More on Conditions

A brief intro into bash conditional details are covered in the more-scripts page. Here is some more on making bash do conditional logic.

More on Conditions

Conditions and Test

test, according to man test says...

SYNOPSIS
       test EXPRESSION
       test
       [ EXPRESSION ]
       [ ]
       [ OPTION

DESCRIPTION
       Exit with the status determined by EXPRESSION.

The result here is an exit code based on an expression.

String Has a length

A -z flag can be added to the test command that tells test to check if a string has a length that is not zero:

# test wont print the output - here's a 2-line approach:
ubuntu@primary:~$ test -z ""
ubuntu@primary:~$ echo $?
0
ubuntu@primary:~$ test -z "asdf"
ubuntu@primary:~$ echo $?
1

# here's a one-line approach
ubuntu@primary:~$ test -z ""; echo $?
0
ubuntu@primary:~$ test -z "water"; echo $?
1

2 Numbers Are Equal

ubuntu@primary:~$ test 15 -eq 2 ; echo $?
1
ubuntu@primary:~$ test 15 -eq 15 ; echo $?
0

2 Strings are Equal

ubuntu@primary:~$ test horse = horse; echo $?
0
ubuntu@primary:~$ test horse = dog; echo $?
1

Numbers are Greater or Less Than

# greater than
ubuntu@primary:~$ test 15 -gt 10; echo $?
0
ubuntu@primary:~$ test 15 -gt 20; echo $?
1

# less than or equal to
ubuntu@primary:~$ test 15 -le 15 ; echo $?
0
ubuntu@primary:~$ test 15 -le 16 ; echo $?
0
ubuntu@primary:~$ test 15 -le 2 ; echo $?
1
0

Files exists

# file exists
ubuntu@primary:~$ test -e ~/some-file.txt; echo $?
0

# file exists AND I can write to it
ubuntu@primary:~$ test -w ~/existing-file.txt; echo $?

Conditions with If and Else

Create a file: to-ten.sh.
This will illustrate an if/else block.
This will expect an argument that is a number & will give a result based on th args value as it relates to the number 10.

if [ $1 -gt 10 ]; then
  echo "greater than 10"
elif [ $1 -lt 10 ]; then
  echo "less than 10"
else
  echo "equals 10"
fi

Conditions that Match One of Many Cases

Create a file: feeling-face.sh.
This will illustrate the case statement by taking an argument & putting the argument through a "case" statement, and depending on the argument the program will return something:

case $1 in
  "happy")
    echo ":)"
  ;;
  "sad")
    echo ":("
  ;;
  "silly")
    echo ":D"
  ;;
  "angry")
    echo "o()xxx[{::::::::::::::>"
  ;;
  "surprised")
    echo "O_o"
  ;;
*)
  echo "I don't understand $1 as a face yet"
  ;;
esac

This program works like

ubuntu@primary:~$ bash faces.sh happy
:)

ubuntu@primary:~$ bash faces.sh angry
o()xxx[{::::::::::::::>

Tags:

linux