How can I script file generation from a template using bash?

Question

I am trying to automate the set up of site creation for our in-house development server.

Currently, this consists of creating a system user, mysql user, database, and apache config. I know how I can do everything in a single bash file, but I wanted to ask if there was a way to more cleanly generate the apache config.

Essentially what I want to do is generate a conf file based on a template, similar to using printf. I could certainly use printf, but I thought there might be a cleaner way, using sed or awk.

The reason I don't just want to use printf is because the apache config is about 20 lines long, and will take up most of the bash script, as well as make it harder to read.

Any help is appreciated.

Keep the prototypical httpd.conf file separately, and have the script edit the template (prototypical) file into the actual file as it runs. Of course, that means two files required - the script and the template. But it keeps the two logically disjoint files actually disjoint. — Jonathan Leffler
– Jonathan Leffler, Commented Jan 12, 2012 at 19:40
Yes, that's what I was asking (sorry if it wasn't clear) but I needed to know how to do that. I think Péter Varga's answer will get me there with some tweaking. — Brendan
– Brendan, Commented Jan 12, 2012 at 20:31

Jonathan Leffler · Accepted Answer · 2012-01-12 21:40:33Z

Choose a way of marking parameters. One possibility is :parameter:, but any similar pair of markers that won't be confused with legitimate text for the template file(s) is good.

Write a sed script (in sed, awk, perl, ...) similar to the following:

sed -e "s/:param1:/$param1/g" \
    -e "s/:param2:/$param2/g" \
    -e "s/:param3:/$param3/g" \
    httpd.conf.template > $HTTPDHOME/etc/httpd.conf

If you get to a point where you need sometimes to edit something and sometimes don't, you may find it easier to create the relevant sed commands in a command file and then execute that:

{
echo "s/:param1:/$param1/g"
echo "s/:param2:/$param2/g"
echo "s/:param3:/$param3/g"
if [ "$somevariable" = "somevalue" ]
then echo "s/normaldefault/somethingspecial/g"
fi
} >/tmp/sed.$$
sed -f /tmp/sed.$$ httpd.conf.template > $HTTPDHOME/etc/httpd.conf

Note that you should use a trap to ensure the temporary doesn't outlive its usefulness:

tmp=/tmp/sed.$$   # Consider using more secure alternative schemes
trap "rm -f $tmp; exit 1" 0 1 2 3 13 15  # aka EXIT HUP INT QUIT PIPE TERM
...code above...
rm -f $tmp
trap 0

This ensures that your temporary file is removed when the script exits for most plausible signals. You can preserve a non-zero exit status from previous commands and use exit $exit_status after the trap 0 command.

tripleee · Accepted Answer · 2012-01-13 06:57:29Z

5

I'm surprised nobody mentioned here documents. This is probably not what the OP wants, but certainly a way to improve legibility of the script you started out with. Just take care to escape or parametrize away any constructs which the shell will perform substitutions on.

#!/bin/sh
# For example's sake, a weird value
# This is in single quotes, to prevent substitution
literal='$%"?*=`!!'
user=me

cat <<HERE >httpd.conf
# Not a valid httpd.conf
User=${user}
Uninterpolated=${literal}
Escaped=\$dollar
HERE

In this context I would recommend ${variable} over the equivalent $variable for clarity and to avoid any possible ambiguity.

answered Jan 13, 2012 at 6:57

tripleee

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P Varga · Accepted Answer · 2012-01-12 19:41:23Z

2

Use sed like for example

sed s/%foo%/$foo/g template.conf > $newdir/httpd.conf

answered Jan 12, 2012 at 19:41

P Varga

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2 Comments

Brendan Over a year ago

How would I replace multiple markers with multiple different values? i.e. %foo% replaced with $foo and %bar% replaced with $bar.

tripleee Over a year ago

Easy: sed "s/%foo%/$foo/g;s/%bar%/$bar/g"

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