list of control operators in linux?

; semicolon

You can put two or more commands on the same line separated by a semicolon ; .

The shell will scan the line until it reaches the semicolon. All the arguments before

this semicolon will be considered a separate command from all the arguments after

the semicolon. Both series will be executed sequentially with the shell waiting for

each command to finish before starting the next one.

[root@redhat ~]$ echo Hello

Hello

[root@redhat ~]$ echo World

World

[root@redhat ~]$ echo Hello ; echo World

Hello

World

[root@redhat ~]$

& ampersand

When a line ends with an ampersand &, the shell will not wait for the command

to finish. You will get your shell prompt back, and the command is executed in

background. You will get a message when this command has finished executing in

background.

[root@redhat ~]$ sleep 20 &

[1] 7925

[root@redhat ~]$

...wait 20 seconds...

[root@redhat ~]$

[1]+ Done sleep 20

&& double ampersand

The shell will interpret && as a logical AND. When using && the second command

is executed only if the first one succeeds (returns a zero exit status).

root@redhat:~$ echo first && echo second

first

second

root@redhat:~$ zecho first && echo second

-bash: zecho: command not found

Another example of the same logical AND principle. This example starts with a

working cd followed by ls, then a non-working cd which is not followed by ls.

[root@redhat ~]$ cd gen && ls

file1 file3 File55 fileab FileAB fileabc

file2 File4 FileA Fileab fileab2

[root@redhat gen]$ cd gen && ls

-bash: cd: gen: No such file or directory

|| double vertical bar

The || represents a logical OR. The second command is executed only when the first

command fails (returns a non-zero exit status).

root@redhat:~$ echo first || echo second ; echo third

first

third

root@redhat:~$ zecho first || echo second ; echo third

-bash: zecho: command not found

second

third

root@redhat:~$

Another example of the same logical OR principle.

[root@redhat ~]$ cd gen || ls

[root@redhat gen]$ cd gen || ls

-bash: cd: gen: No such file or directory

file1 file3 File55 fileab FileAB fileabc

file2 File4 FileA Fileab fileab2

combining && and ||

You can use this logical AND and logical OR to write an if-then-else structure on

the command line. This example uses echo to display whether the rm command was

successful.

root@redhat:~/test$ rm file1 && echo It worked! || echo It failed!

It worked!

root@redhat:~/test$ rm file1 && echo It worked! || echo It failed!

rm: cannot remove `file1': No such file or directory

It failed!

root@redhat:~/test$

# pound sign

Everything written after a pound sign (#) is ignored by the shell. This is useful to

write a shell comment, but has no influence on the command execution or shell

expansion.

root@redhat:~$ mkdir test # we create a directory

root@redhat:~$ cd test #### we enter the directory

root@redhat:~/test$ ls # is it empty ?

root@redhat:~/test$

\ escaping special characters

The backslash \ character enables the use of control characters, but without the shell

interpreting it, this is called escaping characters.

[root@redhat ~]$ echo hello \; world

hello ; world

[root@redhat ~]$ echo hello\ \ \ world

hello world

[root@redhat ~]$ echo escaping \\\ \#\ \&\ \"\ \'

escaping \ # & " '

[root@redhat ~]$ echo escaping \\\?\*\"\'

escaping \?*"'

end of line backslash

Lines ending in a backslash are continued on the next line. The shell does not interpret

the newline character and will wait on shell expansion and execution of the command

line until a newline without backslash is encountered.

[root@redhat ~]$ echo This command line \

> is split in three \

> parts

This command line is split in three parts

[root@redhat ~]$