Loops

[Simon]

How to construct various types of loop

[Simon]

The basic loop around a fixed set of values looks like this:

for X in 1 2 3
do
 echo The value of X is ${X}
done

where the values are actually strings, so we can also have

for X in 34 b purple c 99 a
do
 echo The value of X is ${X}
done

or

for FRUIT in apples oranges pears
do
 echo The current fruit is ${FRUIT}
done

[Simon]

To loop around a range of numerical values, you can use

for X in {1..10}
do
 echo The value of X is ${X}
done

A more flexible way is to use the seq command which allows you to control the increment step size, use non-integer values, and control the format in which the number is printed

for X in $(seq 1 10)
do
 echo The value of X is ${X}
done

or

for X in $(seq -w 1 0.5 6)
do
 echo The value of X is ${X}
done

and so on. Type ‘man seq’ at a bash prompt to read the manual for the seq command.

[Simon]

You might want to load the list of values from a file:

for X in `cat myfile.scp`
do
 echo The value of X is ${X}
done

where myfile.scp is a plain text file with one value per line. This is a good way to loop around a list of files, for example.

[Simon]

You could use a loop to construct a string (e.g., in order to pass it as a command line argument):

S=""
for X in `cat myfile.scp`
do
 S=${S}" -I "${X}
done
echo $S

# ... now do something useful with ${S}

where myfile.scp is a plain text file with one value per line.

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