[shell] Repoint an existing symbolic link (symlink) to a different target

$  ln -nsf <TARGET>  <LINK>

Credit: Commandline-fu.

[perl] Access shell environmental variables in your Perl script

The environmental variables are stored in %ENV hash (“associative array”). Now if you want to access a shell variable, say $HOME, in your Perl script, then you need to refer to it as $ENV{'HOME'}:

#!/usr/bin/perl
$my_home_dir = "$ENV{'HOME'}" ;
$my_shell = "$ENV{'SHELL'}" ;
$my_editor = "$ENV{'EDITOR'}" ;

Don’t forget the curly braces and the single quotes!

Reference: Devdaily blog.

[zsh] Histroy substitution

#For Previous Command (for comparison)
!-1     repeat whole command
!!      repeat (shortcut)
!:0     command
!^      first parameter
!:1     first parameter
!:1-4   first 4 parameters
!$      last parameter
!*      all parameters
!!:s/bash/zsh (or ^bash^zsh)
!^:t    just file name of first parameter
!$:h    just path of last parameter
!-2$:r  just file name without extension of first parameter

For last but one command
!-2     repeat last but one command
!-2^    first parameter last but one command
!-2$    last parameter last but one command
!-2:2   second parameter of second but last command
!-2:s/bash/zsh substitute bash by zsh in the last but one command

Most of them are applicable for bash as well.
Credit: copied from here.

[cli] Find out success status of a command in BASH/ CSH

To know the return value of a command, in BASH run
$ echo $?
and in CSH/ TCSH run
$ echo $status
which will give you the return value (0 or 1) of the last command (the one issued just before the echo).

In both the shells, if the above spits out 0 (zero), it means the previous command was successfully completed, otherwise it’ll give rise to 1 (unsuccessful!) — quite contrary to our concept of 0 being false and 1 being true, eh!

Credit: Oracle Spin.

[unix] Change default login shell

I just stumbled upon a few ways to change the default login shell to bash:

The easiest one is

$ chsh -s /bin/bash username

On some machines you may have to use ypcsh instead of chsh. The full path of the shell executable must be specified.

But if you have root access you may have the option to use either of the following two as well:

$ usermod -s bash username

Change it by editing the entry for the user in /etc/passwd file, manually.

If none of the above works, here’s a workaround!

N.B. Make sure that the desired shell is listed in /etc/shells file.