5.5 KiB
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.
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 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:
$ 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:
$ 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 </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 </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 译者：译者ID 校对：校对者ID