Shell Script: Convert Uppercase to Lowercase

Objective: Convert the contents of a file or the contents of a shell string variable from uppercase to lowercase.

There are too many ways to do that. Let’s take a look at a few of them. Throughout this article, you can assume that the file “uppercase.txt” is the input file and the file “lowercase.txt” is the output file.

Uppercase to Lowercase with tr

The first method is to use the tr command. The below command will read the text found in file uppercase.txt, convert the text to lower case and write the output to file lowercase.txt.

$ tr '[:upper:]' '[:lower:]' < uppercase.txt > lowercase.txt

1	$ tr '[:upper:]' '[:lower:]' < uppercase.txt > lowercase.txt

If the input is stored in a variable, use the echo command and pipe the output to tr command. So, if the variable $UCASE has a string in uppercase, convert it using the syntax below.

$ echo $UCASE | tr '[:upper:]' '[:lower:]'

1	$ echo $UCASE \| tr '[:upper:]' '[:lower:]'

Uppercase to Lowercase with awk

Use the following syntax to convert the contents of the file using awk.

$ awk '{print tolower($0)}' < uppercase.txt > lowercase.txt

1	$ awk '{print tolower($0)}' < uppercase.txt > lowercase.txt

To convert a string variable ($UCASE) from uppercase to lowercase, use the following syntax.

$ echo $UCASE | awk '{print tolower($0)}'

1	$ echo $UCASE \| awk '{print tolower($0)}'

Uppercase to Lowercase with dd

Yes, the dd command is also able to perform the case conversion. Perform the case conversion using the following syntax.

$ dd if=uppercase.txt of=lowercase.txt conv=lcase

1	$ dd if=uppercase.txt of=lowercase.txt conv=lcase

To convert a string variable, and print the result on the screen, use the following syntax.

$ echo $UCASE | dd conv=lcase 2> /dev/null

1	$ echo $UCASE \| dd conv=lcase 2> /dev/null

Uppercase to Lowercase with sed

This works on GNU sed only as it uses GNU sed extension. Perform case conversion by using the following syntax.

$ sed -e 's/\(.*\)/\L\1/' uppercase.txt > lowercase.txt

1	$ sed -e 's/$.*$/\L\1/' uppercase.txt > lowercase.txt

The ‘\1’ matches the whole line since the whole regular expression is within parenthesis. The special sequence ‘\L’ changes text to lowercase.

For a string variable containing uppercase characters, convert it to lowercase with sed by using the following syntax.

$ echo "$UCASE" | sed -e 's/\(.*\)/\L\1/'

1	$ echo "$UCASE" \| sed -e 's/$.*$/\L\1/'

Mohamed Ibrahim

ibrahim = { interested_in(unix, linux, android, open_source, reverse_engineering); coding(c, shell, php, python, java, javascript, nodejs, react); plays_on(xbox, ps4); linux_desktop_user(true); }