I'm trying to write a command that's a one-line for loop containing a command that uses the & operator to put the command in the background:
for dir in $(ls); do command $dir &; done
Where & tells BASH to run command in the background. Yet BASH returns
bash: syntax error near unexpected token `;'
What's going on here? How can I use & in conjunction with ;? I tried enclosing things in quotes, like do "command $dir &" or do command $dir "&" but those have unexpected results.
You don't use the ; with '&', the '&' on its own is enough to finish the command. I believe there is currently no mention of this behaviour in the manual. Your loop would simply be:
for dir in $(ls); do command $dir & done
Additionally, you should consider using a glob instead of $(ls) which will fail if the filename contains whitespace. You can set nullglob to prevent dir being a * if there are no files:
shopt -s nullglob
for dir in *; do command $dir & done
;, &, &&, ||, and \n are all command separators, some of which have different meanings than others.
for dir in $(ls); do command $dir ;; done will throw the same error, as will for f in $(ls); do echo $f |;done etc.
$(ls)in place of*is generally considered a bad practice. Because of the nature of globbing*will allow you to deal with filenames that contain whitespace,$(ls)will not. Consider a list of files, "one a", "one b", "two". With$(ls)you will iterate "one", "a" "one", "b", "two". With*you will iterate the actual file list.