#028 Bash Loops
All about Bash loops
Notes
Iteration is a findamental construct that is often needed for our scripting needs.
Bash provides a number of facilities for loops, including the
built-in commands for, while and until.
Using for
The for command can be used to iterate a set or list of items.
See for_in_list.sh for an example or using a fixed list.
But anything that can product a list can also be used (like cat a file).
for fruit in "apples" "oranges" "pears"
do
echo "loop with fruit==${fruit}"
done
Or can use a ranges. See for_in_range.sh for an example…
for i in {1..9}
do
echo "iteration ${i}"
done
while
A while loop will execute until the test condition is not true.
n1=0
n2=5
while [ $n1 -le $n2 ]
do
echo $n1
n1=$(($n1+1))
done
See while.sh for an example…
$ ./while.sh 3
loop with n1==0
loop with n1==1
loop with n1==2
loop with n1==3
finished now n1==4
until
The until command is the complement of while: the loop will execute until the test condition is true.
n1=0
n2=5
until [ $n1 -ge $n2 ]
do
echo $n1
n1=$(($n1+1))
done
See until.sh for an example…
$ ./until.sh 3
loop with n1==0
loop with n1==1
loop with n1==2
finished now n1==3
Infinite Loops
Just require a condition that always passes into the loop.
e.g. true with a while loop:
while true
do
echo "Press [CTRL+C] to stop.."
sleep 1
done
or with for:
for (( ; ; ))
do
echo "Press [CTRL+C] to stop.."
sleep 1
done
One-liners
Simple loops can usually be collapsed to one-liners that can be used on the command line. For example:
$ for i in {1..3}; do echo "iteration ${i}"; done
iteration 1
iteration 2
iteration 3
Or an infinite loop..
$ echo "infinite loop on a single line [ hit CTRL+C to stop]" ; for (( ; ; )) ; do date ; sleep 1 ; done
infinite loop on a single line [ hit CTRL+C to stop]
Mon Oct 9 22:33:26 SGT 2017
Mon Oct 9 22:33:27 SGT 2017
Mon Oct 9 22:33:28 SGT 2017