Variables as commands in bash scripts

Question

I am writing a very simple bash script that tars a given directory  encrypts the output of that  and then splits the resultant file into multiple smaller files since the backup media don t support huge files   I don t have a lot of experience with bash scripting  I believe I m having issues with quoting my variables properly to allow spaces in the parameters  The script follows       bin bash    This script tars the given directory  encrypts it  and transfers   it to the given directory  likely a USB key    if      -ne 2   then     echo  Usage   basename  0  DIRECTORY BACKUP DIRECTORY      exit 1 fi  DIRECTORY  1 BACKUP DIRECTORY  2 BACKUP FILE   BACKUP DIRECTORY  date   Y- m- dT H- M- S backup    TAR CMD  tar cv  DIRECTORY  SPLIT CMD  split -b 1024m -    BACKUP FILE     ENCRYPT CMD  openssl des3 -salt   echo   TAR CMD    ENCRYPT CMD    SPLIT CMD    TAR CMD    ENCRYPT CMD    SPLIT CMD   say  Done backing up    Running this command fails with      split   foo 2009-04-27T14-32-04 backup aa  No such file or directory   I can fix it by removing the quotes around  BACKUP FILE where I set  SPLIT CMD  But  if I have a space in the name of my backup directory it doesn t work  Also  if I copy and paste the output from the  echo  command directly into the terminal it works fine  Clearly there s something I don t understand about how Bash is escaping things

User · Answer

Quoting spaces inside variables such that the shell will re-interpret things properly is hard. It's this type of thing that prompts me to reach for a stronger language. Whether that's perl or python or ruby or whatever (I choose perl, but that's not always for everyone), it's just something that will allow you to bypass the shell for quoting.

It's not that I've never managed to get it right with liberal doses of eval, but just that eval gives me the eebie-jeebies (becomes a whole new headache when you want to take user input and eval it, though in this case you'd be taking stuff that you wrote and evaling that instead), and that I've gotten headaches in debugging.

With perl, as my example, I'd be able to do something like:

@tar_cmd = ( qw(tar cv), $directory );
@encrypt_cmd = ( qw(openssl des3 -salt) );
@split_cmd = ( qw(split -b 1024m -), $backup_file );

The hard part here is doing the pipes - but a bit of IO::Pipe, fork, and reopening stdout and stderr, and it's not bad. Some would say that's worse than quoting the shell properly, and I understand where they're coming from, but, for me, this is easier to read, maintain, and write. Heck, someone could take the hard work out of this and create a IO::Pipeline module and make the whole thing trivial ;-)

User · Answer

Simply don t put whole commands in variables  You ll get into a lot of trouble trying to recover quoted arguments   Also    Avoid using all-capitals variable names in scripts  Easy way to shoot yourself on the foot  Don t use backquotes  use        instead  it nests better          bin bash  if      -ne 2   then     echo  Usage    basename  0  DIRECTORY BACKUP DIRECTORY      exit 1 fi  directory  1 backup directory  2 current date   date   Y- m- dT H- M- S  backup file    backup directory    current date  backup   tar cv   directory    openssl des3 -salt   split -b 1024m -   backup file

User · Answer

I am not sure  but it might be worth running an eval on the commands first   This will let bash expand the variables  TAR CMD and such to their full breadth just as the echo command does to the console  which you say works   Bash will then read the line a second time with the variables expanded   eval  TAR CMD    ENCRYPT CMD    SPLIT CMD    I just did a Google search and this page looks like it might do a decent job at explaining why that is needed  http   fvue nl wiki Bash  Why use eval with variable expansion 3F

User · Answer

There is a point to only put commands and options in variables       bin bash  if      -ne 2   then     echo  Usage   basename  0  DIRECTORY BACKUP DIRECTORY      exit 1 fi    standard tools      directory  1 backup directory  2 current date   date   Y- m- dT H- M- S  backup file    backup directory    current date  backup     tar create     directory       openssl      split 1024    backup file    You can relocate the commands to another file you source  so you can reuse the same commands and options across many scripts    This is very handy when you have a lot of scripts and you want to control how they all use tools   So standard tools would contain   export tar create  tar cv  export openssl  openssl des3 -salt  export split 1024  split -b 1024m -

User · Answer

eval is not an acceptable practice if your directory names can be generated by untrusted sources  See BashFAQ  48 for more on why eval should not be used  and BashFAQ  50 for more on the root cause of this problem and its proper solutions  some of which are touched on below   If you need to build up your commands over time  use arrays   tar cmd   tar cv   directory    split cmd   split -b 1024m -   backup file    encrypt cmd   openssl des3 -salt      tar cmd           encrypt cmd           split cmd        Alternately  if this is just about defining your commands in one central place  use functions   tar cmd     tar cv   directory     split cmd     split -b 1024m -   backup file     encrypt cmd     openssl des3 -salt    tar cmd   split cmd   encrypt cmd

[bash] Variables as commands in bash scripts

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