SHELL - Variables

SHELL/Bash pre-defined variables

There are quite a few predefined variables in BASH/Shell. These variables can be viewed by the command ‘env’ or ‘set’. Among them, there are quite a few variables we need to understand well.

• $HOME: home directory
• $PWD: the current path
• $SHELL: The shell which you are using
• $UID: the UID
• $LOGIN: the login name for the current user
• $HOSTNAME: the hostname of the server

There are some tricky variables we need to understand as well.

• $$: the current PID
• $#: the parameters number
• $*: all the parameters
• $!:  the last run background PID
• $?:  the exit code for the last command
• $_: the final argument of previous command executed.

Set variables in BASH

Usually it is suggested using all capitalized letters by convention.

To set an variable in bash, just simple use =

eg:     go=’this’                                   #it will set a variable called go, let it equals ‘this’

alternatively, you can use declare command to set an variable.

eg:    declare go=’that’                   #it will set a variable called go, let it equals ‘that’

to use the variable, you can use $variable name or ${variable}otherletters

eg:    echo ${go}inthemiddle

to delete the variable. You can use unset command. But not applied for readonly variables.

eg: unset $go

Set a value to variable

There are a few ways of change the variables.

        1. read V1 V2 V3
            read from standard input
        2. VARIABLE=`command`
            Set the variable equal to the command ‘s output.
    

 
3. Also for the BASH script we can get the parameters from the command line parameters using $1 $2 $3 etc

Variable output:

1. Simply use echo $VARIABLE to get the value
2. Use printf to get the formatted value

• %s: string
• %f: float
• %d: integer
• -: left align , + : right align

Array variables.

1.Declare an array variables:

ARRAY=(element1 element2 elment3)
Or
ARRAY[0]=element0
ARRAY[1]=element1
ARRAY[2]=element2

2. Get the value of the Array by index
${ARRAY[index]}

3. Tricky tips:
How to get the length of the array ${#ARRAY[*]}
How to copy the array: NEWARRAY=(“${ARRAY[@]}”)