Bash tips & tricks – Expanding parameters

A parameter is something that stores value and is reference by a name. This are also known as bash variable or simply variables.

Often we’d like to get values from the parameters, this procedure is called parameter expansion.

The most common usage to get the value of a parameter is to reference it by a “$” symbol.

For example,

srvstva@x3400-m3:~$ name="Ankur"
srvstva@x3400-m3:~$ echo $name
Ankur

Or,
srvstva@x3400-m3:~$ echo ${name}
Ankur

But there are many other tricks that can be quite helpful.

Case modification

${VARIABLE^} – converts the first character to upper case

srvstva@x3400-m3:~$ name="ankur"
srvstva@x3400-m3:~$ echo ${name^} 
Ankur

${VARIABLE^^} – converts the whole string to upper case

srvstva@x3400-m3:~$ name="ankur"
srvstva@x3400-m3:~$ echo ${name^^} 
ANKUR

${VARIABLE,} – converts the first character to lower case

srvstva@x3400-m3:~$ name=ANKUR
srvstva@x3400-m3:~$ echo ${name,} 
aNKUR

${VARIABLE,,} – converts the whole string to lower case

srvstva@x3400-m3:~$ name=ANKUR
srvstva@x3400-m3:~$ echo ${name,,} 
ankur

Variable name expansion

Variables can be expanded to print their own names. In the preceding example, the variable name contained the string “ANKUR”

Now lets see how expansion works for this cases. A trailing “!” and a leading “*” or “@” does the trick.

srvstva@x3400-m3:~$ name="ANKUR"
srvstva@x3400-m3:~$ echo ${!name*} 
name
srvstva@x3400-m3:~$ echo ${!name@} 
name

String length

You can easily find the length of string using this ${#VARIABLE}

srvstva@x3400-m3:~$ name="ANKUR"
srvstva@x3400-m3:~$ echo ${#name} 
5

 Finding substrings

Its as easy as making a cup of coffee 😉 Just look at the examples to understand

${VARIABLE:Offset} Or ${VARIABLE:Offset:Length}

srvstva@x3400-m3:~$ name="ANKUR"
srvstva@x3400-m3:~$ echo ${name:2} 
KUR
srvstva@x3400-m3:~$ echo ${name:2:2} 
KU
srvstva@x3400-m3:~$ echo ${name:1:4} 
NKUR
srvstva@x3400-m3:~$ echo ${name:0} 
ANKUR

 Search and replace

Searching and replacing can done using following ways.

${VARIABLE/PATTERN/STRING_TO_REPLACE} – Replace the pattern with the string
${VARIABLE/PATTERN} – Print all minus the pattern

srvstva@x3400-m3:~$ name="ANKUR" 
srvstva@x3400-m3:~$ echo ${name/NK/nk} 
AnkUR
srvstva@x3400-m3:~$ echo ${name/NK} 
AUR

Hope you enjoyed the simple bash tricks.

Happy copying & pasting!