Conditionals

If with test

#!/usr/bin/env bash

# with test
if test $1 -gt 5
then
    echo "It is greater than 5"
fi 

if test $1 -lt 5
then 
    echo "It is less than 5"
else
    echo "It is not less than 5"
fi

if with brackets

#!/bin/bash

# test but using brackets
# must put space around the brackets
if [ $1 -lt 10 ]
then 
    echo "$1 is less than 10"
elif [ $1 -gt 10 ]
then
    echo "$1 is greater than 10"
else 
    echo "$1 is 10"
fi 

Comparison Operators

# don't run this file
# just showing the operators

### integer comparisons

[ 3 -ge 2 ]  # greater than or equal to
(( 5 >= 3 )) # option with paren

[ 3 -gt 2 ]  # greater than
(( 5 > 2 ))  # with paren

[ 5 -lt 10 ] # less than
(( 5 < 3 ))  # with paren

[ 5 -le 10 ] # less than or equal to
(( 5 <= 10 )) # with paren

[ 5 -eq 5 ]   # equal to
              # no == for ints

[ 5 -ne 5 ]   # not equal to

##### String comparisons
# equality
[ "apple" == "apple" ] # returns true

myvar="hi you"
[ $myvar == "hi you" ] # returns false, performs word splitting on "hi you"

[[ $myvar == "hi you" ]] # returns true
[ "$myvar" == "hi you" ] # or put the variable in quotes to return true

# double brackets can help with pattern matching
[[ "apple" == a* ]]  # returns ture, bc apple starts with an a (* for wildcard)

#  checking for ASCII alphabetical order with > or <
[[ "apple" < "banana" ]] # returns true
[ "apple" \< "banana" ]  # can use \< if using single brackets

# check for an empty string with -z
avar=""
[ -z $avar ]  # returns true

# check if a string is not null/empty
[ -n $avar ]  # return false

File Testing

#!/usr/bin/env bash

## Three file names to test
## Be sure each exists in the directory here
## Comment/Uncomment each to test the conditionals that follow

# filename="hello world.txt" 
# filename="04 File Testing.sh"
filename="test directory"

# -e checks if a file or directory exists with the given name
# note that we are using double brackets with the variable to prevent word splitting 
# The filename hello world.txt would cause word splitting because there is a space in it
printf "File/Directory Existence: "
[[ -e $filename ]] && echo "True" || echo "False"  

# -f checks if the string matches a regular file (i.e. actual file, not directory or device)
printf "File Only Existence: "
[[ -f $filename ]] && echo "True" || echo "False"   

# -r checks if the string matches a readable file
printf "Readable File: "                   
[[ -r $filename ]] && echo "True" || echo "False"   

# -d checks if the string matches a directory
printf "Directory: "
[[ -d $filename ]] && echo "True" || echo "False"    

# -L checks if the string matches a symbolic link
printf "Is it a symbolic link: "
[[ -L $filename ]] && echo "True" || echo "False"    

# -x checks if the string matches an executable
printf "Is it executable: "
[[ -x $filename ]] && echo "True" || echo "False"    

# there are many more of these... use `man test` to look them up (or the internet)

Logical Operators

# and
[ 1 -eq 1 -a 2 -eq 2 ]  # -a for and
[[ 1 -eq 1 && 2 -eq 2 ]]  # double amp in double bracket
[ 1 -eq 1 ] && [2 -eq 2 ] # double amp outside of single bracket

# or
[ 1 -eq 1 -o 2 -eq 2 ]  # -o for or
[[ 1 -eq 1 || 2 -eq 2 ]]  # double pipe in double bracket
[ 1 -eq 1 ] || [2 -eq 2 ] # double pipe outside of single bracket

# not is ! (just put it out front)
[ ! -f "hello.txt" ]

Existance Example

#!/bin/bash

if [ -f "hello world.txt" ]
then
    echo "The file hello world.txt exists"
else
    echo "The file does not exist"
fi

# check if some directories exist
if [ -d "../01 Intro" -a -d "../02 Args" ]
then 
    echo "They exist"
else  
    echo "They don't"
fi