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Case Conversion
Recent versions of bash have support for upper/lowercase conversion of strings. bash
has four parameter expansions and two options to the declare command to support it.
So what is case conversion good for? Aside from the obvious aesthetic value, it has an important role in programming. Let's consider the case of a database look-up. Imagine that a user has entered a string into a data input field that we want to look up in a data - base. It's possible the user will enter the value in all uppercase letters or lowercase letters or a combination of both. We certainly don't want to populate our database with every possible permutation of upper and lower case spellings. What to do?
A common approach to this problem is to normalize the user's input. That is, convert it
into a standardized form before we attempt the database look-up. We can do this by con- verting all of the characters in the user's input to either lower or uppercase and ensure that the database entries are normalized the same way.
The declare command can be used to normalize strings to either upper or lowercase. Using declare, we can force a variable to always contain the desired format no matter what is assigned to it:
#!/bin/bash
# ul-declare: demonstrate case conversion via declare declare -u upper
declare -l lower
if [[ $1 ]]; then
upper="$1" lower="$1" echo $upper echo $lower
fi
#!/bin/bash
# ul-declare: demonstrate case conversion via declare declare -u upper
declare -l lower
if [[ $1 ]]; then
upper="$1" lower="$1" echo $upper echo $lower
fi
In the above script, we use declare to create two variables, upper and lower. We assign the value of the first command line argument (positional parameter 1) to each of the variables and then display them on the screen:
[me@linuxbox ~]$ ul-declare aBc
ABC
abc
[me@linuxbox ~]$ ul-declare aBc
ABC
abc
As we can see, the command line argument ("aBc") has been normalized.
In addition to declare, there are four parameter expansions that perform upper/lower- case conversion:
Table 34-1: Case Conversion Parameter Expansions
Format Result
Format Result
${parameter,,} Expand the value of parameter into all lowercase.
${parameter,} Expand the value of parameter changing only the first
character to lowercase.
${parameter^^} Expand the value of parameter into all uppercase letters.
${parameter^} Expand the value of parameter changing only the first
character to uppercase (capitalization).
Here is a script that demonstrates these expansions:
#!/bin/bash
# ul-param: demonstrate case conversion via parameter expansion if [[ $1 ]]; then
echo ${1,,}
echo ${1,} echo ${1^^} echo ${1^}
fi
#!/bin/bash
# ul-param: demonstrate case conversion via parameter expansion if [[ $1 ]]; then
echo ${1,,}
echo ${1,} echo ${1^^} echo ${1^}
fi
Here is the script in action:
[me@linuxbox ~]$ ul-param aBc
abc aBc ABC
ABc
[me@linuxbox ~]$ ul-param aBc
abc aBc ABC
ABc
Again, we process the first command line argument and output the four variations sup- ported by the parameter expansions. While this script uses the first positional parameter, parameter may be any string, variable, or string expression.