Converting between uppercase and lowercase on the Linux command line

Converting text between uppercase and lowercase can be very tedious, especially when you want to avoid inadvertent misspellings. Fortunately, Linux provides a handful of commands that can make the job very easy.

language / alphabet / letters / characters
andy.brandon50 (CC BY-SA 2.0)

There are many ways to change text on the Linux command line from lowercase to uppercase and vice versa. In fact, you have an impressive set of commands to choose from. This post examines some of the best commands for the job and how you can get them to do just what you want.

Using tr

The tr (translate) command is one of the easiest to use on the command line or within a script. If you have a string that you want to be sure is in uppercase, you just pass it through a tr command like this:

$ echo Hello There | tr [:lower:] [:upper:]
HELLO THERE

Below is an example of using this kind of command in a script when you want to be sure that all of the text that is added to a file is in uppercase for consistency:

#!/bin/bash

echo -n "Enter department name: "
read dept
echo $dept | tr [:lower:] [:upper:] >> depts

Switching the order to [:upper:] [:lower:] would have the opposite effect, putting all the department names in lowercase:

echo $dept | tr [:upper:] [:lower:] >> depts

Similarly, you could use the sed command's A-Z and a-z strings to accomplish the same thing:

echo $dept | tr a-z A-Z >> depts

As you undoubtedly suspect, reversing the order of the a-z and A-Z strings will have the opposite effect, turning the text to all lowercase.

Using awk

The awk command lets you do the same thing with its toupper and tolower options. The command in the script shown in the previous example could be done this way instead:

echo $dept | awk '{print toupper($0)}' >> depts

The reverse (switching to lowercase) would look like this:

echo $dept | awk '{print tolower($0)}' >> depts

Using sed

The sed (stream editor) command also does a great job of switching between upper- and lowercase. This command would have the same effect as the first of the two shown above.

echo $dept | sed 's/[a-z]/\U&/g' >> depts

Switching from uppercase to lowercase would simply involve replacing the U near the end of the line with an L.

echo $dept | sed 's/[A-Z]/\L&/g' >> depts

Manipulating text in a file

Both awk and sed also allow you to change the case of text for entire files. So, you just found out your boss wanted those department names in all lowercase? No problem. Just run a command like this with the file name provided:

$ awk '{print tolower($0)}' depts
finance
billing
bookkeeping

If you want to overwrite the depts file, instead of just displaying its contents in lowercase, you would need to do something like this:

$ awk '{print tolower($0)}' depts > depts-
$ mv depts- depts

Making the change with sed, however, you can avoid that last step because sed can edit a file "in place" as shown here, leaving the file intact, but the text in all lowercase:

$ sed 's/[A-Z]/\L&/g' depts

Capitalizing first letters only

To capitalize only the first letters of words in a string, you can do something like this:

$ echo design \& engineering| sed -e "s/\b\(.\)/\u\1/g"
Design & Engineering

That command will ensure that first letters are capitalized, but won't change the rest of the letters.

Making sure only first letters are uppercase

It's a little more challenging when you want to change text so that only first letters are in uppercase. Say you're manipulating a list of staff members' names and you want them to be formatted in the normal Firstname Lastname manner.

with sed

You could use a considerably more complex sed command to ensure this result:

$ echo design \& ENGINEERING | sed 's/\b\([[:alpha:]]\)\([[:alpha:]]*\)\b/\u\1\L\2/g'
Design & Engineering

with python

If you have python loaded, you can run a command like this that also formats text so that only the first letters of each word are capitalized and the command may be a little easier to parse than the sed command shown above:

$ echo -n "design & engineering" | python3 -c "import sys; print(sys.stdin.read().title())"
Design & Engineering

There are many ways to change the formatting of text between upper- and lowercase. Which works best depends in part of whether you're manipulating a single string or an entire file and how you want the end result to look.

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