Variable
System variables
  • Created and maintained by Linux itself, this type of variable defined in CAPITAL LETTERS
  • set, check system variables
    • #!/bin/bash

echo $BASH
echo $BASH_VERSION
echo $COLUMNS
echo $HOME
echo $PATH
echo '$USER is: '"$USER"
    User defined variables
  • Created and maintained by user, this type of variable defined in lower letters
  • Don't put spaces on either side of the equal sign when assigning value to variable
    • #!/bin/bash

name='Lin Chen'
echo $name
val=bus
echo $val
vech= # define NULL variable
echo $vech
unset val # unset variable
    Special variables
    #!/bin/bash

echo $$ # process id
echo $? # The exit status of the last command executed
echo $! # The process number of the last background command
    Variable Substitution
    #!/bin/bash

# If var is null or unset, print the default value
echo ${var:-"Not defined ..."}

# If var is null or unset, var is set to the specified value
echo ${var:="Set value ..."}
echo $var

# If var is null or unset, message is printed to standard error
unset var
echo ${var:?"Warning message ..."} # print warning and exit

# If var is set, word is substituted for var
var="Hello"
echo ${var:+"Substitution ..."} # Substitution
echo $var # Hello
    Reference
  • Tutoriaspoint