Environment variable in csh, tcsh, bash, zsh
Introduction
Environment variables are an essential part of any operating system. They are used to store system-wide configurations and information that can be accessed by programs and scripts that run on the system. In this blog post, we’ll be discussing environment variables in csh, tcsh, bash, and zsh.
csh tcsh
## variable
set A=B
## environment variable
setenv VAR VALUE
setenv PATH "/bin:/usr/bin:/usr/sbin:/usr/local/bin"
In csh and tcsh, environment variables are declared and set using the setenv command. For example, to declare and set the PATH environment variable, you would use the following command: setenv PATH /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin.
To view all the environment variables that are currently set, you can use the env command. This will display a list of all the environment variables and their values.
One important thing to note is that csh and tcsh use different syntax for referencing environment variables. In csh, you reference a variable using the $ symbol followed by the variable name, like this: $PATH. In tcsh, you use the syntax ${VARNAME}, like this: ${PATH}.
sh bash zsh
## variable
A=B
## environment variable
export PATH="/usr/local/bin:${PATH}"
In bash, environment variables are declared and set using the export command. For example, to declare and set the PATH environment variable, you would use the following command: export PATH=/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin.
To view all the environment variables that are currently set, you can use the env command, just like in csh and tcsh.
In bash, you reference a variable using the same syntax as in csh: $VARNAME.
Conclusion
Environment variables are an important part of any operating system, and understanding how to work with them in different shells is essential for any system administrator or developer. In this blog post, we’ve discussed how to declare, set, and reference environment variables in csh, tcsh, bash, and zsh. While there are differences in syntax between the shells, the basic concepts are the same.