Oh ok so basically for the most part backticks are not required. But for some reason, I remember when I was writing another program, I had to put the backticks in or it just wasn't working. Is there a scenario where backticks are required?
You have used backticks probably like below (in echo (print) statement ) but not csh because of its hang may cause of a bug when use bactick with tcsh..
you can maybe update tcsh with related patch or upgrade with a newer version then try again with backticks
Code:
#!/bin/bash
clear
echo ".-----------------------------------------------------------------."
echo "| |"
echo "| Please specify which Shell you want to switch to: |"
echo "| 1. Bash |"
echo "| 2. Korn Shell |"
echo "| 3. T Shell |"
echo "| |"
echo "'-----------------------------------------------------------------'"
echo " "
echo "Enter the number of your choice: "; read shell
case $shell in
1) echo "You are now in BASH" ; bash ;;
2) echo "You are now in KSH" ; ksh ;;
3) echo "You are now in TCSH" ; tcsh ;;
esac
clear
#!/bin/bash
echo "Please select which Shell you want to switch to:"
select shell in bash ksh tcsh
do
echo "You are now running $shell"
exec $shell
done
Backticks have one and precisely one function: They turn a program's output into a parameter or string.
Code:
# doesn't work
STRING=programname
# works
STRING=`programname`
# doesn't work
myprogram program-printing-parameters
# works
myprogram `program-printing-parameters`
If you're throwing around backticks everywhere until a program "works" you are misusing them. Only use them where you want to capture a program's output into a parameter or string.
parent shell is waiting for execute of return value (file , command , variable , or any result..) that in backticks expression.firstly a new shell is executed and switching in.in new child shell , but commands are executed that is not efficiently work so does not produce any output.however parent shell is waiting still the result of it's and therefore probably commands can not see on stdoutput in this shell (probably non-access to terminal device via stdoutput because of ` ` backticks are still open so uncompleted and subshell has not it's own stdoutput fd!! )
firstly lets see wait issue effect these with backticks
for this i added the end of file an `echo`
Code:
# sed -n '$p' /etc/bashrc
echo UNIX.com
and then execute a new shell
Code:
# `bash`
i am in a new shell now
# echo "wheres is results my commands "
nothing displayed..but commands are running!
parent shell is try to execute result in backticks so it will try to execute "UNIX.com" when exit the subshell
Code:
# exit
exit
-bash: UNIX.com: command not found
in subshell
Code:
# echo "test"
nothing displayed again!.
try vim
Code:
# vim /usr/local/bin/test
Vim: Warning: Output is not to a terminal
Besides, as seen in above there is not a map with terminal and stdoutput in new shell with backticks..
last of all , in your script bash and ksh is seem success , csh is hanging (but in fact none of the successful)
as a result backticks are un-needed when they already will execute the commands by current shell.
Code:
#!/bin/bash
clear
echo ".-----------------------------------------------------------------."
echo "| |"
echo "| Please specify which Shell you want to switch to: |"
echo "| 1. Bash |"
echo "| 2. Korn Shell |"
echo "| 3. T Shell |"
echo "| |"
echo "'-----------------------------------------------------------------'"
echo " "
echo "Enter the number of your choice: "; read shell
case $shell in
1) echo "You are now in BASH" ; exec bash ;;
2) echo "You are now in KSH" ; exec ksh ;;
3) echo "You are now in TCSH" ; exec tcsh ;;
esac
clear
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