This post is Part Three of Bash Programming. Today let’s look at flow control.
Conditions
if
Format:
1
2
3
4
5
6
7
| if [[ ... ]]; then
...
elif [[ ... ]]; then
...
else
...
fi
|
e.g. Input number & 7:
1
2
3
4
5
6
7
8
9
| read -p "Input: " a
if [[ $a > 7 ]]; then
echo Bigger
elif [[ $a = 7 ]]; then
echo Equal
else
echo Smaller
fi
|
case
Format:
1
2
3
4
5
6
| case <VAR> in
..1)
... ;;
..2)
... ;;
esac
|
e.g. Month & Half-year:
1
2
3
4
5
6
7
8
9
10
| read -p "Input: " a
case $a in
1 | 2 | 3 | 4 | 5 | 6)
echo "1st half of the year";;
7 | 8 | 9 | 10 | 11 | 12)
echo "2nd half of the year";;
*)
echo "Input error";;
esac
|
Loops
for
Bash-Type:
1
2
3
4
5
6
| sum=0
for n in {1..5}; do
let "sum += n"
done
echo "Total is: $sum"
|
C-Type:
1
2
3
4
5
6
| sum=0
for (( n = 1; n <=5; n++ )); do
let "sum += n"
done
echo "Total is: $sum"
|
while
e.g. Calculate sum of 1 - 10:
1
2
3
4
5
6
7
| s=0
n=1
while (( $n <= 10 )); do // while n ≤ 10
let "s += n"
let "n++"
done
echo "Total is: $s"
|
until
e.g. Calculate sum of 1 - 10:
1
2
3
4
5
6
7
| s=0
n=1
until (( $n > 10 )); do // until n > 10
let "s += n"
let "n++"
done
echo "Total is: $s"
|
select
e.g. Select words
1
2
3
4
5
6
7
8
9
10
| PS3="Select: "
select w in hello world; do
echo -e "Result: \n $w"
done
>>> 1) hello
>>> 2) world
>>> Select: 1
>>> Result:
>>> hello
|
Note that echo -e: Recognize \, escape.
PS3 is the prompt. Default is #?.
Nested Loop
1
2
3
4
5
6
7
8
| n=3
while [[ $n > 0 ]]; do
echo "Outer $n"
let "n--"
for (( i=2; i>0; i-- )); do
echo "Inner $i"
done
done
|
break
Out of all loops
1
2
3
4
5
6
7
8
9
10
11
12
| n=0
while [[ $n <= 4 ]]; do
echo $n
if [[ $n == 2 ]]; then
break // break AFTER echo
fi
let n++
done
>>> 0
1
2
|
Note: break after echo
continue
Out of current loop
1
2
3
4
5
6
7
8
9
10
11
| for n in {0..4}; do
if [[ $n == 2 ]]; then
continue // continue BEFORE echo
fi
echo $n
done
>>> 0
1
3
4
|
Note: continue before echo