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(26) More on Shells

More on shells

The Environment

The environment variables are a way to pass information to programs - any program can examine these strings and modify its behaviour (e.g. PRINTER is looked at by the lpr program. Some of these environment variables can be changed, others are preset by the system.

env
HOME=/classes/rcomp1 SHELL=/usr/local/bin/tcsh TERM=vt100 USER=rcomp1 PATH=/bin:/usr/bin:/usr/local/bin:/usr/bin/X11:/usr/ucb:. HOSTTYPE=sun4 VENDOR=sun OSTYPE=solaris MACHTYPE=sparc SHLVL=1 PWD=/classes/rcomp1 LOGNAME=rcomp1 GROUP=users HOST=sunray.dartmouth.edu MANPATH=/usr/local/man:/usr/man:/usr/share/man PRINTER=berry-public

Command Line Arguments

The other way information is passed to applications is through the command line arguments. Everything following the program name on the command line, except things interpreted directly by the shell, is made available to the program. Typically these are option flags (to modify the behaviour of the program) and filenames (for the program to act on). The program may then also have a direct interaction with the use via a text or graphical interface. Most program use all of the methods.

Many featuresof Unix are designed to help in automating procedures (Unix users are lazy). Passing information via the environment and the command line is scriptable, while direct interaction via the keyboard and mouse is not.

Job control

Shells also allow you to place jobs in the background and run multiple processes from one shell session, although multiple windows are easier for this.

ls -FCR > save_it &: The "&" says make this a background job.
^Z: control-Z sends a "suspend" signal to a currently running process
bg: continue execution of a suspended process in the background (must not try to read from the keyboard, but can write to the screen)
fg: resume execution of a suspended program in the foreground
jobs: list background jobs running in current shell session.

slide27.src last modified Jan 31, 2011

Introduction

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