UNIX Tools and the UNIX File System CMSC 121 Introduction to UNIX

UNIX Tools and the
UNIX File System
CMSC 121 Introduction to UNIX
The material on submitexec was taken from
Dan Hood’s CMSC 121 Lecture Notes.
UNIX File System: Links
Every file in the UNIX file system has an associated
block of information called an i-node.
The i-node contains such all information about a file, except
its name.
The i-node’s information includes: date created, user id,
group id, file size, timestamps, the number of links, etc. It
also contains the physical location of the file’s blocks on the
The i-node table maps the filename to the specific inode. ls –i will display the i-node for a file.
As each file has its own i-node, there is no reason that
more than one name can’t point to the same i-node in
the table. This is the idea behind links.
UNIX File System: Links
The ln command creates two names to the same file.
linux2 [4]# touch temp
linux2 [5]# ln temp temp_link
These are called “hard links” because they are of equal
status. Type ls –i and compare the files’ i-nodes.
A file isn’t actually deleted until all of the hard links to
it have been removed. So, rm temp will not remove
the file, only this one link to it.
One important thing to note is that you cannot hard
link across file systems, as i-node numbers are not
unique across file systems.
You also cannot hard link directories.
UNIX File System: Symbolic Links
A symbolic link is a sort of alias to a file. Symlinks
have a much lower status than hard links.
The ln -s command creates a symlink to a file.
linux2 [5]# ln –s temp temp_symlink
ls –L will dereference symlinks to the actual files,
instead of giving information about where symlinks
Symlinks can work with directories and across file
systems, as this is their main purpose.
Note that some programs (e.g. tar) provide options for
dealing with symlinks so that they do not break.
tar’s –h ensures that symlinks don’t break by saving the
referenced files instead of the symlinks.
UNIX Tools Philosophy
Each tool should do one thing very well,
nothing more and nothing less.
These tools should support chaining using
streams. The output of one program should be
considered to be the input to another program.
Use existing tools in combination to do very
complex tasks instead of building specialized
xargs: chaining commands
xargs takes the output of one command and passes it
as command line arguments to another command.
find . -name “*.txt” | xargs grep John
Using single quotes may overflow the command line, so
you shouldn’t use this form.
grep John ‘find . -name “*.txt”’
Another use of xargs:
Example: take all the files whose names are in the file
“names” and concatenate their contents:
xargs cat < names > contents
enscript: more flexible printing
The enscript command converts ascii files to
postscript and then spools the postscript file to the
printer. It is a more flexible way of printing than lpr.
enscript options:
-G : a “gaudy” header with a timestamp, file name, and page
-r : rotate the page to a landscape orientation
-2 : print in two columns
-a : print only the specified pages
-p : print to the specified file instead of the printer
linux2 [5]# enscript -2Gr -p myfile.ps
Working with ps and pdf files
ps2pdf generates a pdf file (Adobe Acrobat)
from a ps (Postscript) file.
linux2 [10]# ps2pdf input.ps output.pdf
Viewers: gv, xpdf.
Working with text files
ispell interactively spell checks a file.
linux2 [9]# ispell filename.txt
cut extracts specific columns of data from
sed is an advanced tool for editing a stream.
awk is a powerful data manipulation tool.
Working with graphics
gimp is a powerful graphics software package.
convert changes images from one format to
xv is an interactive image viewer.
Many people wish to do things like compile and run projects
once they are submitted. There have been many instances where
a student has forgotten to submit a small part of a project, such
as a header file, and loses a bunch of points for it not compiling.
Dan Hood has written a script that allows you to execute
arbitrary commands from within your submit directory. Some of
courses provide such program, but many of them are written for
a specific course and are compiled for a particular architecture
(such as Linux and not IRIX or Solaris). So, Dan Hood has
written a cross platform (works on any of the UNIX systems
that we have here) script that you can use for any course.
It is in his public directory:
and the name of the script is called submitexec
The usage for this script is similar to all of the other submit
utilities. The first command line argument is the course, the
second is the project, and then any remaining arguments are the
command(s) that you want to execute.
This is a very versatile script. You can do anything from an ls
command to compiling and running your program where the
grader will do so. I suggest making a copy of this script and
placing it in your own bin directory.
Note: some of you may have used submitmake or heard of it.
This script is also capable of doing that as well. If you rename
the script (or create a symbolic link) with the name submitmake
it will act like the submitmake provided in other courses. Again,
it is no longer limited to a particular course or a particular
linux2 [3]# submit cs331 proj2 hello.c
Submitting hello.c...OK
linux2 [4]# submitexec cs331 proj2 ls
Executing the command: ls
Execution complete: exit status = 0
linux2 [5]# submitexec cs331 proj2 ls –l
Executing the command: ls –l
total 1
-rw-r--r-- 1 dhood2 general 78 Apr 20 10:42 hello.c
Execution complete: exit status = 0
linux2 [6]# submitexec cs331 proj2 gcc -Wall -ansi hello.c
Executing the command: gcc -Wall -ansi hello.c
Execution complete: exit status = 0
linux2 [7]# submitexec cs331 proj2 a.out
Executing the command: a.out
Hello World!
Execution complete: exit status = 256
Common Programming Tools
gcc – the GNU compiler for C,C++, and Objective-C.
gdb – the GNU debugger.
make – automates compilation and execution
java/javac – the Java virtual machine and compiler
An excellent guide to UNIX tools for the edit-compilelink-debug programming cycle:
indent: proper whitespace
indent formats C/C++ files by adjusting the
whitespace to make it easier to read. It is also
useful for enforcing consistent formatting for
large projects.
linux2 [8]# indent file.c –o file.out
linux2 [9]# indent –st file.c > file.out
make: automating compilation
and execution
make runs using the build instructions in a file called “Makefile”
or “makefile” in the working directory.
The makefile includes variable definitions, and build rules for
each component.
Variable definitions facilitate future makefile modifications:
CC = gcc
CFLAGS = -g -I~/classes/cmsc121/include
A build rule consists of one line specifying dependency
information and a second line specifying the command to build
that component.
proj1.o : proj1.c proj1.h aux.h
TAB$(CC) $(CFLAGS) –c proj1.c
# Makefile
PROJ = Proj1
SUBMITCLASS = cs121_fall05
PRINTPGM = /usr/lib/print/lptops
PRINTFLAGS = -G -U -H -M2 -O0.4pt
- rm -f *~
- make clean; rm -f *.o
- make cleaner; rm -f $(PROJ)
$(PROJ).C \
Aux1.H \
Aux1.C \
Aux2.H \
Aux1.o \
Aux1.o: Aux1.C Aux1.H
$(CC) $(CCFLAGS) -c Aux1.C
Aux2.o: Aux2.C Aux2.H
$(CC) $(CCFLAGS) -c Aux2.C
print: $(SOURCES)
submit $(SUBMITCLASS) $(PROJ) $(SOURCES) Makefile