scripting loops

(Written by Paul Cobbaut, https://github.com/paulcobbaut/, with contributions by: Alex M. Schapelle, https://github.com/zero-pytagoras/)

test [ ]

The test command can test whether something is true or false. Let\'s start by testing whether 10 is greater than 55.

[student@linux ~]$ test 10 -gt 55 ; echo $?
1
[student@linux ~]$

The test command returns 1 if the test fails. And as you see in the next screenshot, test returns 0 when a test succeeds.

[student@linux ~]$ test 56 -gt 55 ; echo $?
0
[student@linux ~]$

If you prefer true and false, then write the test like this.

[student@linux ~]$ test 56 -gt 55 && echo true || echo false
true
[student@linux ~]$ test 6 -gt 55 && echo true || echo false
false

The test command can also be written as square brackets, the screenshot below is identical to the one above.

[student@linux ~]$ [ 56 -gt 55 ] && echo true || echo false
true
[student@linux ~]$ [ 6 -gt 55 ] && echo true || echo false
false

Below are some example tests. Take a look at man test to see more options for tests.

[ -d foo ]             Does the directory foo exist ?
[ -e bar ]             Does the file bar exist ?
[ '/etc' = $PWD ]      Is the string /etc equal to the variable $PWD ?
[ $1 != 'secret' ]     Is the first parameter different from secret ?
[ 55 -lt $bar ]        Is 55 less than the value of $bar ?
[ $foo -ge 1000 ]      Is the value of $foo greater or equal to 1000 ?
[ "abc" < $bar ]       Does abc sort before the value of $bar ?
[ -f foo ]             Is foo a regular file ?
[ -r bar ]             Is bar a readable file ?
[ foo -nt bar ]        Is file foo newer than file bar ?
[ -o nounset ]         Is the shell option nounset set ?

Tests can be combined with logical AND and OR.

student@linux:~$ [ 66 -gt 55 -a 66 -lt 500 ] && echo true || echo false
true
student@linux:~$ [ 66 -gt 55 -a 660 -lt 500 ] && echo true || echo false
false
student@linux:~$ [ 66 -gt 55 -o 660 -lt 500 ] && echo true || echo false
true

if then else

The if then else construction is about choice. If a certain condition is met, then execute something, else execute something else. The example below tests whether a file exists, and if the file exists then a proper message is echoed.

#!/bin/bash

if [ -f isit.txt ]
then echo isit.txt exists!
else echo isit.txt not found!
fi

If we name the above script \'choice\', then it executes like this.

[student@linux scripts]$ ./choice 
isit.txt not found!
[student@linux scripts]$ touch isit.txt
[student@linux scripts]$ ./choice 
isit.txt exists!
[student@linux scripts]$

if then elif

You can nest a new if inside an else with elif. This is a simple example.

#!/bin/bash
count=42
if [ $count -eq 42 ]
then
  echo "42 is correct."
elif [ $count -gt 42 ]
then
  echo "Too much."
else
  echo "Not enough."
fi

for loop

The example below shows the syntax of a classical for loop in bash.

for i in 1 2 4
do
   echo $i
done

An example of a for loop combined with an embedded shell.

#!/bin/ksh
for counter in `seq 1 20`
do
   echo counting from 1 to 20, now at $counter
   sleep 1
done

The same example as above can be written without the embedded shell using the bash {from..to} shorthand.

#!/bin/bash
for counter in {1..20}
do
   echo counting from 1 to 20, now at $counter
   sleep 1
done

This for loop uses file globbing (from the shell expansion). Putting the instruction on the command line has identical functionality.

kahlan@solexp11$ ls
count.ksh  go.ksh
kahlan@solexp11$ for file in *.ksh ; do cp $file $file.backup ; done
kahlan@solexp11$ ls                                                 
count.ksh  count.ksh.backup  go.ksh  go.ksh.backup

while loop

Below a simple example of a while loop.

i=100;
while [ $i -ge 0 ] ;
do
   echo Counting down, from 100 to 0, now at $i;
   let i--;
done

Endless loops can be made with while true or while : , where the colon is the equivalent of no operation in the Korn and bash shells.

#!/bin/ksh
# endless loop
while :
do
 echo hello
 sleep 1
done

until loop

Below a simple example of an until loop.

let i=100;
until [ $i -le 0 ] ;
do
   echo Counting down, from 100 to 1, now at $i;
   let i--;
done

practice: scripting tests and loops

Write a script that uses a for loop to count from 3 to 7.
Write a script that uses a for loop to count from 1 to 17000.
Write a script that uses a while loop to count from 3 to 7.
Write a script that uses an until loop to count down from 8 to 4.
Write a script that counts the number of files ending in .txt in the current directory.
Wrap an if statement around the script so it is also correct when there are zero files ending in .txt.

solution: scripting tests and loops

Write a script that uses a for loop to count from 3 to 7.

#!/bin/bash

for i in 3 4 5 6 7
do
   echo "Counting from 3 to 7, now at ${i}"
done

Write a script that uses a for loop to count from 1 to 17000.

#!/bin/bash

for i in `seq 1 17000`
do
    echo "Counting from 1 to 17000, now at ${i}"
done

Write a script that uses a while loop to count from 3 to 7.

#!/bin/bash

i=3
while [ $i -le 7 ]
do
 echo "Counting from 3 to 7, now at ${i}"
 let i=i+1
done

Write a script that uses an until loop to count down from 8 to 4.

#!/bin/bash

i=8
until [ $i -lt 4 ]
do
 echo "Counting down from 8 to 4, now at ${i}"
 let i=i-1
done

Write a script that counts the number of files ending in .txt in the current directory.

#!/bin/bash

let i=0
for file in *.txt
do
    let i++
done
echo "There are ${i} files ending in .txt"

Wrap an if statement around the script so it is also correct when there are zero files ending in .txt.

#!/bin/bash

ls *.txt > /dev/null 2>&1
if [ $? -ne 0 ] 
then echo "There are 0 files ending in .txt"
else
    let i=0
    for file in *.txt
    do
        let i++
    done
    echo "There are ${i} files ending in .txt"
fi