How can I convert a string from uppercase to lowercase in Bash?

Question

I have been searching to find a way to convert a string value from uppercase to lowercase. All the search results show approaches of using the tr command.

The problem with the tr command is that I am able to get the result only when I use the command with the echo statement. For example:

y="HELLO"
echo $y| tr '[:upper:]' '[:lower:]'

The above works and results in 'hello', but I need to assign the result to a variable as below:

y="HELLO"
val=$y| tr '[:upper:]' '[:lower:]'
string=$val world

When assigning the value like above it gives me an empty result.

PS: My Bash version is 3.1.17

for files: cat file_name | tr '[:upper:]' '[:lower:]' >> lower — mccurcio, Oct 14 '21 at 19:12
I needed to use `val=$(echo $y | tr '[:upper:]' '[:lower:]')` rather than just `val=$y| tr '[:upper:]' '[:lower:]'` — mikemtnbikes, Mar 25 '23 at 17:32

score 234 · Answer 1 · edited Jun 21 '22 at 11:03

234

If you are using Bash 4, you can use the following approach:

x="HELLO"
echo $x  # HELLO

y=${x,,}
echo $y  # hello

z=${y^^}
echo $z  # HELLO

Use only one , or ^ to make the first letter lowercase or uppercase.

edited Jun 21 '22 at 11:03

Peter Mortensen

30,738
21
105
131

answered Jul 09 '12 at 09:24

kev

155,172
47
273
272

2

Thanks for your immediate response. When i tried the above it says ${y,,}--bad substitution. Any how i tried another approach of y="HI" val = $( tr '[A-Z]' '[a-z]' <<< $y) and this worked for me.Thanks once again – raga Jul 09 '12 at 09:55
10

Your bash version too low. It doesn't support those features. – kev Jul 09 '12 at 09:58
Yes...Am trying to do the same logic in ant build.xml file.Trying to convert a string from uppercase to lowercase.How do I achievce that.Tried scriptdef pattern and others and it dint work.. – raga Jul 09 '12 at 10:15
bash string substitution doesn't work on centos 5.9 – nurettin Apr 02 '13 at 12:54
8

The ,, ^^ substitutions only work on bash 4, not bash 3 so you'd need to upgrade bash or use the tr approach. – Ian Oct 21 '13 at 09:52
2

If bad substitution message is returned, also check if you accidentally wrote '$' before variable name (${$y,,} instead of ${y,,}), which results in the same error as when bash version is too low (you can check it with 'bash --version'). – BartekM Oct 17 '16 at 12:15
5

I was confused for a moment because I was doing `bash --version` and getting 5.0.3, but then realized that was brew version of bash (at `/usr/local/bin/bash`). My script had my system bash in the header with `#!/bin/bash` which was version 3 :( – combinatorist Jan 21 '20 at 19:59
bash version 4.4.20(1)-release `bad substitution` – yuklia Dec 20 '21 at 09:58
version 5.1.12(1)-release in /usr/local/bin/bash (brew) also failing on the CLI with the above error – volvox Jan 04 '22 at 15:35

score 88 · Answer 2 · edited Jun 21 '22 at 11:05

One way to implement your code is

y="HELLO"
val=$(echo "$y" | tr '[:upper:]' '[:lower:]')
string="$val world"

This uses $(...) notation to capture the output of the command in a variable. Note also the quotation marks around the string variable -- you need them there to indicate that $val and world are a single thing to be assigned to string.

If you have Bash 4.0 or higher, a more efficient & elegant way to do it is to use Bash built-in string manipulation:

y="HELLO"
string="${y,,} world"

score 34 · Answer 3 · edited Jun 21 '22 at 11:21

34

Note that tr can only handle plain ASCII, making any tr-based solution fail when facing international characters.

Same goes for the Bash 4-based ${x,,} solution.

The AWK tool, on the other hand, properly supports even UTF-8 / multibyte input.

y="HELLO"
val=$(echo "$y" | awk '{print tolower($0)}')
string="$val world"

Answer courtesy of liborw.

edited Jun 21 '22 at 11:21

Peter Mortensen

30,738
21
105
131

answered Dec 15 '14 at 09:17

DevSolar

67,862
21
134
209

1

Thanks. Options with `tr` didn't work for me. Might be a MacOS/BSD thing. – TastyWheat Mar 02 '22 at 15:37

score 26 · Answer 4 · edited Jun 21 '22 at 11:02

26

Execute in backticks:

 x=`echo "$y" | tr '[:upper:]' '[:lower:]'`

This assigns the result of the command in backticks to the variable x. (I.e., it's not particular to tr, but it is a common pattern/solution for shell scripting.)

You can use $(..) instead of the backticks. See here for more info.

edited Jun 21 '22 at 11:02

Peter Mortensen

30,738
21
105
131

answered Jul 09 '12 at 09:22

Brian Agnew

268,207
37
334
440

5

Thanks a lot for your answer. the above works..to make it better i tried y="HI" val = $( tr '[A-Z]' '[a-z]' <<< $y) and it worked fine for me..Thank you once again for your suggestion – raga Jul 09 '12 at 09:51
1

@raga `<<<` - love it!!! much more elegant than `echo` "overkill" ☺︎ – msciwoj Dec 05 '14 at 12:09
1

Just remember that `\$`, double backslash and backslash backtick will not be treated literally with backticks, but they will inside `$(..)`. That could be good or bad depending on your usecase. http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_03_04.html Section 3.4.5. – Jeremy Jun 05 '15 at 20:55

score 10 · Answer 5 · edited Jun 21 '22 at 11:25

10

I'm on Ubuntu 14.04 (Trusty Tahr), with Bash version 4.3.11. However, I still don't have the fun built-in string manipulation ${y,,}

This is what I used in my script to force capitalization:

CAPITALIZED=`echo "${y}" | tr '[a-z]' '[A-Z]'`

edited Jun 21 '22 at 11:25

Peter Mortensen

30,738
21
105
131

answered Feb 18 '16 at 16:32

BoomShadow

912
1
18
31

Does the shebang line at the top of your script point to an older version of bash? – Tan Wang Feb 19 '17 at 18:55
No, I only have one version installed on my system, so it points to the system default bash. – BoomShadow Feb 21 '17 at 14:11
I think that the `[]` are not required: https://unix.stackexchange.com/questions/51983/how-to-uppercase-the-command-line-argument#comment801846_344791 – Ciro Santilli OurBigBook.com May 09 '18 at 16:27
1

`${y,,}` won't work in Ubuntu if you use `#!/bin/sh` since the default shell is Dash. You must use `#!/bin/bash` – David Lopez Mar 22 '21 at 15:44

score 6 · Answer 6 · edited Jun 21 '22 at 11:51

6

If you define your variable using declare (old: typeset) then you can state the case of the value throughout the variable's use.

declare -u FOO=AbCxxx
echo $FOO

Output:

ABCXXX

Option -l to declare does lowercase:

When the variable is assigned a value, all upper-case characters are converted to lower-case. The upper-case attribute is disabled.

edited Jun 21 '22 at 11:51

Peter Mortensen

30,738
21
105
131

answered Nov 06 '17 at 17:29

Steven Powell

115
2
7

2

Not in Bash 3.2 – Amit Naidu May 24 '19 at 21:49
2

Bash 3.2 [was released](https://en.wikipedia.org/wiki/Bash_%28Unix_shell%29#Release_history) on 2006-10-11 (nearly 16 years ago). That was before [Shellshock](https://en.wikipedia.org/wiki/Shellshock_%28software_bug%29) was (officially) discovered (2014). – Peter Mortensen Jun 21 '22 at 11:37
@Amit Naidu: What system are you using? – Peter Mortensen Jun 21 '22 at 11:52

score 4 · Answer 7 · edited Jun 21 '22 at 11:16

4

Building on Rody's answer, this worked for me.

y="HELLO"
val=$(echo $y | tr '[:upper:]' '[:lower:]')
string="$val world"

One small modification: if you are using underscore next to the variable, you need to encapsulate the variable name in {}.

string="${val}_world"

edited Jun 21 '22 at 11:16

Peter Mortensen

30,738
21
105
131

answered Sep 26 '14 at 18:16

Ray Moncada

63
1
3

Is it only underscore (not a rhetorical question)? What about other punctuation, like comma? – Peter Mortensen Jun 21 '22 at 11:11

How can I convert a string from uppercase to lowercase in Bash?

7 Answers7

Linked