Thema: How to 'filter' on two lines, in a file ?

Hi Lucifer,

i think i'am not the best teacher but i will try to explain it;

programs in linux are returns an exit status and if all is ok this exit status is 0 (zero)
if the program fails then the exit status is nonzero as example 1 or whatever.

to try, write only the command mv in telnet and press the enter button, then you can see following;

Code:

BusyBox v1.24.1 (2017-03-16 11:12:22 CET) multi-call binary.

Usage: mv [-fin] SOURCE DEST
or: mv [-fin] SOURCE... DIRECTORY

Rename SOURCE to DEST, or move SOURCE(s) to DIRECTORY

        -f      Don't prompt before overwriting
        -i      Interactive, prompt before overwrite
        -n      Don't overwrite an existing file

ok, and now make echo $? in telnet then you got the following output;

Code:

1

this is the exit status from the command mv which is failed in this case because mv need options too.
and now make echo "take it" in telnet and as next again echo $?;
if the command was successful you got now the output;

Code:

0

now you know the command echo "take it" was successful since the exit status is zero.

so a simple way to check exit status with telnet as you can see is the command echo $?

in scripts i personally use only exit if a script is successful or exit 1 if a command in the script fails.
you can check the exit status in scripts as following also;

Code:

        if [ "$?" = "0" ] ; then
            echo "blablabla"
        else
              echo "failed" && exit 1
        fi

as example (from my dvdcopy script);

Code:

if [ ! -f $DVDBACKUP ] ; then
    opkg install $DVDBACKUP_PACKAGE
    
        if [ "$?" = "0" ] ; then
            echo -e "Installation Programm $DVDBACKUP_PACKAGE erfolgreich\n"
        else
            echo "ABBRUCH ! Installation Programm $DVDBACKUP_PACKAGE fehlgeschlagen !"
            echo "Eventuell weil die Paketmanagement Aktualisierung zuvor fehlschlug"
            echo -e "Bitte Programm $DVDBACKUP_PACKAGE manuell installieren, und das Script erneut starten\n" && exit 1
        fi
        
else
    echo -e "Programm $DVDBACKUP_PACKAGE ist vorhanden\n"
fi

if the command to install $DVDBACKUP_PACKAGE fails so you can see at the else branch under if [ "$?" = "0" ] ; then i use exit 1
you do not have to use exit 1 if the script (command) fails at this part, the script ends also if you use exit or exit 0 or exit 1 but in this case exit 1 is correct.

and at last here a link to the exit status;
Exit Codes With Special Meanings


regards
Pike