Looping your way through bash

There are a lot of options for looping in bash whether on the command line or in a script. The choice depends on what you’re trying to do.

You may want to loop indefinitely or quickly run through the days of the week. You might want to loop once for every file in a directory or for every account on a server. You might want to loop through every line in a file or have the number of loops be a choice when the script is run. Let’s check out some of the options.

Simple loops

Probably the simplest loop is a for loop like the one below. It loops as many times as there are pieces of text on the line. We could as easily loop through the words cats are smart as the numbers 1, 2, 3 and 4.

#!/bin/bash

for num in 1 2 3 4
do
    echo $num
done

And, to prove it, here’s a similar loop run on the command line:

$ for word in cats are smart
> do
>    echo $word
> done
cats
are
smart

for vs while

Bash provides both a for and a while looping command. In while loops, some condition is tested each time through the loop to determine whether the loop should continue. This example is practically the same as the one before in how it works, but imagine what a difference it would make if we wanted to loop 444 times instead of just 4.

#!/bin/bash

n=1

while [ $n -le 4 ]
do
    echo $n
    ((n++))
done

Looping through value ranges

If you want to loop through every letter of the alphabet or some more restricted range of letters, you can use syntax like this:

#!/bin/bash

for x in {a..z}
do
    echo $x
done

If you used {d..f}, you would only loop three times.

Looping inside loops

There’s also nothing stopping you from looping inside a loop. In this example, we’re using a for loop inside a while loop.

#!/bin/bash

n=1

while [ $n -lt 6 ]
do
    for l in {a..d}
    do
        echo $n$l
    done
    ((n++))
done

The output would in this example include 1a, 1b, 1c, 1d, 2a and so on, ending at 5d. Note that ((n++)) is used to increment the value of $n so that while has a stopping point.

Looping through variable data

If you want to loop through every account on the system, every file in a directory or some other kind of variable data, you can issue a command within your loop to generate the list of values to loop through. In this example, we loop through every account (actually every file) in /home – assuming, as we should expect, that there are no other files or directories in /home.

#!/bin/bash

for user in `ls /home`
do
    echo $user
done

If the command were date instead of ls /home, we’d run through each of the 7 pieces of text in the output of the date command.

$ for word in `date`
> do
>     echo $word
> done
Thu
31
Oct
2019
11:59:59
PM
EDT

Looping by request

It’s also very easy to allow the person running the script to determine how many times a loop should run. If you want to do this, however, you should test the response provided to be sure that it’s numeric. This example shows three ways to do that.

#!/bin/bash

echo -n "How many times should I say hello? "
read ans

if [ "$ans" -eq "$ans" ]; then
    echo ok1
fi

if [[ $ans = *[[:digit:]]* ]]; then
    echo ok2
fi

if [[ "$ans" =~ ^[0-9]+$ ]]; then
    echo ok3
fi

The first option above shown might look a little odd, but it works because the -eq test only works if the values being compared are numeric. If the test came down to asking if “f” -eq “f”, it would fail. The second test uses the bash character class for digits. The third tests the variable to ensure that it contains only digits.

Of course, once you’ve selected how you prefer to test a user response to be sure that it’s numeric, you need to follow through on the loop. In this next example, we’ll print “hello” as many times as the user wants to see it. The le does a “less than or equal” test.

#!/bin/bash

echo -n "How many times should I say hello? "
read ans

if [ "$ans" -eq "$ans" ]; then
  n=1
  while [ $n -le $ans ]
  do
    echo hello
    ((n++))
  done
fi

Looping through the lines in a file

If you want to loop through the contents of a file line by line (i.e., NOT word by word), you can use a loop like this one:

#!/bin/bash

echo -n "File> "
read file
n=0

while read line; do
  ((n++))
  echo "$n: $line"
done 
The word "line" used in the above script is for clarity, but you could use any variable name. The while read and the redirection of the file content on the last line of the script is what provides the line-by-line reading.
Looping forever
If you want to loop forever or until, well, someone gets tired of seeing the script's output and decides to kill it, you can simple use the while true syntax.
#!/bin/bash

while true
do
    echo -n "Still running at "
    date
    sleep 10
done

The examples shown above are basically only (excuse the pun) "shells" for the kind of real work that you might need to do and are meant simply to provide the basic syntax for running undoubtedly far more useful commands.
Now see: