command line fun – flow control

Shell scripting is like any other programming language as far as control statements go.  To be honest, all your favorite control statements can be whipped up using either if/then statement or a loop.

The good news is that we don’t need to create this from scratch but can instead use all the same control statements as most modern languages.

IF / THEN / ELSE

The general statement is the same as other languages but the actual comparison verbs are slightly different.

The comparison verbs are different depending on whether comparing numbers or strings.

#!/bin/bash
A=1
B=4

if [ $A -eq $B ]
then
  echo $A
  echo A equals B
else
  echo $A $B
  echo A not equal B
fi

WHILE

The while construct simply does a comparison at the beginning and executes the body if the statement is true.  The comparison verbs are the same list of verbs as seen above for the if/then/else statement.

Just like in C, you can use either break to exit the loop completely, the continue statement causes the program control to pass to the conditional tests.

#!/bin/bash
A=1
B=4

while [ $A -le $B ]
do
  echo hi $A
  A=$(($A + 1))
done

FOR LOOP

The for loop, when it can be used, is perhaps one of the best control structures to use.  It could be a personal hangup of mine, but the advantage is that the initialization, test and increment is all conveniently located in one spot.  This makes it hard to forget to setup one of these three crucial components.

#!/bin/bash
for (( idx=1 ; $idx < 9 ; idx=`expr $idx + 1` )) ; 
do 
   echo "some neat message here $idx" 
done

Oddly enough, I almost never use the for loop in this manner.  The much more convenient way of using it is almost like some sort of set operator or iterator.  Rather than counting something, the for loop is used to iterate through a small collection of items.

#!/bin/bash
VALS="1 2 3 5 8 13 21"
for i in $VALS
do
   echo $i
done

This particular example is pretty unimaginative and unrealistic.  The better case is to collect some data from a file or file-system and then to process each one.  This could be to run a special program against each file individually, or to perform some special steps one at a time.

#!/bin/bash
CSVFILE=output.csv
FILES=`ls -1 control-data-2016*`
for i in $FILES
do    
   CNT=`wc -l it.sh | sed -e 's/ .*//'`
   echo $i,$CNT >> $CSVFILE
   gzip -9 $i
done

This small script is probably not the simplest way to count the number of lines in a file and prepare the output for a comma delimited file but it shows one iterative method of processing files.

SWITCH STATEMENT

The switch statement is much nicer than a really big if/then/else block.  Simply separate each case block with two semicolons.

#!/bin/bash
argument=$1

case "$argument" in

  start) 
  echo starting service 
  ;;

  stop) 
  echo stopping service 
  ;;

  *) 
  echo unknown command $argument encountered 
  ;;

esac

Depending on the problem, I am sure there are a number of uses for this structure but I don’t normally have much use for it.

The main reason is due to quite a few other operators that can be used with the if/then/else or while structures.  It makes it easy to verify that files or directories exist and are readable or writable.