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

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 2

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 3

Note:

• the following condenses and organizes other answers in this thread, esp the excellent work of Stefan Lasiewski and Serge Stroobandt
• Lasiewski and I recommend Ch 19 (Here Documents) in the Advanced Bash-Scripting Guide

The question (how to write a here document (aka heredoc) to a file in a bash script?) has (at least) 3 main independent dimensions or subquestions:

1. Do you want to overwrite an existing file, append to an existing file, or write to a new file?
2. Does your user or another user (e.g., root) own the file?
3. Do you want to write the contents of your heredoc literally, or to have bash interpret variable references inside your heredoc?

(There are other dimensions/subquestions which I don't consider important. Consider editing this answer to add them!) Here are some of the more important combinations of the dimensions of the question listed above, with various different delimiting identifiers--there's nothing sacred about EOF, just make sure that the string you use as your delimiting identifier does not occur inside your heredoc:

1. To overwrite an existing file (or write to a new file) that you own, substituting variable references inside the heredoc:

   cat << EOF > /path/to/your/file
   This line will write to the file.
   ${THIS} will also write to the file, with the variable contents substituted.
   EOF
   

2. To append an existing file (or write to a new file) that you own, substituting variable references inside the heredoc:

   cat << FOE >> /path/to/your/file
   This line will write to the file.
   ${THIS} will also write to the file, with the variable contents substituted.
   FOE
   

3. To overwrite an existing file (or write to a new file) that you own, with the literal contents of the heredoc:

   cat << 'END_OF_FILE' > /path/to/your/file
   This line will write to the file.
   ${THIS} will also write to the file, without the variable contents substituted.
   END_OF_FILE
   

4. To append an existing file (or write to a new file) that you own, with the literal contents of the heredoc:

   cat << 'eof' >> /path/to/your/file
   This line will write to the file.
   ${THIS} will also write to the file, without the variable contents substituted.
   eof
   

5. To overwrite an existing file (or write to a new file) owned by root, substituting variable references inside the heredoc:

   cat << until_it_ends | sudo tee /path/to/your/file
   This line will write to the file.
   ${THIS} will also write to the file, with the variable contents substituted.
   until_it_ends
   

6. To append an existing file (or write to a new file) owned by user=foo, with the literal contents of the heredoc:

   cat << 'Screw_you_Foo' | sudo -u foo tee -a /path/to/your/file
   This line will write to the file.
   ${THIS} will also write to the file, without the variable contents substituted.
   Screw_you_Foo
   

Answer 4

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 5

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 6

For future people who may have this issue the following format worked:

(cat <<- _EOF_
        LogFile /var/log/clamd.log
        LogTime yes
        DatabaseDirectory /var/lib/clamav
        LocalSocket /tmp/clamd.socket
        TCPAddr 127.0.0.1
        SelfCheck 1020
        ScanPDF yes
        _EOF_
) > /etc/clamd.conf

Answer 7

For those looking for a pure bash solution (or a need for speed), here's a simple solution without cat:

# here-doc tab indented
{ read -r -d '' || printf >file '%s' "$REPLY"; } <<-EOF
        foo bar
EOF

or for an easy "mycat" function (and avoid leaving REPLY in environment):

mycat() {
  local REPLY
  read -r -d '' || printf '%s' "$REPLY"
}
mycat >file <<-EOF
        foo bar
EOF

Quick speed comparison of "mycat" vs OS cat (1000 loops >/dev/null on my OSX laptop):

mycat:
real    0m1.507s
user    0m0.108s
sys     0m0.488s

OS cat:
real    0m4.082s
user    0m0.716s
sys     0m1.808s

NOTE: mycat doesn't handle file arguments, it just handles the problem "write a heredoc to a file"

Answer 8

As instance you could use it:

First(making ssh connection):

while read pass port user ip files directs; do
    sshpass -p$pass scp -o 'StrictHostKeyChecking no' -P $port $files $user@$ip:$directs
done <<____HERE
    PASS    PORT    USER    IP    FILES    DIRECTS
      .      .       .       .      .         .
      .      .       .       .      .         .
      .      .       .       .      .         .
    PASS    PORT    USER    IP    FILES    DIRECTS
____HERE

Second(executing commands):

while read pass port user ip; do
    sshpass -p$pass ssh -p $port $user@$ip <<ENDSSH1
    COMMAND 1
    .
    .
    .
    COMMAND n
ENDSSH1
done <<____HERE
    PASS    PORT    USER    IP
      .      .       .       .
      .      .       .       .
      .      .       .       .
    PASS    PORT    USER    IP    
____HERE

Third(executing commands):

Script=$'
#Your commands
'

while read pass port user ip; do
    sshpass -p$pass ssh -o 'StrictHostKeyChecking no' -p $port $user@$ip "$Script"

done <<___HERE
PASS    PORT    USER    IP
  .      .       .       .
  .      .       .       .
  .      .       .       .
PASS    PORT    USER    IP  
___HERE

Forth(using variables):

while read pass port user ip fileoutput; do
    sshpass -p$pass ssh -o 'StrictHostKeyChecking no' -p $port $user@$ip fileinput=$fileinput 'bash -s'<<ENDSSH1
    #Your command > $fileinput
    #Your command > $fileinput
ENDSSH1
done <<____HERE
    PASS    PORT    USER    IP      FILE-OUTPUT
      .      .       .       .          .
      .      .       .       .          .
      .      .       .       .          .
    PASS    PORT    USER    IP      FILE-OUTPUT
____HERE

Answer 9

I like the following method of basic redirection for its concision, readability and presentation in an indented script:

<<-End_of_file >file
→       foo bar
End_of_file

Where →        is a tab character.

This is standard Bourne shell redirection without forking any cat or tee process. But it is not working with current bash even when called through /bin/sh. It is still working with /bin/zsh since more than 20 years.

Answer 10

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 11

In addition, if you're writing to a file, it can be a good idea to check whether or not your write succeeded for failed. For example:

if ! echo "contents" > ./file ; then
    echo "ERROR: failed to write to file" >& 2
    exit 1
fi

To do the same with heredoc, there are two possible approaches.

1)

if ! cat > ./file << EOF
contents
EOF
then
    echo "ERROR: failed to write to file" >& 2
    exit 1
fi

if ! cat > ./file ; then
    echo "ERROR: failed to write to file" >& 2
    exit 1
fi << EOF
contents
EOF

You can test the error case in the above code by replacing the destination file ./file with /file (assuming you're not running as root).