2 min read

For-loop and strings manipulation in bash scripting

2016-09-28

Passing variable to a regular for-loop

Considers the for-loop expression below:

for ((i=0; i<$Index; i++))

one can essentially pass the output from the word count command wc -l to the Index variable, to loop through files/folders.

Might not be the simplest / most efficient way for coding up for-loop, but it does works for carrying out this type of tasks, and it’s relatively clear in my opinion to see how it works.

Strings manipulation: echo and grep

echo and grep are being used A LOT. Familiarizing with these two commands are the key to build up your confidence to manipulate strings in Linux.

echo

In many cases there’s a need of testing a piece of scripts before actually running it. “echo ...” always helps by returning the output as a string of text in the console, and in that way, I know what’s exactly going on.

Say you want to remove some temporary files at the end of a script, however, you don’t want to delete anything yet, and for testing purpose you just want to peek what the script will do. To demonstrate a use case for echo, and this is merely one way of achieving the goal:

echo rm file1 file2
# Commented out for testing
# rm file1 file2

## rm file1 file2

Once checking the output is correct, you can now safely un-comment the actual code (`rm file1 file2``).

grep

On the other hand, the grep commands allows identifying the matched pattern.

  • grep "..." can find the pattern that matches ... and return the entire line in the console.

I can then quickly locate the specific contents I am looking for, often from large chunks of texts.

You can find more details about grep from this post.

Strings manipulation: the cut command

cut is quite a neat and handy command for for strings manipulation. Lots of tutorials out there, this is a good one.

  • cut -c 5- will obtain a substring starting from the 5th character and omit the rest beforehand.

Working example:

a="helloworld"
echo $a

## helloworld

a="helloworld"
echo $a | cut -c 6-

## world

String manipulation: # and %

  • # removes prefix
  • % removes suffix

These two symbols can sometimes be confusing in string manipulation if not being used often.

Working examples (Note the slight different order between the wildcard and underline for these two cases):

a="hello_world"

# remove prefix
echo ${a#*_}

## world

the prefix hello ending in “_” is removed.

a="hello_world"

# remove suffix
echo ${a%_*}

## hello

the suffix hello starting with “_” is removed.