Using IF in shell scripts
test command or [ expr ]
test command or [ expr ] is used to see if an expression is true, and if it is true it return zero(0), otherwise returns nonzero for false.
Syntax:
test expression OR [ expression ]
Example:
Following script determine whether given argument number is positive.
$ cat > ispostive
#!/bin/sh
#
# Script to see whether argument is positive
#
if test $1 -gt 0
then
echo "$1 number is positive"
fi
Run it as follows
$ chmod 755 ispostive
$ ispostive 5 5 number is positive
$ispostive -45 Nothing is printed
$ispostive ./ispostive: test: -gt: unary operator expected
Detailed explanation The line, if test $1 -gt 0 , test to see if first command line argument($1) is greater than 0. If it is true(0) then test will return 0 and output will printed as 5 number is positive but for -45 argument there is no output because our condition is not true(0) (no -45 is not greater than 0) hence echo statement is skipped. And for last statement we have not supplied any argument hence error ./ispostive: test: -gt: unary operator expected, is generated by shell , to avoid such error we can test whether command line argument is supplied or not.
test or [ expr ] works with
1.Integer ( Number without decimal point)
2.File types
3.Character strings
For Mathematics, use following operator in Shell Script
Mathematical Operator in Shell Script | Meaning | Normal Arithmetical/ Mathematical Statements | But in Shell |
---|---|---|---|
For test statement with if command | For [ expr ] statement with if command | ||
-eq | is equal to | 5 == 6 | if test 5 -eq 6 |
-ne | is not equal to | 5 != 6 | if test 5 -ne 6 |
-lt | is less than | 5 < 6 | if test 5 -lt 6 |
-le | is less than or equal to | 5 <= 6 | if test 5 -le 6 |
-gt | is greater than | 5 > 6 | if test 5 -gt 6 |
-ge | is greater than or equal to | 5 >= 6 | if test 5 -ge 6 |
NOTE: == is equal, != is not equal.
For string Comparisons use
Operator | Meaning |
---|---|
string1 = string2 | string1 is equal to string2 |
string1 != string2 | string1 is NOT equal to string2 |
string1 | string1 is NOT NULL or not defined |
-n string1 | string1 is NOT NULL and does exist |
-z string1 | string1 is NULL and does exist |
Shell also test for file and directory types
Test | Meaning |
---|---|
-s file | Non empty file |
-f file | Is File exist or normal file and not a directory |
-d dir | Is Directory exist and not a file |
-w file | Is writeable file |
-r file | Is read-only file |
-x file | Is file is executable |
Logical Operators
Logical operators are used to combine two or more condition at a time
Operator | Meaning |
---|---|
! expression | Logical NOT |
expression1 -a expression2 | Logical AND |
expression1 -o expression2 | Logical OR |
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