mv: missing destination file operand after with grep

Question

Kia ora,

I have a build script where I do a bunch of file operations and if they succeed, I print stuff to stdout as part of a report generated for the build. However I am getting the error "mv: missing destination file operand after" a lot.

if mv $(ls *.md | grep -v _index.md) markdown; then
    echo "    Put Markdown files back in 'markdown'"
fi

I have been reading that it is usually people have just missed the destination part of the command, but in this case I don't think that I have.

Any advice would be appreciated!

You should really avoid constructs like $(ls *.md | grep -v _index.md) - in bash, you could more robustly set the nullglob and extglob options i.e. shopt -s nullglob extglob and then use mv -t markdown/ !(_index).md I think — steeldriver, Sep 09 '20 at 22:59
... the error could simply be because ls *.md | grep -v _index.md returns no matches, so that your command becomes mv markdown — steeldriver, Sep 09 '20 at 23:42
I think that is what is happening, too! Is there a way to avoid the grumbly error? — Finn LeSueur, Sep 09 '20 at 23:46

score 3 · Answer 1 · edited Sep 11 '20 at 02:32

Likely the error is because $(ls *.md | grep -v _index.md) evaluates to an empty string (i.e. there are no .md files that do not match _index.md) so that your command becomes

mv markdown

In general, you should avoid constructions like $(ls *.md | grep -v _index.md) for the reasons discussed here:

Bash has a couple of nice "globbing" features that allow you to avoid such problems:

#!/bin/bash
shopt -s nullglob extglob
mdfiles=( !(_index).md )
if (( ${#mdfiles[@]} > 0 )); then
  if mv -t markdown/ -- "${mdfiles[@]}"; then
    echo "    Put Markdown files back in 'markdown'"
  fi
else
  echo "    No Markdown files to move"
fi

This should work unless the number of .md files is large enough to overflow the shell's ARG_MAX.

mv: missing destination file operand after with grep

1 Answers1