A variable is a value that can change depending on conditions or data passed to the program. To work with the variable in bash, here are what you need to know:
- A variable name cannot start with a number, and cannot contain spaces.
- It can be alphanumeric
- It can start with an underscore
To declare a variable, type the name you want and set its value using the equals sign (=), make sure there is no space between the variable name and the equal sign or between the equals sign and the value, otherwise it would throw an error.
#!/bin/bash b="How are You" c=15
To use this variable, you call it with a dollar sign in front of their name.
#!/bin/bash b="How are You" c=15 echo $b echo $c
This is my output:
user@server:~/bin$ hw4.sh How are You 15
These variables can also work inside other variable or strings, just make sure they are double quoted, e.g:
#!/bin/bash b="How are You" c=15 echo "b! You are $c old."
user@server:~/bin$ hw4.sh How are You! You are 15 old.
Also, you can give variable a special attribute, e.g declaring it as an integer or as a read-only, meaning it can't be modified later arithmetically or with stream manipulation.
The one I find interesting is converting strings to a lowercase or uppercase:
- declare -i b=700 #b is an integer
- declare -r e=300 #e is read-only
- declare -l j="Lowercase" #j would be converted to lowercase
- declare -u k="Uppercase" #k would be converted to UPPERCASE
Major Built-in Variable
- echo $PWD - Returns the current directory
- echo $HOME - User home directory
- echo $MACHTYPE - Returns the machine type, handy when working on a different platform
- echo $HOSTNAME - Returns system name