Shell Programming and Scripting

It's a good example of arrays. Whether arrays are actually needed here is the question.

Novice programmers tend to store entire documents:

Code:

# Store every line
while read LINE
do
        ARR[L]="$LINE"
        let L=L+1
done < inputfile

# Modify and print each line
for((M=0; M<$L; M++))
do
        V="${LINE/A/B}"
        echo "$V"
done > outputfile

...when there was no need:

Code:

# Read, modify, and print
while read LINE
do
        echo "${LINE/A/B}"
done < inputfile > outputfile

For instance, in classic processing, you sort all the input files so you can evalutate the current lines and then discard on or the other. In the old days, machines were sold with as little as 2K, so programs had to line up their ducks before the shot them. Sometimes there were multiple sorts to join input transactions to many differently keyed masters. The merge process may also produce output transactions that may need sorting to go to the next, differently keyed master.

Note that shell arrays come in two flavors, the integer indexed simple flavor, and the alphanumeric string indexed, hash map associative arrays. Sometimes the total environment is system limited to about a meg., so robust solutions do not put into 'memory' any set that can have unpredictably large size.

Code:

$ (s=1 x=x
while [ 1 ]
do
  echo $s >/dev/tty
  (( s += s ))
 x="$x$x"
done)
1
2
4
8
16
32
64
128
256
512
1024
2048
4096
8192
16384
32768
65536
131072
262144
524288
1048576
2097152
4194304
8388608
16777216
33554432
67108864
134217728
ksh[6]: no space
$

Somewhere around 100M here.