UNIX for Dummies Questions & Answers

can anyone tell me what the following mean.....
1. ! bash/bin and what is its significance...
2. what are runlevels like 1, 2, 3, 4....what do they mean???
this was picked up from the /etc/init.d/crond file......its an initscript.

#!/bin/bash tells your OS what program to run the script with. in this case, it is bash. in others, it could be #!/bin/perl. it depends on what language the script was written in.

runlevels are a nice concept to have. this is from my inittab file on suse linux 8.2:

# runlevel 0 is System halt (Do not use this for initdefault!)
# runlevel 1 is Single user mode
# runlevel 2 is Local multiuser without remote network (e.g. NFS)
# runlevel 3 is Full multiuser with network
# runlevel 4 is Not used
# runlevel 5 is Full multiuser with network and xdm
# runlevel 6 is System reboot (Do not use this for initdefault!)


in the /etc/inittab file you can specify default runlevel, of which are described above. from there you might have an /etc/rc.d/ directory. from here you can customize your runlevels. each runlevel starts services according to what is defined in the /etc/rc.d/ directory. you can add or take away services that are run at boot to customize your system however you want.

Also see What does "#! /usr/bin/ksh" mean?

norsk hedensk is very detail very informative.

Something to note on top of this, is that what each runlevel does out of the box can vary between systems, for example Slackware Linux has Multiuser + X as runlevel 4 rather than 5.

right. i forgot to mention that, but that is why i specified my system. so remember that it is important to make sure you know how your particular system is configured.