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#050 Bash Loops

All about Bash loops

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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

Credits and References

About LCK#27 Bash
Project Source on GitHub Return to the Project Catalog

This page is a web-friendly rendering of my project notes shared in the LittleCodingKata GitHub repository.

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