How can I write a heredoc to a file in Bash script?

Question

How can I write a here document to a file in Bash script?

Answer 1

Instead of using cat and I/O redirection it might be useful to use tee instead:

tee newfile <<EOF
line 1
line 2
line 3
EOF

It's more concise, plus unlike the redirect operator it can be combined with sudo if you need to write to files with root permissions.

Answer 2

To build on @Livven's answer, here are some useful combinations.

1. variable substitution, leading tab retained, overwrite file, echo to stdout

   tee /path/to/file <<EOF
   ${variable}
   EOF
   

2. no variable substitution, leading tab retained, overwrite file, echo to stdout

   tee /path/to/file <<'EOF'
   ${variable}
   EOF
   

3. variable substitution, leading tab removed, overwrite file, echo to stdout

   tee /path/to/file <<-EOF
       ${variable}
   EOF
   

4. variable substitution, leading tab retained, append to file, echo to stdout

   tee -a /path/to/file <<EOF
   ${variable}
   EOF
   

5. variable substitution, leading tab retained, overwrite file, no echo to stdout

   tee /path/to/file <<EOF >/dev/null
   ${variable}
   EOF
   

6. the above can be combined with sudo as well

   sudo -u USER tee /path/to/file <<EOF
   ${variable}
   EOF
   

Answer 3

Read the Advanced Bash-Scripting Guide Chapter 19. Here Documents.

Here's an example which will write the contents to a file at /tmp/yourfilehere

cat << EOF > /tmp/yourfilehere
These contents will be written to the file.
        This line is indented.
EOF

Note that the final 'EOF' (The LimitString) should not have any whitespace in front of the word, because it means that the LimitString will not be recognized.

In a shell script, you may want to use indentation to make the code readable, however this can have the undesirable effect of indenting the text within your here document. In this case, use <<- (followed by a dash) to disable leading tabs (Note that to test this you will need to replace the leading whitespace with a tab character, since I cannot print actual tab characters here.)

#!/usr/bin/env bash

if true ; then
    cat <<- EOF > /tmp/yourfilehere
    The leading tab is ignored.
    EOF
fi

If you don't want to interpret variables in the text, then use single quotes:

cat << 'EOF' > /tmp/yourfilehere
The variable $FOO will not be interpreted.
EOF

To pipe the heredoc through a command pipeline:

cat <<'EOF' |  sed 's/a/b/'
foo
bar
baz
EOF

Output:

foo
bbr
bbz

... or to write the the heredoc to a file using sudo:

cat <<'EOF' |  sed 's/a/b/' | sudo tee /etc/config_file.conf
foo
bar
baz
EOF

Answer 4

When root permissions are required

When root permissions are required for the destination file, use |sudo tee instead of >:

cat << 'EOF' |sudo tee /tmp/yourprotectedfilehere
The variable $FOO will *not* be interpreted.
EOF

cat << "EOF" |sudo tee /tmp/yourprotectedfilehere
The variable $FOO *will* be interpreted.
EOF

Answer 5

If you want to keep the heredoc indented for readability:

$ perl -pe 's/^\s*//' << EOF
     line 1
     line 2
EOF

The built-in method for supporting indented heredoc in Bash only supports leading tabs, not spaces.

Perl can be replaced with awk to save a few characters, but the Perl one is probably easier to remember if you know basic regular expressions.

Answer 6

I like this method for concision, readability and presentation in an indented script:

<<-End_of_file >file
→       foo bar
End_of_file

Where →        is a tab character.