bash script to write content into file. File content requires bash variables. How to do that?

Question

The following is a piece of content my bash script will write into the nginx configuration file.

WEBAPP=example.com
APPFOLDER=abcapp
CONFFILENAME=abc.conf

read -r -d '' FILECONTENT <<'ENDFILECONTENT'
server {
        listen 80;
        client_max_body_size 2M;
        server_name $WEBAPP;
        root /var/virtual/stage.$WEBAPP/current/src/$APPFOLDER/webroot;
}

ENDFILECONTENT
echo "$FILECONTENT" > /etc/nginx/sites-available/$CONFFILENAME

The code works successfully to write the content inside /etc/nginx/sites-available/abc.conf.

However, I have two bash variables in $WEBAPP and $APPFOLDER. They are manifested exactly like that inside the file instead of example.com and abcapp which is what I intended.

How do I make the script work as intended?

Do yourself a favor and forget read exists. It is rarely necessary and often misused. — msw
– msw, Commented Sep 6, 2014 at 5:24

rici · Accepted Answer · 2013-07-29 04:28:33Z

5

bash allows newlines in quoted strings. It does parameter replacement (that is, $FOO gets replaced with the value of FOO) inside double-quoted strings ("$FOO") but not inside single-quoted strings ('$FOO').

So you could just do this:

FILECONTENT="server {
        listen 80;
        client_max_body_size 2M;
        server_name $WEBAPP;
        root /var/virtual/stage.$WEBAPP/current/src/$APPFOLDER/webroot;
}"
echo "$FILECONTENT" > /etc/nginx/sites-available/$CONFFILENAME

You don't really need the FILECONTENT parameter, since you could just copy directly to the file:

cat >/etc/nginx/sites-available/$CONFFILENAME <<ENDOFCONTENT
server {
        listen 80;
        client_max_body_size 2M;
        server_name $WEBAPP;
        root /var/virtual/stage.$WEBAPP/current/src/$APPFOLDER/webroot;
}
ENDOFCONTENT

In the second example, using <<ENDOFCONTENT indicates that the $VARs should be replaced with their values <<'ENDOFCONTENT' would prevent the parameter replacement.

edited Jul 29, 2013 at 4:28

answered Jul 29, 2013 at 4:20

rici

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1 Comment

Jeff Bowman Over a year ago

Do you mean cat instead of cp in your second example? On my machine, cat > bar <<< foo works, but cp > bar <<< foo doesn't.

Jeff Bowman · Accepted Answer · 2013-07-29 04:19:47Z

1

You're actually deliberately turning off parameter subsitution by enclosing 'ENDFILECONTENT' in quotes. See this excerpt from example 19-7 of the advanced Bash scripting guide on Heredocs, slightly reformatted:

# No parameter substitution when the "limit string" is quoted or escaped.
# Either of the following at the head of the here document would have
# the same effect.
#
# cat <<"Endofmessage"
# cat <<\Endofmessage

Remove the single quotes around 'ENDFILECONTENT' and BASH will replace the variables as expected.

answered Jul 29, 2013 at 4:19

Jeff Bowman

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