Syntax for a single-line while loop in 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

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 13

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 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 &"

Answer 15

in bash 5 (or perhaps even earlier), you can also reverse the role of the : by running everything in the loop criteria but using : in the loop body instead :

while ( gdate +"%c ( %s.%-N )" && sleep 0.71  ) do :; done  # discards side-effects
      
while { gdate +"%c ( %s.%-N )" && sleep 0.71; } do :; done  # no sub-shell needed

---

+ gdate '+%c ( %s.%-N )'
Sun Apr 30 12:38:25 2023 ( 1682872705.498728 )
+ sleep 0.71
+ :

+ gdate '+%c ( %s.%-N )'
Sun Apr 30 12:38:26 2023 ( 1682872706.218152 )
+ sleep 0.71

... 

---

meanwhile zsh is even more forgiving, and willing to loop even without the :

while; do gdate +'%c ( %s.%-N )' && sleep 0.31 ; done

Sun Apr 30 12:46:09 2023 ( 1682873169.469092 )
Sun Apr 30 12:46:09 2023 ( 1682873169.789560 )
Sun Apr 30 12:46:10 2023 ( 1682873170.105635 )
Sun Apr 30 12:46:10 2023 ( 1682873170.424766 )