Behaviour of echo commands used by Linux admins Post: 303023888

Sponsored Content

Top Forums UNIX for Beginners Questions & Answers Behaviour of echo commands used by Linux admins Post 303023888 by kraljic on Tuesday 25th of September 2018 07:51:22 AM

09-25-2018

Registered User

Behaviour of echo commands used by Linux admins

version info : Fedora 28 (Kernel version: 4.16.12-300)
shell : bash

Using echo command , if I redirect a text like "Chocolate" to a file , all the contents in the file are overwritten as shown below.

Code:

[root@john215 ~]# cat /tmp/someTest 
Hello world 
One more Hello world 
myLine3 
 
[root@john215 ~]# echo Chocolate > /tmp/someTest 
[root@john215 ~]# cat /tmp/someTest 
Chocolate 
[root@john215 ~]# file /tmp/someTest 
/tmp/someTest: ASCII text

But, I have seen Linux system admins using echo commands like below to scan for new LUNs.

Why is "/sys/class/scsi_host/host1/scan" file not being overwritten with "- - -" or "1" by the echo commands below ?

Code:

 
# echo "1" > /sys/class/fc_host/host/issue_lip 
# echo "- - -" > /sys/class/scsi_host/host1/scan

Source:

1.8. Scanning Storage Interconnects - Red Hat Customer Portal

Code:

# file /sys/class/scsi_host/host1/scan 
/sys/class/scsi_host/host1/scan: writable, regular file, no read permission

kraljic

View Public Profile for kraljic

Find all posts by kraljic

9 More Discussions You Might Find Interesting

1. UNIX for Dummies Questions & Answers

Longer commands and strange behaviour on ksh

Hi, I was trying to customize this archaic HP-UX box. only shell available is ksh and that too seems to be pretty old and doesn't completely conform to what I read on the web about ksh. Anyway here are my issues: - I wanted to have a dynamic title on xterm or dtterm. I put the following lines...

2. Shell Programming and Scripting

Wierd behaviour setting stty echo

Hi all, Encountered a wierd behaviour which I am unable to understand. I have a function doing the follow: function RETRIEVE_PASSWORD { if (( $DC_ACCOUNT )) then clear printf "\nEnter Password for ${ConfiguredUser}" printf...

3. Shell Programming and Scripting

SSH, Remote Commands and echo, oh my!

So, HostB has a SSH trust via pre-shared keys from HostA. HostA> ssh HostB hostname HostB HostA> ssh HostB echo `hostname` HostA HostA> ssh HostB 'echo `hostname`' `hostname` HostA> ssh HostB "echo `hostname`" HostA HostA> ssh HostB echo $PS1 user@HostA:$PWD HostA> ssh HostB...

4. Shell Programming and Scripting

Advice using cut & echo combination commands

Hi, I am cutting data from a fixed length test file and then writing out a new record using the echo command, the problem I have is how to stop multiple spaces from being written to the output file as a single space. Example: cat filea | while read line do field1=`echo $line | cut -c1-2` ...

5. Linux

Linux/Weblogic Admins?

Please forgive me if I've chosen the wrong forum... but I'm just wondering how likely it is to find a Linux sys admin with a vast BEA Weblogic skillset or vice versa (a Weblogic admin with vast Linux admin skills)? The reason I ask is because our Linux Sys Admin is leaving for a position with...

6. UNIX for Dummies Questions & Answers

Having a hard time with the sed/echo commands?

Hello, well what I'm trying to do is to remove underscores from filenames and leaving empty spaces instead: arturas@Universe:/windows/Center/training$ ls big_file failas su shudu arturas@Universe:/windows/Center/training$ a=big_file arturas@Universe:/windows/Center/training$ mv $a `echo...

7. Shell Programming and Scripting

echo and grep commands

Hey im new in this...anything will be helpful... The user will input the word or phrase .... I want to search the user input in file (by lines) but not all then with this line search on another file ( with the specific line) and show to the user. Example: file1.txt ======= a aa aaa...

8. UNIX for Dummies Questions & Answers

Echo out running commands

Is there any way in a script to print out the commands being ran? In DOS script, there is the "@echo on" and "@echo off". so I have a script like this: #!/bin/ksh echo "hello there. moving files." <turn on echoing here> cp thisfile.txt thatfile.txt cp whatfile.prop whyfile.prop <turn...

9. Shell Programming and Scripting

Strange behaviour on my Linux Server.

Hi, I keep getting this error when i run my script: No matter how many times i run this rm -rf /siebel/sfs/tmp/dump it succeeds when i expect it only the first time to succeed becoz i expect the second to fail as the directory should be gone in the first run. i had earlier mentioned...

9 More Discussions You Might Find Interesting

1. UNIX for Dummies Questions & Answers

Longer commands and strange behaviour on ksh

Discussion started by: anurags

2. Shell Programming and Scripting

Wierd behaviour setting stty echo

Discussion started by: srage

3. Shell Programming and Scripting

SSH, Remote Commands and echo, oh my!

Discussion started by: Wrathe

4. Shell Programming and Scripting

Advice using cut & echo combination commands

Discussion started by: dc18

5. Linux

Linux/Weblogic Admins?

Discussion started by: rm -r *

6. UNIX for Dummies Questions & Answers

Having a hard time with the sed/echo commands?

Discussion started by: arcelivez

7. Shell Programming and Scripting

echo and grep commands

Discussion started by: Sundown

8. UNIX for Dummies Questions & Answers

Echo out running commands

Discussion started by: ronron5477

9. Shell Programming and Scripting

Strange behaviour on my Linux Server.

Discussion started by: mohtashims