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
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..."
