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
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
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
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
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
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;
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.