Syntax for a single-line while loop in Bash - bash

Question

I am having trouble coming up with the right combination of semicolons and/or braces. I'd like to do this, but as a one-liner from the command line:
while [ 1 ]
do
foo
sleep 2
done

Answer 1

while true; do foo; sleep 2; done
By the way, if you type it as a multiline (as you are showing) at the command prompt and then call the history with arrow up, you will get it on a single line, correctly punctuated.
$ while true
> do
> echo "hello"
> sleep 2
> done
hello
hello
hello
^C
$ <arrow up> while true; do echo "hello"; sleep 2; done

Answer 2

It's also possible to use sleep command in while's condition. Making one-liner looking more clean imho.
while sleep 2; do echo thinking; done

Answer 3

Colon is always "true":
while :; do foo; sleep 2; done

Answer 4

You can use semicolons to separate statements:
$ while [ 1 ]; do foo; sleep 2; done

Answer 5

You can also make use of until command:
until ((0)); do foo; sleep 2; done
Note that in contrast to while, until would execute the commands inside the loop as long as the test condition has an exit status which is not zero.
Using a while loop:
while read i; do foo; sleep 2; done < /dev/urandom
Using a for loop:
for ((;;)); do foo; sleep 2; done
Another way using until:
until [ ]; do foo; sleep 2; done

Answer 6

Using while:
while true; do echo 'while'; sleep 2s; done
Using for Loop:
for ((;;)); do echo 'forloop'; sleep 2; done
Using Recursion, (a little bit different than above, keyboard interrupt won't stop it)
list(){ echo 'recursion'; sleep 2; list; } && list;

Answer 7

A very simple infinite loop.. :)
while true ; do continue ; done
Fr your question it would be:
while true; do foo ; sleep 2 ; done

Answer 8

For simple process watching use watch instead

Answer 9

I like to use the semicolons only for the WHILE statement,
and the && operator to make the loop do more than one thing...
So I always do it like this
while true ; do echo Launching Spaceship into orbit && sleep 5s && /usr/bin/launch-mechanism && echo Launching in T-5 && sleep 1s && echo T-4 && sleep 1s && echo T-3 && sleep 1s && echo T-2 && sleep 1s && echo T-1 && sleep 1s && echo liftoff ; done

Answer 10

If you want the while loop to stop after some condition, and your foo command returns non-zero when this condition is met then you can get the loop to break like this:
while foo; do echo 'sleeping...'; sleep 5; done;
For example, if the foo command is deleting things in batches, and it returns 1 when there is nothing left to delete.
This works well if you have a custom script that needs to run a command many times until some condition. You write the script to exit with 1 when the condition is met and exit with 0 when it should be run again.
For example, say you have a python script batch_update.py which updates 100 rows in a database and returns 0 if there are more to update and 1 if there are no more. The the following command will allow you to update rows 100 at a time with sleeping for 5 seconds between updates:
while batch_update.py; do echo 'sleeping...'; sleep 5; done;

Answer 11

You don't even need to use do and done. For infinite loops I find it more readable to use for with curly brackets. For example:
for ((;;)) { date ; sleep 1 ; }
This works in bash and zsh. Doesn't work in sh.

Answer 12

If I can give two practical examples (with a bit of "emotion").
This writes the name of all files ended with ".jpg" in the folder "img":
for f in *; do if [ "${f#*.}" == 'jpg' ]; then echo $f; fi; done
This deletes them:
for f in *; do if [ "${f#*.}" == 'jpg' ]; then rm -r $f; fi; done
Just trying to contribute.

Answer 13

You can try this too
WARNING: this you should not do but since the question is asking for infinite loop with no end...this is how you could do it.
while [[ 0 -ne 1 ]]; do echo "it's looping"; sleep 2; done

Answer 14

You can also put that loop in the background (e.g. when you need to disconnect from a remote machine)
nohup bash -c "while true; do aws s3 sync xml s3://bucket-name/xml --profile=s3-profile-name; sleep 3600; done &"