Linux shell programming : array operations
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Array operations - iterating array
Let's start with the simplest array indexing:
#!/bin/bash
arr=(10 5 300 4000 22000)
len=${#arr[@]}
for ((i=0;i<$len;i++))
do
# index element of the array length of the element
echo "$i ${arr[$i]} ${#arr[$i]}"
done
Output:
0 10 2 1 5 1 2 300 3 3 4000 4 4 22000 5
Here is a more comprehensive array shell code:
#!/bin/bash
stars=(Andromeda Circunus Lupus Sagitta Virgo)
for item in "${stars[@]}"
do
echo $item
done
# first element
echo ${stars[0]}
echo ${stars:0}
# all elements
echo ${stars[@]}
echo ${stars[@]:0}
# all elements but not the first one
echo ${stars[@]:1}
# elements in a range
echo ${stars[@]:1:3}
# length of the first element
echo ${#stars[0]}
echo ${#stars}
# number of elements
echo ${#stars[*]}
echo ${#stars[@]}
# replacing substring
echo ${stars[@]//t/T}
The output looks like this:
$ ./op.sh Andromeda Circunus Lupus Sagitta Virgo Andromeda Andromeda Andromeda Circunus Lupus Sagitta Virgo Andromeda Circunus Lupus Sagitta Virgo Circunus Lupus Sagitta Virgo Circunus Lupus Sagitta 9 9 5 5 Andromeda Circunus Lupus SagiTTa Virgo
Array operations - Reverse a string
To reverse a string, we can do with a command rev:
$ echo abcde | rev
To do the same thing in shell script, we need to know how to extract a single character from a string:
#!/bin/bash
s="abcde"
len=${#s}
echo "1st string at index = 0, ${s:0:1}"
echo "last string at index = 0, ${s:len-1:1}"
Note that the substring syntax looks like this:
## syntax ##
${parameter:offset:length}
Output:
1st string at index = 0, a last string at index = 0, e
Now, let's run a loop - extract a character from the end:
#!/bin/bash
s="abcde"
len=${#s}
for ((i=len-1;i>=0;i--))
do
echo "index=$i, ${s:$i:1}"
done
Output:
index=4, e index=3, d index=2, c index=1, b index=0, a
It's time to reverse the string, constructing it one by one from the end:
#!/bin/bash
s="abcde"
len=${#s}
for ((i=len-1;i>=0;i--))
do
rev="$rev${s:$i:1}"
echo "index=$i, rev=$rev"
done
Output:
index=4, rev=e index=3, rev=ed index=2, rev=edc index=1, rev=edcb index=0, rev=edcba
Array operations - iterating array by index
In this section, we'll loop through via array index:
#!/bin/bash
stars=(Andromeda Circunus Lupus Sagitta Virgo)
for (( i=0; i<${#stars[@]}; i++ ))
do
echo ${stars[i]}
done
echo
printf "%s\n" "${stars[@]}"
Output:
Andromeda Circunus Lupus Sagitta Virgo Andromeda Circunus Lupus Sagitta Virgo
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