmp/filename </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 </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 </dev/null Status of backup as on $(date) Backing up files $HOME and /etc/ EOFecho "Starting backup using rsync..." + +## A note about using in-memory here-docs + +Here is my updated script: +``` +#!/bin/bash +OUT=/tmp/output.txt + +## in memory here docs +## thanks https://twitter.com/freebsdfrau +exec 9<$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)