0

I'm trying to write a Bash shell script that copies all the files in the current directory, minus a few exceptions, into another directory. The script builds a command, stores it in a variable, and runs it. Here's the baffling part: when the command it builds is run in a shell script, it fails with the message find: paths must precede expression: `|xargs' If I remove the space before the |, I instead see find: unknown predicate `-i' . But when I echo the command it builds and run it, it works fine! To make it even weirder, echo "$cmd"|bash fails with the same error message!

The script (synchronize.sh):

#! /bin/bash

EXCLUDED_FILES=(folder1 file.txt synchronize.sh)
FLAGS='-r'
PROJECT_NAME=project

TARGET_DIR=$HOME/Documents/IDE/$PROJECT_NAME/assets
cmd='find ./* -maxdepth 0'
cmd_end="|xargs -i cp -r {} -t $TARGET_DIR"

for file in ${EXCLUDED_FILES[@]}
do
    cmd+=" ! -name \"$file\""
done

cmd+=$cmd_end

$cmd               #Running the command directly fails
#echo  $cmd        #Yet copying and pasting the output of this command will work just fine
#echo "$cmd"|bash  #Though this inexplicably fails, with the same message as just $cmd

None of the files or folders have spaces in their name, or any special characters except underscores and periods.

VHS
  • 101
  • 5
  • 2
    (1) Your code does not depend on any input not known in advance, right? It could be a plain `find … | xargs …` line without this `cmd` contraption. But if you want it then: [How can we run a command stored in a variable?](https://unix.stackexchange.com/q/444946/108618) (2) "None of the files or folders have spaces in their name, or any special characters except underscores and periods." – Don't let it be your excuse. It's a virtue to do things in the right way anyway. [Why does my shell script choke on whitespace or other special characters?](https://unix.stackexchange.com/q/131766/108618) – Kamil Maciorowski May 03 '22 at 05:36
  • Thank you SO much for that first linked answer! I hardly ever write any shell scripts, and all the idiosyncrasies and minor distinctions are really confusing to someone who doesn't write much Bash and only uses the terminal for simple tasks like running Make, moving files, wget'ing a bunch of images, and opening a file browser to a deep subfolder. – VHS May 03 '22 at 07:10
  • This question is deceivingly tricky. I ended up spending some time poking through this after I posted a pretty lousy response. I assume you came to the same conclusion that I did -- concatenating variables in the `find` command is...not ideal. I ended up rewriting your script using an array comparison just because I was curious how the workflow looked. Since it's off-topic to the subject of this question I'm just adding a gist. https://gist.github.com/iamwpj/12cb157d82578a2383b28ec1fa259c3e – iamwpj May 06 '22 at 08:00

2 Answers2

-1

So, in the answer linked by @KamilMaciorowski , I learned that Bash treats pipes differently in strings that run commands. I solved the problem by changing the line $cmd to eval "$cmd" and it works great now!

Be warned, eval can lead to code injection issues, but that's not relevant for this particular script.

VHS
  • 101
  • 5
  • 2
    No, that's the _wrong_ solution. The correct one is to use `cmd=( find ./* -maxdepth 0 )` then to extend that array with e.g. `cmd+=( ! -name "$file" )` etc. and finally to run using `"${cmd[@]}"`. Note that it makes no sense to store the `xargs` command in a variable. Just use `cmd+=( -exec cp -t "$TARGET_DIR" -r {} + )` to avoid piping to `xargs` at all. – Kusalananda May 03 '22 at 07:03
  • 1
    Note that if you build `cmd` with `cmd+=" ! -name \"$file\""` as in the question, you'll end up with a string like `! -name "filename1" ! -name "filename2 ...`, which will break apart if the filenames contain double quotes, dollar signs or backslashes. If you know they don't, it should work, though. – ilkkachu May 03 '22 at 07:37
-2

You have a good idea about loading a list to a command, but there are better ways to combine variables and commands.

@KamilMaciorowski provided some good context here, but you're probably still hunting around for a more definitive answer. You can see a good example for using printf to format your array into the find command by reading more here.

What this will look like in your case would be something like this:

find ./* -maxdepth 0 $(printf "! -name %s " $EXCLUDED_FILES) -exec cp -r {} $TARGET_DIR \;
iamwpj
  • 107
  • 3
  • if `EXCLUDED_FILES` is an array, like in the code in the question, `$EXCLUDED_FILES` will only take the first element of the array, and we could just have `! -name "$EXCLUDED_FILES"` without the printf and command substitution. If you make `$EXCLUDED_FILES` a whitespace-separated list of filenames, it would basically work (as long as the filenames don't contain glob characters either. Just that it would be impossible to have filenames that contain whitespace themselves in the list. – ilkkachu May 03 '22 at 07:35