Linux Commands | Shell Script | How to Use if, for, and while Statements and Conditions

if Statement Usage and Conditions

Basic Format

if [value condition value condition...]
then
    statements
elif [value condition value condition...]
then
    statements
else
    statements
fi

Types of Conditions

Condition	Description
`-z`	True if the string length is 0
`-n`	True if the string length is not 0
`-eq`	True if the values are equal. Same as the `=` operator
`-ne`	True if the values are different
`-gt`	value1 > value2
`-ge`	value1 >= value2
`-lt`	value1 < value2
`-le`	value1 <= value2
`-a`	Same as the `&&` operator, AND operation
`-o`	Same as the `
`-d`	True if the file is a directory
`-e`	True if the file exists
`-L`	True if the file is a symbolic link
`-r`	True if the file is readable
`-s`	True if the file size is greater than 0
`-w`	True if the file is writable
`-x`	True if the file is executable
file1 -nt file2	True if file1 is newer than file2
file1 -ot file2	True if file1 is older than file2
file1 -ef file2	True if file1 and file2 are the same file

#!/bin/bash

value=0 # If you write value = 0 with spaces between the variable and value, a syntax error occurs.

if [ $value = 0 ]
then
    echo "value is 0"
else
    echo "value is not 0"
fi

Execution result:

$ ./test.sh
value is 0

for Statement Usage

Basic Format

for [variable] in [loop condition]
do
    [statement]
done

Usage Example

#!/bin/bash

for i in 1 2 3
do
    echo "$i"
done

Execution result

$ ./test.sh
1
2
3

while Statement Usage and Conditions

Basic Format

while [ value1 condition value2 ]
do
   [statement]
   [statement]
done

Usage Example

i=0

while [ $i -lt 3 ]
do
    echo $i
     i=$(($i+1))
done

Execution result:

$ ./test.sh 
0
1
2