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Old 05-22-2017, 08:26 AM   #1
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Post Bash - while IFS and external variable management

Hello everyone,

I'm trying to get familiar with bash and Scripting and i would to realize something that get some command (with variable) from an external file and elaborate it in a cycle in a bash.

This is what i've made (and the problem i'm not able to solve atm).

in the main bash i declare some variables like:
Dirname (a specific directory)
and then a LogDir=Logs"_"$DirName"_"$DateYmd"_"$DateHMS

then i wrote:

while IFS='' read -r line || [[ -n "$line" ]]; do
if $line > /dev/null 2>&1;then
echo "Success....... - $line"
echo "FAILED........ - $line"
done < "$1"

I launch this with the commands.txt

in the "commands.txt" there's something like
/bin/mkdir "$LogDir"
/cp /etc/hosts "$LogDir"

the problem is that this cycle is not able to let bash manage the $LogDir variable and it creates a dir named $LogDir

if i do not use variables everything works well. There's any way to let Bash manage it?

Thanks in advance

Last edited by Cobraone72; 05-22-2017 at 08:28 AM. Reason: modified whife with while :)
Old 05-22-2017, 09:36 AM   #2
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Please use [code][/code] tags around code/data

You could use eval on your 'line' variable of course the danger is if the commands.txt has a nasty piece of code in it you have no way to know until after it is run and the damage is done.

Alternatively, you could source the commands.txt file, but this would run all commands and again, you would want to know the contents prior so not to run malicious code.

I would add that you do not need to set IFS in this case as by having a single variable passed to read, the entire line will be read into that variable.
Also, the use of '-n' would seem mute because if read couldn't get any data from the file, there would be nothing in 'line'. If the line is empty then 'line' will be empty but it will still
be a successful read, hence the '-n' would not be looked at.
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Old 05-22-2017, 10:02 AM   #3
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I think we need to see some more of the script to understand how you are assigning variables here. But you definitely do not need to have quotes around that variable name when you execute the mkdir and cp commands. Also, it is typically not /cp. Either fine the real path for cp, or don't try to explicitly specify that and let the system path variable take care of this.

One thing I always recommend is just after the shebang line, to add a "set -xv" to enable verbose debugging for your script. You can comment that line out using the # character at the start of the line to option this debug on and off as you debug the script. Further, you can move it lower in the script to enable debugging only in places where you are testing and debugging certain code. "set +xv" also reverts the state of that.
/bin/mkdir $LogDir
cp /etc/hosts $LogDir
should work fine
Old 05-23-2017, 02:12 AM   #4
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Complete script

I try to explain exactly what i'm doing a complete way..

There are several systems that has an appliance installed and some paths are different (example log path is /var/log/DirSite1 /var/log/DirSite2 etc etc). Those systems are in some networks that could not access to internet so i wanted to create a script that collects all configuration files, gzip them so che customer could send me everything.

The idea is simple.. I declare some variables (sitename,time, date and the new log dir path created with Logs_Sitename_date_time format, and then i go to the paths to collect the files i need. The main script goes to read into a file nammed command.txt the "relative" paths and copy them into the new dir log i've crated and then zip everything

The problem is that in the While Ifs cycle the variables i've declared in the initial part of the script are lost

this is the example code (complete)
Note: the DirName variable gets the name of the site because there's a dir in home named siteName1 SiteName2, the result will be Name1 or Name2)

# Declare system fixed variables
DirName=$(ls /home/ | grep -i site | awk '{print substr($0,5)}')
DateYmd=$(date +%Y%m%d)
DateHMS=$(date +%H%M%S)
# Create start point in Getlogs.log
echo Site: $DirName - Date $DateYmd - Time $DateHMS | tee -a GetLogs.log
# Execute commands in commands.txt
while IFS='' read -r line || [[ -n "$line" ]]; do
$line > /dev/null 2>&1;then
echo "Success....... - $line" | tee -a GetLogs.log
echo "FAILED........ - $line" | tee -a GetLogs.log
done < "$1"

so i create a txt file named commands.txt where into you could put those lines

/bin/mkdir "$LogDir"
/bin/cp /etc/hosts "$LogDir"
/bin/cp /etc/fstab "$LogDir"
/bin/cp /etc/init/site"$DirName"/* "$LogDir"

(i tired with or without " and the result is the same)

Pratically when a command is passed into the while cycle the system is unable to retain the variable value and create a dir named $LogDir and copies files into a new file named $LogDir

i tried to move the variables into the cycle after while but the result is the same. It is like reading from an external file let variables to be lost.

Any solution or there's no way to solve them?

Old 05-23-2017, 05:52 AM   #5
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you can simply:
source commands.txt
to do that job


bash, ifs, variables

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