问题
I have been searching to find a way to convert a string value from upper case to lower case. All the search results show approaches of using tr command.
The problem with the tr command is that I am able to get the result only when I use the command with 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
回答1:
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 $z # HELLO
Use only one , or ^ to make the first letter lowercase or uppercase.
回答2:
The correct way to implement your code is
y="HELLO"
val=$(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 $val and world are a single thing to be assigned to string.
If you have bash 4.0 or higher, a more efficient & elegant way to do it is to use bash builtin string manipulation:
y="HELLO"
string="${y,,} world"
回答3:
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.
回答4:
Why not 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 is a common pattern/solution for shell scripting)
You can use $(..) instead of the backticks. See here for more info.
回答5:
I'm on Ubuntu 14.04, 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]'`
回答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 $FOO
ABCXXX
"-l" does lc.
回答7:
This worked for me. Thank you Rody!
y="HELLO"
val=$(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"
来源：https://stackoverflow.com/questions/11392189/how-to-convert-a-string-from-uppercase-to-lowercase-in-bash