Linux Tutorial Series – 193 – Command substitution

Here is the video version, if you prefer it:

You can take a result of a command and store it in a variable or use the result of that first command as an argument for another command. This is called command substitution. (Ward, 2014)⁠

Here is an example of command substitution in a sample script:

#!/bin/bash

LINES=$(grep 'a' aba.txt)

for word in $LINES

do

echo $word

done

We first take every line that has the character a in the file aba.txt and then we store it in the variable LINES. Then we iterate over each word in the variable LINES and we print it. Why does LINES contain words, not lines? It contains lines indeed, but words that make up those lines are separated by a space (you can see so yourself by putting echo $LINES just before the for loop) and since each space is interpreted as a delimiter in a sequence, we get individual words.

Here is the output (along with aba.txt contents):

mislav@mislavovo-racunalo:~/Linux_folder$ cat aba.txt

Mustard is how we transfer wealth

Some stuff Abba Mustard

Mustard Mustard

Mustard Mustard

It's the Mustard

In the Rich Man's World

AB

Mustard is how we transfer wealth

Ab

aB

ab

mislav@mislavovo-racunalo:~/Linux_folder$ ./tutorialScript.sh

Mustard

is

how

we

transfer

wealth

…

A simple example of using the result of the first command as an input to another command is echo $(ls). We use the output of ls and we echo it.

Thank you for reading!

References

Ward, B. (2014). How Linux Works: What Every Superuser Should Know (2nd ed.). No Starch Press. Pages 263-264