The fundamental process for an interactive shell is
You can get a list of the current shell's environmental variables in various ways: env, printenv; (in Bash, you can also use set, but that also gives additional shell variables that aren't actually in the process's environment.)
Important environment variables:
In addition to environmental variables which are passed on to child processes, you can create local variables.
% export x=12 # create an environmental variable % y=15 # create a local variable % echo x = $x and y = $y x = 12 and y = 15 % bash # now create a new child process % echo x = $x and y = $y x = 12 and y =
Bourne shell, Korn shell, Bash, and others in that family have two primary prompt variables, PS1 and PS2
export PS1="# " export PS2="... "
I have already mentioned input redirection in the context of creating files. Overall, there are three important redirection operators: <, >, and >>
The < operator lets you redirect standard in; for instance, you can do
sort < /etc/passwd
and this will have sort takes its standard in from the file /etc/passwd.
As we have seen already, the > operator lets you redirect standard out; for instance, you can take the output of cal and save it:
cal 1752 > /tmp/unusual-cal.txt
cal 1753 >> /tmp/unusual-cal.txt
You can also specifically name which file descriptor to use with the form "n>" and "n<".
This is particularly useful when you want to split, say, stdout and stderr data out to two different places.
(ls -R | wc -l) 2>/dev/null
ls -lR / 1>&2
This sends all of file descriptor 1 (stdout) also to file descriptor 2 (stderr).
Another very useful type of redirection, particularly in shell scripts, is the "here" document.
The syntax looks like
<<'EOF' data... data... data... data... EOF
$ cat <<'EOF' > this is stuff that > we want to echo > until the > EOF this is stuff that we want to echo until the $
In addition to simple file redirection, we have the concept of a "pipe", which is a creation of the kernel. It's actually just a simple "buffer".
The syntax for creating a pipe between two processes is the vertical bar ("|").
sort < /etc/passwd | cut -d: -f 1
The tee: there's a nice program call tee that will let you intercept "mid-pipe" data.
$ sort < /etc/passwd | tee /tmp/sorted-passwd | cut -d: -f 1 $ cat /tmp/sorted-passwd
$ pv /etc/services | wc 19.1kB 0:00:00 [ 434MB/s] [==================================>] 100% 611 2662 19558 $
echo The date and time is `date`
This can be particularly valuable when you save the output to a variable:
% x=`date --iso-8601` % mkdir new-$x/ old-$x/ cur-$x/ % ls -d *$x cur-2013-01-29 new-2013-01-29 old-2013-01-29 %
% x=$(data --iso-8601) % mkdir new-$x/ old-$x/ cur-$x/ % ls -d *$x cur-2013-01-29 new-2013-01-29 old-2013-01-29 %
Once we have the concept of a bytestream flowing through pipes, a natural metaphor for programs that modify the bytestream is to call them a "filter".
There are a very large number of standard filters; some of the most useful are here.
Here's a filter example derived from page 107 of Kernighan and Pike's The Unix Programming Environment that shows the 8 most frequently used words in Pilgrim's Progress by John Bunyan (book contents courtesy of Project Gutenberg):
% tr -sc A-Za-z '\012' < pg39452.txt | sort | uniq -c | sort -n | tail -8 1454 in 1567 a 1910 I 2289 that 2570 of 3078 to 3479 and 4397 the
Here's a "one-liner" I created to list all of the system calls on a given system:
% ls /usr/share/man/man2 | sed -e s/.2.gz//g | \ > xargs man -s 2 -k | sort | \ > grep -v 'unimplemented system calls'
One of the great strengths of Unix has been its very clean "job control". You can easily send a process into the background with a simple ampersand:
% ( ls -R / | wc -l ) 2>/dev/null & % jobs + Running ( ls -R / | wc -l ) 2> /dev/null &
You can start many jobs:
% cat &  7200 % cat &  7201 % cat &  7203 % cat &  7209 % fg %1 # now bring job #1 back cat some input some input [CTRL-D] % jobs # okay, we see that job #1 has finished  Stopped cat - Stopped cat + Stopped cat % fg %3 # now bring job #3 cat more input more input [CTRL-D] % jobs # and now we see that it's finished also - Stopped cat + Stopped cat % # and so forth...
If you get stuck in a LaTeX session (or emacs, for that matter), you can often use CTRL-Z to get out. Once you do, you can use kill to get rid of it:
pdflatex file1.tex This is pdfTeX, Version 3.1415926-1.40.10 (TeX Live 2009/Debian) entering extended mode ! I can't find file `file1.tex'. <*> file1.tex (Press Enter to retry, or you will never exit!) Please type another input file name: + Stopped pdflatex file1.tex [CTRL-Z] % kill -9 %1