[SOLVED] >>> and Output redirection

mmhs · 10-02-2010, 11:56 PM

hi Guys i have two questions about redirection in linux

first : what does it mean in linux >>> i know we have this type of redirection but it's undocumented .

second : i want to write a shell script cuz when a user use Output redirect if file has a Contents give an error to user an dont overwrite on this file . ( and i think we have a command in linux that do this when u use this command it's output redirect and if file has content give an error to user ) !!!!

i dont want to use appned !!

malekmustaq · 10-03-2010, 12:16 AM

mmhs,

Really hard to answer a hardly understandable question.

Quote:

first : what does it mean in linux >>> i know we have this type of redirection but it's undocumented .

Pipe. e.g. 'echo "The quick brown fox" >> txt.file' The sentence is written into txt.file.

mmhs · 10-03-2010, 12:41 AM

thx man but u dont understand my question >> use to append but i want to know what about this >>> in linux ( we have >>> in linux but it's undocumented (same as here document ( << ) )

and what about my second question !!

grail · 10-03-2010, 01:12 AM

I am not sure about >>>, but <<< is called a here string, see here for some detail

mmhs · 10-03-2010, 01:27 AM

Quote:

Originally Posted by grail

I am not sure about >>>, but <<< is called a here string, see here for some detail

thx alot man but what about my second question ??

catkin · 10-03-2010, 01:32 AM

Please give an example of >>>. It is not known by bash 3.1.7:

Code:

c@CW8:/tmp$ echo bar >>> foo
bash: syntax error near unexpected token `>'

To prevent > overwriting existing files, use set -o noclobber. Here's a terminal scrape to illustrate its usage

Code:

c@CW8:/tmp$ echo bar > foo
c@CW8:/tmp$ ls -l foo
-rw-r--r-- 1 c users 4 2010-10-03 11:54 foo
c@CW8:/tmp$ set -o noclobber
c@CW8:/tmp$ echo bar > foo
bash: foo: cannot overwrite existing file
c@CW8:/tmp$ set +o noclobber
c@CW8:/tmp$ echo barbar > foo
c@CW8:/tmp$ ls -l foo
-rw-r--r-- 1 c users 7 2010-10-03 11:54 foo

The "here document" use of << is in the bash man page and here.

mmhs · 10-03-2010, 01:58 AM

Quote:

Originally Posted by catkin

Please give an example of >>>. It is not known by bash 3.1.7:

Code:

c@CW8:/tmp$ echo bar >>> foo
bash: syntax error near unexpected token `>'

To prevent > overwriting existing files, use set -o noclobber. Here's a terminal scrape to illustrate its usage

Code:

c@CW8:/tmp$ echo bar > foo
c@CW8:/tmp$ ls -l foo
-rw-r--r-- 1 c users 4 2010-10-03 11:54 foo
c@CW8:/tmp$ set -o noclobber
c@CW8:/tmp$ echo bar > foo
bash: foo: cannot overwrite existing file
c@CW8:/tmp$ set +o noclobber
c@CW8:/tmp$ echo barbar > foo
c@CW8:/tmp$ ls -l foo
-rw-r--r-- 1 c users 7 2010-10-03 11:54 foo

The "here document" use of << is in the bash man page and here.

thx alot man but i have no example for >>> i hear it before an now i want to know what's this !

if i want to write a shell script to check it for dont overwrite when user use > how can i do ???

thx alot guys for help

Wim Sturkenboom · 10-03-2010, 02:40 AM

Quote:

Originally Posted by mmhs

if i want to write a shell script to check it for dont overwrite when user use > how can i do ???

thx alot guys for help

You can use 'stat' and check the return value (the '$?' in examples below)

example for existing file

Code:

fortyfourgalena@desktop1:~$ stat version-check.sh
  File: `version-check.sh'
  Size: 1186      	Blocks: 8          IO Block: 4096   regular file
Device: 809h/2057d	Inode: 7768977     Links: 1
Access: (0755/-rwxr-xr-x)  Uid: ( 1005/fortyfourgalena)   Gid: ( 1005/fortyfourgalena)
Access: 2010-08-10 17:00:55.000000000 +0200
Modify: 2009-08-04 20:05:44.000000000 +0200
Change: 2010-07-18 20:30:09.000000000 +0200
fortyfourgalena@desktop1:~$ echo $?
0
fortyfourgalena@desktop1:~$

example for non-existing file

Code:

fortyfourgalena@desktop1:~$ stat version-chk.sh
stat: cannot stat `version-chk.sh': No such file or directory
fortyfourgalena@desktop1:~$ echo $?
1
fortyfourgalena@desktop1:~$

check the man page if necessary

mmhs · 10-03-2010, 03:22 AM

Quote:

Originally Posted by Wim Sturkenboom

You can use 'stat' and check the return value (the '$?' in examples below)

example for existing file

Code:

fortyfourgalena@desktop1:~$ stat version-check.sh
  File: `version-check.sh'
  Size: 1186      	Blocks: 8          IO Block: 4096   regular file
Device: 809h/2057d	Inode: 7768977     Links: 1
Access: (0755/-rwxr-xr-x)  Uid: ( 1005/fortyfourgalena)   Gid: ( 1005/fortyfourgalena)
Access: 2010-08-10 17:00:55.000000000 +0200
Modify: 2009-08-04 20:05:44.000000000 +0200
Change: 2010-07-18 20:30:09.000000000 +0200
fortyfourgalena@desktop1:~$ echo $?
0
fortyfourgalena@desktop1:~$

example for non-existing file

Code:

fortyfourgalena@desktop1:~$ stat version-chk.sh
stat: cannot stat `version-chk.sh': No such file or directory
fortyfourgalena@desktop1:~$ echo $?
1
fortyfourgalena@desktop1:~$

check the man page if necessary

thx man but i want check if file exists and has content give user and error and protect file from overwrite

catkin · 10-03-2010, 04:30 AM

Quote:

Originally Posted by mmhs

thx alot man but i have no example for >>> i hear it before an now i want to know what's this !

Your informant may have been mistaken. You could ask them to show you an example.

Quote:

Originally Posted by mmhs

thx man but i want check if file exists and has content give user and error and protect file from overwrite

What's wrong with set -o noclobber? If I understand what you are asking for then it does exactly what you want.

mmhs · 10-03-2010, 04:35 AM

it's work fine man but i want to write a shell script work same as set -o noclobber !!!

catkin · 10-03-2010, 04:43 AM

Quote:

Originally Posted by mmhs

it's work fine man but i want to write a shell script work same as set -o noclobber !!!

Which you can do by using set -o noclobber in your shell script before using any >.

EDIT: like this

Code:

#!/bin/bash

set -o noclobber
echo foo > /tmp/myfile
[[ $? -ne 0 ]] && exit 1
exit 0

If /tmp/myfile does not exist, the script writes foo to it and exits normally with an exitcode of 0 (which you can show by running echo $? after running the script).

If /tmp/myfile does exist, the script generates a error message and exits with an exitcode of 1 (which you can show by running echo $? after running the script).