Bash Templating: How to build configuration files from templates with Bash?

Try envbust

Try envsubst

$ cat envsubst-template.txt
Variable FOO is (${FOO}).
Variable BAR is (${BAR}).

$ FOO=myfoo

$ BAR=mybar

$ export FOO BAR

$ cat envsubst-template.txt | envsubst
Variable FOO is (myfoo).
Variable BAR is (mybar).

---

A heredoc is a builtin way to template a conf file.

STATUS_URI="/hows-it-goin";  MONITOR_IP="10.10.2.15";

cat >/etc/apache2/conf.d/mod_status.conf <<EOF
<Location ${STATUS_URI}>
    SetHandler server-status
    Order deny,allow
    Deny from all
    Allow from ${MONITOR_IP}
</Location>
EOF

Regarding yottsa's answer: envsubst was new to me. Fantastic.

---

You can use this:

perl -p -i -e 's/\$\{([^}]+)\}/defined $ENV{$1} ? $ENV{$1} : $&/eg' < template.txt

to replace all ${...} strings with corresponding enviroment variables (do not forget to export them before running this script).

For pure bash this should work (assuming that variables do not contain ${...} strings):

#!/bin/bash
while read -r line ; do
    while [[ "$line" =~ (\$\{[a-zA-Z_][a-zA-Z_0-9]*\}) ]] ; do
        LHS=${BASH_REMATCH[1]}
        RHS="$(eval echo "\"$LHS\"")"
        line=${line//$LHS/$RHS}
    done
    echo "$line"
done

. Solution that does not hang if RHS references some variable that references itself:

#!/bin/bash
line="$(cat; echo -n a)"
end_offset=${#line}
while [[ "${line:0:$end_offset}" =~ (.*)(\$\{([a-zA-Z_][a-zA-Z_0-9]*)\})(.*) ]] ; do
    PRE="${BASH_REMATCH[1]}"
    POST="${BASH_REMATCH[4]}${line:$end_offset:${#line}}"
    VARNAME="${BASH_REMATCH[3]}"
    eval 'VARVAL="$'$VARNAME'"'
    line="$PRE$VARVAL$POST"
    end_offset=${#PRE}
done
echo -n "${line:0:-1}"

WARNING: I do not know a way to correctly handle input with NULs in bash or preserve the amount of trailing newlines. Last variant is presented as it is because shells “love” binary input:

read will interpret backslashes. read -r will not interpret backslashes, but still will drop the last line if it does not end with a newline. "$(…)" will strip as many trailing newlines as there are present, so I end … with ; echo -n a and use echo -n "${line:0:-1}": this drops the last character (which is a) and preserves as many trailing newlines as there was in the input (including no).

---

I agree with using sed: it is the best tool for search/replace. Here is my approach:

$ cat template.txt
the number is ${i}
the dog's name is ${name}

$ cat replace.sed
s/${i}/5/
s/${name}/Fido/

$ sed -f replace.sed template.txt > out.txt

$ cat out.txt
the number is 5
the dog's name is Fido

---

I have a bash solution like mogsie but with heredoc instead of herestring to allow you to avoid escaping double quotes

eval "cat <<EOF
$(<template.txt)
EOF
" 2> /dev/null

---

Try eval

I think eval works really well. It handles templates with linebreaks, whitespace, and all sorts of bash stuff. If you have full control over the templates themselves of course:

$ cat template.txt
variable1 = ${variable1}
variable2 = $variable2
my-ip = \"$(curl -s ifconfig.me)\"

$ echo $variable1
AAA
$ echo $variable2
BBB
$ eval "echo \"$(<template.txt)\"" 2> /dev/null
variable1 = AAA
variable2 = BBB
my-ip = "11.22.33.44"

This method should be used with care, of course, since eval can execute arbitrary code. Running this as root is pretty much out of the question. Quotes in the template need to be escaped, otherwise they will be eaten by eval.

You can also use here documents if you prefer cat to echo

$ eval "cat <<< \"$(<template.txt)\"" 2> /dev/null

@plockc provoded a solution that avoids the bash quote escaping issue:

$ eval "cat <<EOF
$(<template.txt)
EOF
" 2> /dev/null

Edit: Removed part about running this as root using sudo...

Edit: Added comment about how quotes need to be escaped, added plockc's solution to the mix!

---

Edit Jan 6, 2017

I needed to keep double quotes in my configuration file so double escaping double quotes with sed helps:

render_template() {
  eval "echo \"$(sed 's/\"/\\\\"/g' $1)\""
}

I can't think of keeping trailing new lines, but empty lines in between are kept.

Although it is an old topic, IMO I found out more elegant solution here: http://pempek.net/articles/2013/07/08/bash-sh-as-template-engine/

#!/bin/sh

# render a template configuration file
# expand variables + preserve formatting
render_template() {
  eval "echo \"$(cat $1)\""
}

user="Gregory"
render_template /path/to/template.txt > path/to/configuration_file

All credits to Grégory Pakosz.

---

Instead of reinventing the wheel go with envsubst Can be used in almost any scenario, for instance building configuration files from environment variables in docker containers.

If on mac make sure you have homebrew then link it from gettext:

brew install gettext
brew link --force gettext

./template.cfg

# We put env variables into placeholders here
this_variable_1 = ${SOME_VARIABLE_1}
this_variable_2 = ${SOME_VARIABLE_2}

./.env:

SOME_VARIABLE_1=value_1
SOME_VARIABLE_2=value_2

./configure.sh

#!/bin/bash
cat template.cfg | envsubst > whatever.cfg

Now just use it:

# make script executable
chmod +x ./configure.sh
# source your variables
. .env
# export your variables
# In practice you may not have to manually export variables 
# if your solution depends on tools that utilise .env file 
# automatically like pipenv etc. 
export SOME_VARIABLE_1 SOME_VARIABLE_2
# Create your config file
./configure.sh

---

I'd have done it this way, probably less efficient, but easier to read/maintain.

TEMPLATE='/path/to/template.file'
OUTPUT='/path/to/output.file'

while read LINE; do
  echo $LINE |
  sed 's/VARONE/NEWVALA/g' |
  sed 's/VARTWO/NEWVALB/g' |
  sed 's/VARTHR/NEWVALC/g' >> $OUTPUT
done < $TEMPLATE

---

If you want to use Jinja2 templates, see this project: j2cli.

It supports:

Templates from JSON, INI, YAML files and input streams

Templating from environment variables

---

A longer but more robust version of the accepted answer:

perl -pe 's;(\\*)(\$([a-zA-Z_][a-zA-Z_0-9]*)|\$\{([a-zA-Z_][a-zA-Z_0-9]*)\})?;substr($1,0,int(length($1)/2)).($2&&length($1)%2?$2:$ENV{$3||$4});eg' template.txt

This expands all instances of $VAR or ${VAR} to their environment values (or, if they're undefined, the empty string).

It properly escapes backslashes, and accepts a backslash-escaped $ to inhibit substitution (unlike envsubst, which, it turns out, doesn't do this).

So, if your environment is:

FOO=bar
BAZ=kenny
TARGET=backslashes
NOPE=engi

and your template is:

Two ${TARGET} walk into a \\$FOO. \\\\
\\\$FOO says, "Delete C:\\Windows\\System32, it's a virus."
$BAZ replies, "\${NOPE}s."

the result would be:

Two backslashes walk into a \bar. \\
\$FOO says, "Delete C:\Windows\System32, it's a virus."
kenny replies, "${NOPE}s."

If you only want to escape backslashes before $ (you could write "C:\Windows\System32" in a template unchanged), use this slightly-modified version:

perl -pe 's;(\\*)(\$([a-zA-Z_][a-zA-Z_0-9]*)|\$\{([a-zA-Z_][a-zA-Z_0-9]*)\});substr($1,0,int(length($1)/2)).(length($1)%2?$2:$ENV{$3||$4});eg' template.txt

---

Here is another solution: generate a bash script with all the variables and the contents of the template file, that script would look like this:

word=dog           
i=1                
cat << EOF         
the number is ${i} 
the word is ${word}

EOF                

If we feed this script into bash it would produce the desired output:

the number is 1
the word is dog

Here is how to generate that script and feed that script into bash:

(
    # Variables
    echo word=dog
    echo i=1

    # add the template
    echo "cat << EOF"
    cat template.txt
    echo EOF
) | bash

Discussion

The parentheses opens a sub shell, its purpose is to group together all the output generated

Within the sub shell, we generate all the variable declarations

Also in the sub shell, we generate the cat command with HEREDOC

Finally, we feed the sub shell output to bash and produce the desired output

If you want to redirect this output into a file, replace the last line with: ) | bash > output.txt

---

Here's another pure bash solution:

it's using heredoc, so: complexity doesn't increase because of additionaly required syntax template can include bash code that also allows you to indent stuff properly. See below.

complexity doesn't increase because of additionaly required syntax

template can include bash code that also allows you to indent stuff properly. See below.

that also allows you to indent stuff properly. See below.

it doesn't use eval, so: no problems with the rendering of trailing empty lines no problems with quotes in the template

no problems with the rendering of trailing empty lines

no problems with quotes in the template

$ cat code

#!/bin/bash
LISTING=$( ls )

cat_template() {
  echo "cat << EOT"
  cat "$1"
  echo EOT
}

cat_template template | LISTING="$LISTING" bash

Input:
$ cat template (with trailing newlines and double quotes)

<html>
  <head>
  </head>
  <body> 
    <p>"directory listing"
      <pre>
$( echo "$LISTING" | sed 's/^/        /' )
      <pre>
    </p>
  </body>
</html>

Output:

<html>
  <head>
  </head>
  <body> 
    <p>"directory listing"
      <pre>
        code
        template
      <pre>
    </p>
  </body>
</html>

---

Taking the answer from ZyX using pure bash but with new style regex matching and indirect parameter substitution it becomes:

#!/bin/bash
regex='\$\{([a-zA-Z_][a-zA-Z_0-9]*)\}'
while read line; do
    while [[ "$line" =~ $regex ]]; do
        param="${BASH_REMATCH[1]}"
        line=${line//${BASH_REMATCH[0]}/${!param}}
    done
    echo $line
done

---

If using Perl is an option and you're content with basing expansions on environment variables only (as opposed to all shell variables), consider Stuart P. Bentley's robust answer.

This answer aims to provide a bash-only solution that - despite use of eval - should be safe to use.

The goals are:

Support expansion of both ${name} and $name variable references.

Prevent all other expansions: command substitutions ($(...) and legacy syntax `...`) arithmetic substitutions ($((...)) and legacy syntax $[...]).

command substitutions ($(...) and legacy syntax `...`)

arithmetic substitutions ($((...)) and legacy syntax $[...]).

Allow selective suppression of variable expansion by prefixing with \ (\${name}).

Preserve special chars. in the input, notably " and \ instances.

Allow input either via arguments or via stdin.

Function expandVars():

expandVars() {
  local txtToEval=$* txtToEvalEscaped
  # If no arguments were passed, process stdin input.
  (( $# == 0 )) && IFS= read -r -d '' txtToEval
  # Disable command substitutions and arithmetic expansions to prevent execution
  # of arbitrary commands.
  # Note that selectively allowing $((...)) or $[...] to enable arithmetic
  # expressions is NOT safe, because command substitutions could be embedded in them.
  # If you fully trust or control the input, you can remove the `tr` calls below
  IFS= read -r -d '' txtToEvalEscaped < <(printf %s "$txtToEval" | tr '`([' '\1\2\3')
  # Pass the string to `eval`, escaping embedded double quotes first.
  # `printf %s` ensures that the string is printed without interpretation
  # (after processing by by bash).
  # The `tr` command reconverts the previously escaped chars. back to their
  # literal original.
  eval printf %s "\"${txtToEvalEscaped//\"/\\\"}\"" | tr '\1\2\3' '`(['
}

Examples:

$ expandVars '\$HOME="$HOME"; `date` and $(ls)'
$HOME="/home/jdoe"; `date` and $(ls)  # only $HOME was expanded

$ printf '\$SHELL=${SHELL}, but "$(( 1 \ 2 ))" will not expand' | expandVars
$SHELL=/bin/bash, but "$(( 1 \ 2 ))" will not expand # only ${SHELL} was expanded

For performance reasons, the function reads stdin input all at once into memory, but it's easy to adapt the function to a line-by-line approach.

Also supports non-basic variable expansions such as ${HOME:0:10}, as long as they contain no embedded command or arithmetic substitutions, such as ${HOME:0:$(echo 10)} Such embedded substitutions actually BREAK the function (because all $( and ` instances are blindly escaped). Similarly, malformed variable references such as ${HOME (missing closing }) BREAK the function.

Such embedded substitutions actually BREAK the function (because all $( and ` instances are blindly escaped).

Similarly, malformed variable references such as ${HOME (missing closing }) BREAK the function.

Due to bash's handling of double-quoted strings, backslashes are handled as follows: \$name prevents expansion. A single \ not followed by $ is preserved as is. If you want to represent multiple adjacent \ instances, you must double them; e.g.: \\ -> \ - the same as just \ \\\\ -> \\ The input mustn't contain the following (rarely used) characters, which are used for internal purposes: 0x1, 0x2, 0x3.

\$name prevents expansion.

A single \ not followed by $ is preserved as is.

If you want to represent multiple adjacent \ instances, you must double them; e.g.: \\ -> \ - the same as just \ \\\\ -> \\

\\ -> \ - the same as just \

\\\\ -> \\

The input mustn't contain the following (rarely used) characters, which are used for internal purposes: 0x1, 0x2, 0x3.

There's a largely hypothetical concern that if bash should introduce new expansion syntax, this function might not prevent such expansions - see below for a solution that doesn't use eval.

If you're looking for a more restrictive solution that only supports ${name} expansions - i.e., with mandatory curly braces, ignoring $name references - see this answer of mine.

Here is an improved version of the bash-only, eval-free solution from the accepted answer:

The improvements are:

Support for expansion of both ${name} and $name variable references.

Support for \-escaping variable references that shouldn't be expanded.

Unlike the eval-based solution above, non-basic expansions are ignored malformed variable references are ignored (they don't break the script)

non-basic expansions are ignored

malformed variable references are ignored (they don't break the script)

 IFS= read -d '' -r lines # read all input from stdin at once
 end_offset=${#lines}
 while [[ "${lines:0:end_offset}" =~ (.*)\$(\{([a-zA-Z_][a-zA-Z_0-9]*)\}|([a-zA-Z_][a-zA-Z_0-9]*))(.*) ]] ; do
      pre=${BASH_REMATCH[1]} # everything before the var. reference
      post=${BASH_REMATCH[5]}${lines:end_offset} # everything after
      # extract the var. name; it's in the 3rd capture group, if the name is enclosed in {...}, and the 4th otherwise
      [[ -n ${BASH_REMATCH[3]} ]] && varName=${BASH_REMATCH[3]} || varName=${BASH_REMATCH[4]}
      # Is the var ref. escaped, i.e., prefixed with an odd number of backslashes?
      if [[ $pre =~ \\+$ ]] && (( ${#BASH_REMATCH} % 2 )); then
           : # no change to $lines, leave escaped var. ref. untouched
      else # replace the variable reference with the variable's value using indirect expansion
           lines=${pre}${!varName}${post}
      fi
      end_offset=${#pre}
 done
 printf %s "$lines"

---

Try shtpl

Perfect case for shtpl. (project of mine, so it is not widely in use and lacks in documentation. But here is the solution it offers anyhow. May you want to test it.)

Just execute:

$ i=1 word=dog sh -c "$( shtpl template.txt )"

Result is:

the number is 1
the word is dog

Have fun.

---

This page describes an answer with awk

awk '{while(match($0,"[$]{[^}]*}")) {var=substr($0,RSTART+2,RLENGTH -3);gsub("[$]{"var"}",ENVIRON[var])}}1' < input.txt > output.txt

---

# Usage: template your_file.conf.template > your_file.conf
template() {
        local IFS line
        while IFS=$'\n\r' read -r line ; do
                line=${line//\\/\\\\}         # escape backslashes
                line=${line//\"/\\\"}         # escape "
                line=${line//\`/\\\`}         # escape `
                line=${line//\$/\\\$}         # escape $
                line=${line//\\\${/\${}       # de-escape ${         - allows variable substitution: ${var} ${var:-default_value} etc
                # to allow arithmetic expansion or command substitution uncomment one of following lines:
#               line=${line//\\\$\(/\$\(}     # de-escape $( and $(( - allows $(( 1 + 2 )) or $( command ) - UNSECURE
#               line=${line//\\\$\(\(/\$\(\(} # de-escape $((        - allows $(( 1 + 2 ))
                eval "echo \"${line}\"";
        done < "$1"
}

This is the pure bash function adjustable to your liking, used in production and should not break on any input. If it breaks - let me know.

---

To follow up on plockc's answer on this page, here is a dash-suitable version, for those of you looking to avoid bashisms.

eval "cat <<EOF >outputfile
$( cat template.in )
EOF
" 2> /dev/null

---

You can also use bashible (which internally uses the evaluating approach described above/below).

There is an example, how to generate a HTML from multiple parts:

https://github.com/mig1984/bashible/tree/master/examples/templates

---

Look at simple variables substitution python script here: https://github.com/jeckep/vsubst

It is very simple to use:

python subst.py --props secure.properties --src_path ./templates --dst_path ./dist

---

Here's a bash function that preserves whitespace:

# Render a file in bash, i.e. expand environment variables. Preserves whitespace.
function render_file () {
    while IFS='' read line; do
        eval echo \""${line}"\"
    done < "${1}"
}

---

Here's a modified perl script based on a few of the other answers:

perl -pe 's/([^\\]|^)\$\{([a-zA-Z_][a-zA-Z_0-9]*)\}/$1.$ENV{$2}/eg' -i template

Features (based on my needs, but should be easy to modify):

Skips escaped parameter expansions (e.g. \${VAR}).

Supports parameter expansions of the form ${VAR}, but not $VAR.

Replaces ${VAR} with a blank string if there is no VAR envar.

Only supports a-z, A-Z, 0-9 and underscore characters in the name (excluding digits in the first position).

---

You can also use printf to fill a template.

#!/bin/bash

IFS='' read -rd '' TEMPL <<-'EOB'
The number is %d
The word is "%s"
Birds of Massachusetts:
    %s




EOB

N=12
WORD="Bird"
MULTILINE="Eastern Bluebirds
Common Grackles"

echo "START"
printf "${TEMPL}" ${N} ${WORD} "${MULTILINE}"
echo "END"

Here's the output, with quotes and whitespace intact:

START
The number is 12
The word is "Bird"
Birds of Massachusetts:
    Eastern Bluebirds
Common Grackles




END

---