How to substitute shell variables in complex text files

Question

I have several text files in which I have introduced shell variables   VAR1 or  VAR2 for instance    I would like to take those files  one by one  and save them in new files where all variables would have been replaced   To do this  I used the following shell script  found on StackOverflow    while read line do     eval echo   line   gt  gt  destination txt done  lt   source txt    This works very well on very basic files   But on more complex files  the  eval  command does too much    Lines starting with     are skipped XML files parsing results in tons of errors   Is there a better way to do it   in shell script    I know this is easily done with Ant for instance   Kind regards

User · Answer

Call the perl binary  in search and replace per line mode   the -pi   by running the perl code   the -e  in the single quotes  which iterates over the keys of the special  ENV hash containing the exported variable names as keys and the exported variable values as the keys  values and for each iteration simple replace a string containing a   lt  lt key gt  gt  with its  lt  lt value gt  gt    perl -pi -e  foreach  key sort keys  ENV   s    key  ENV  key  g   file  Caveat  An additional logic handling is required for cases in which two or more vars start with the same string

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Define your ENV variable     export MY ENV VAR congratulation    Create template file  in txt  with following content    MY ENV VAR   You can also use all other ENV variables defined by your system like  in linux   TERM   SHELL   HOME      Run this command to raplace all env-variables in your in txt file and to write the result to out txt     envsubst   printf     s      sh -c  env cut -d    -f1      lt  in txt  gt  out txt    Check the content of out txt file     cat out txt   and you should see  congratulation

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I know this topic is old  but I have a simpler working solution without export the variables  Can be a oneliner  but I prefer to split using   on line end   var1  myVar1   var2 2  var3   var1   envsubst   var1  var3   lt   source txt   gt   destination txt                                                             define which to replace   input            output   The variables need to be defined to the same line as envsubst is to get considered as environment variables   The   var1  var3  is optional to only replace the specified ones  Imagine an input file containing   VARIABLE USED BY JENKINS  which should not be replaced

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If you really only want to use bash  and sed   then I would go through each of your environment variables  as returned by set in posix mode  and build a bunch of -e  regex  for sed from that  terminated by a -e  s    a-zA-Z   a-zA-Z0-9     g   then pass all that to sed   Perl would do a nicer job though  you have access to the environment vars as an array and you can do executable replacements so you only match any environment variable once

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In reference to answer 2  when discussing envsubst  you asked      How can I make it work with the variables that are declared in my  sh script    The answer is you simply need to export your variables before calling envsubst   You can also limit the variable strings you want to replace in the input using the envsubst SHELL FORMAT argument  avoiding the unintended replacement of a string in the input with a common shell variable value - e g   HOME    For instance   export VAR1  somevalue  VAR2  someothervalue  MYVARS   VAR1  VAR2   envsubst   MYVARS   lt source txt  gt destination txt   Will replace all instances of  VAR1 and  VAR2  and only VAR1 and VAR2  in source txt with  somevalue  and  someothervalue  respectively

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Looking  it turns out on my system there is an envsubst command which is part of the gettext-base package    So  this makes it easy    envsubst  lt   source txt   gt   destination txt    Note if you want to use the same file for both  you ll have to use something like moreutil s sponge  as suggested by Johnny Utahh  envsubst  lt   source txt    sponge  source txt    Because the shell redirect will otherwise empty the file before its read

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while IFS     read -r name value   do       Print line if found variable     sed -n       quot   name  quot    p  docker-compose yml       Replace variable with value       sed -i  s     quot   name  quot      quot   value  quot     docker-compose yml done  lt   lt  env   Note  Variable name or value should not contain  quot   quot   because it is used as a delimiter

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envsubst seems exactly like something I wanted to use  but -v option surprised me a bit   While envsubst  lt  template txt was working fine  the same with option -v was not working     cat  etc redhat-release Red Hat Enterprise Linux Server release 7 1  Maipo    envsubst -V envsubst  GNU gettext-runtime  0 18 2 Copyright  C  2003-2007 Free Software Foundation  Inc  License GPLv3   GNU GPL version 3 or later  lt http   gnu org licenses gpl html gt  This is free software  you are free to change and redistribute it  There is NO WARRANTY  to the extent permitted by law  Written by Bruno Haible    As I wrote  this was not working     envsubst -v  lt  template txt envsubst  missing arguments   cat template txt   envsubst -v envsubst  missing arguments   I had to do this to make it work   TEXT  cat template txt   envsubst -v   TEXT    Maybe it helps someone

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If you want env variables to be replaced in your source files while keeping all of the non env variables as they are  you can use the following command   envsubst    printf     s      env   sed  s            lt  source txt  gt  destination txt   The syntax for replacing only specific variables is explained here  The command above is using a sub-shell to list all defined variables  and then passing it to the envsubst  So if there s a defined env variable called  NAME  and your source txt file looks like this   Hello  NAME Your balance is 123   USD    The destination txt will be   Hello Arik Your balance is 123   USD    Notice that the  NAME is replaced and the  USD is left untouched

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Export all the needed variables and then use a perl onliner   TEXT   echo   TEXT  perl -wpne  s       w       ENV  1       amp   ge     This will replace all the ENV variables present in TEXT with actual values  Quotes are also preserved

User · Answer

Actually you need to change your read to read -r which will make it ignore backslashes   Also  you should escape quotes and backslashes  So  while read -r line  do   line    line              line    line              line    line              eval echo     line    done  gt  destination txt  lt  source txt   Still a terrible way to do expansion though

[linux] How to substitute shell variables in complex text files

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