IT 3110 : Advanced System Administration

Variables

Variables

Remember, they are containers that hold strings and numbers.

    a=3
    name=tom

Though, traditionally vars are written in ALL CAPS.

Variables

• Remember, no spaces around the equal sign

• Otherwise the shell would think the lhs is a command

• Refer to a var with a $ in front of it:

   echo $a
  

• Just remember that since everything in bash is a string, we need the dollar sign to indicate a variable reference

Scripting

• Comments start with #
• indenting using four spaces per level, with no tabs and especially no mixed tabs and spaces
• Use meaningful names for variables and functions, and spell them out

Variables

Use the full syntax for a variable reference, which includes not just the dollar sign, but also braces around the variable name:

    somescript /tmp/rep${FN}port.txt

Export Variables

Export variables that you want to pass on to other scripts:

• export SOMEVAR #basically make it a env var

   #var.sh
   #!/bin/bash
   
   myvar="IT3110"
   export myvar
   
   ./myscript.sh
   
   #somescript.sh
   #!/bin/bash
   
   echo "my var is ${myvar}"
   exit 0
  

Environment Variables & Shell vars

• env shows all exported variables. (Environment variables)
• set shows all shell variables (only available to current shell)

Try:

    foovar=3
    env | grep foovar; set grep foovar
    export foovar
    env | grep foovar; set grep foovar

Using parameters in a shell script

• All strings present after the command invocation will be stored as numeric variables within the script.

   ./somecmd i like peaches
  
   #somecmd
   echo ${1}
  
   prints an 'i'
  

• What would you change the script to, to print out 'peaches'?

• You can use a $* to refer to all command line arguments.

• You can use a $@ to be more robust (see 5.10 in book for more info)

Variables

Default values:

• Use the `${:-} syntax
  • foo=${1:-something} #gets first command line argument if there, otherwise "something"

Variable Examples

    a=3
    name='tom'
    user=$(cat /etc/passwd | grep 1000 | awk '{ print $1}')
    mydate=$(date)