Operadores shell

Arithmetic Operators:

There are following arithmetic operators supported by Bourne Shell.

Assume variable a holds 10 and variable b holds 20 then:

Show Examples

Operator Description Example
+ Addition – Adds values on either side of the operator `expr $a + $b` will give 30
Subtraction – Subtracts right hand operand from left hand operand `expr $a – $b` will give -10
* Multiplication – Multiplies values on either side of the operator `expr $a \* $b` will give 200
/ Division – Divides left hand operand by right hand operand `expr $b / $a` will give 2
% Modulus – Divides left hand operand by right hand operand and returns remainder `expr $b % $a` will give 0
= Assignment – Assign right operand in left operand a=$b would assign value of b into a
== Equality – Compares two numbers, if both are same then returns true. [ $a == $b ] would return false.
!= Not Equality – Compares two numbers, if both are different then returns true. [ $a != $b ] would return true.

It is very important to note here that all the conditional expressions would be put inside square braces with one spaces around them, for example [ $a == $b ] is correct where as [$a==$b] is incorrect.

All the arithmetical calculations are done using long integers.

Relational Operators:

Bourne Shell supports following relational operators which are specific to numeric values. These operators would not work for string values unless their value is numeric.

For example, following operators would work to check a relation between 10 and 20 as well as in between “10” and “20” but not in between “ten” and “twenty”.

Assume variable a holds 10 and variable b holds 20 then:

Show Examples

Operator Description Example
-eq Checks if the value of two operands are equal or not, if yes then condition becomes true. [ $a -eq $b ] is not true.
-ne Checks if the value of two operands are equal or not, if values are not equal then condition becomes true. [ $a -ne $b ] is true.
-gt Checks if the value of left operand is greater than the value of right operand, if yes then condition becomes true. [ $a -gt $b ] is not true.
-lt Checks if the value of left operand is less than the value of right operand, if yes then condition becomes true. [ $a -lt $b ] is true.
-ge Checks if the value of left operand is greater than or equal to the value of right operand, if yes then condition becomes true. [ $a -ge $b ] is not true.
-le Checks if the value of left operand is less than or equal to the value of right operand, if yes then condition becomes true. [ $a -le $b ] is true.

It is very important to note here that all the conditional expressions would be put inside square braces with one spaces around them, for example [ $a <= $b ] is correct where as [$a <= $b] is incorrect.

Boolean Operators:

There are following boolean operators supported by Bourne Shell.

Assume variable a holds 10 and variable b holds 20 then:

Show Examples

Operator Description Example
! This is logical negation. This inverts a true condition into false and vice versa. [ ! false ] is true.
-o This is logical OR. If one of the operands is true then condition would be true. [ $a -lt 20 -o $b -gt 100 ] is true.
-a This is logical AND. If both the operands are true then condition would be true otherwise it would be false. [ $a -lt 20 -a $b -gt 100 ] is false.

String Operators:

There are following string operators supported by Bourne Shell.

Assume variable a holds “abc” and variable b holds “efg” then:

Show Examples

Operator Description Example
= Checks if the value of two operands are equal or not, if yes then condition becomes true. [ $a = $b ] is not true.
!= Checks if the value of two operands are equal or not, if values are not equal then condition becomes true. [ $a != $b ] is true.
-z Checks if the given string operand size is zero. If it is zero length then it returns true. [ -z $a ] is not true.
-n Checks if the given string operand size is non-zero. If it is non-zero length then it returns true. [ -z $a ] is not false.
str Check if str is not the empty string. If it is empty then it returns false. [ $a ] is not false.

File Test Operators:

There are following operators to test various properties associated with a Unix file.

Assume a variable file holds an existing file name “test” whose size is 100 bytes and has read, write and execute permission on:

Show Examples

Operator Description Example
-b file Checks if file is a block special file if yes then condition becomes true. [ -b $file ] is false.
-c file Checks if file is a character special file if yes then condition becomes true. [ -b $file ] is false.
-d file Check if file is a directory if yes then condition becomes true. [ -d $file ] is not true.
-f file Check if file is an ordinary file as opposed to a directory or special file if yes then condition becomes true. [ -f $file ] is true.
-g file Checks if file has its set group ID (SGID) bit set if yes then condition becomes true. [ -g $file ] is false.
-k file Checks if file has its sticky bit set if yes then condition becomes true. [ -k $file ] is false.
-p file Checks if file is a named pipe if yes then condition becomes true. [ -p $file ] is false.
-t file Checks if file descriptor is open and associated with a terminal if yes then condition becomes true. [ -t $file ] is false.
-u file Checks if file has its set user id (SUID) bit set if yes then condition becomes true. [ -u $file ] is false.
-r file Checks if file is readable if yes then condition becomes true. [ -r $file ] is true.
-w file Check if file is writable if yes then condition becomes true. [ -w $file ] is true.
-x file Check if file is execute if yes then condition becomes true. [ -x $file ] is true.
-s file Check if file has size greater than 0 if yes then condition becomes true. [ -s $file ] is true.
-e file Check if file exists. Is true even if file is a directory but exists. [ -e $file ] is true.

C Shell Operators:

Following link would give your brief idea on C Shell Operators.

C Shell Operators

Anuncios
Publicado en shell script
One comment on “Operadores shell
  1. Muchas gracias por esta información, la verdad que es bueno conseguir paginas como esta, ahora mismo iniciaré un proyecto que se relaciona bastante con esto.

Responder

Introduce tus datos o haz clic en un icono para iniciar sesión:

Logo de WordPress.com

Estás comentando usando tu cuenta de WordPress.com. Cerrar sesión / Cambiar )

Imagen de Twitter

Estás comentando usando tu cuenta de Twitter. Cerrar sesión / Cambiar )

Foto de Facebook

Estás comentando usando tu cuenta de Facebook. Cerrar sesión / Cambiar )

Google+ photo

Estás comentando usando tu cuenta de Google+. Cerrar sesión / Cambiar )

Conectando a %s

A %d blogueros les gusta esto: