Table of ContentsTable of Contents 
Previous Chapter Other shells available at SLAC include the Bourne shell, the Korn shell (ksh) and the Bourne Again shell (bash). The latter two are each upwardly compatible extensions of the Bourne shell, which was the original UNIX shell; all three share a syntax that is somewhat different from that of csh and tcsh. On most systems, the Bourne shell is synonymous with the "default shell", and is referred to simply as "sh"; however, IBM's version of UNIX, AIX, uses the Korn shell as the default, and this is assumed in all the IBM documentation. Moreover, starting in Version 4 of AIX, the command name "sh" invokes the Korn rather than the Bourne shell. In order to invoke the Bourne shell on an AIX 4 system, you must use the "bsh" command (note that the bsh command does not exist in all versions of UNIX).
Although shells normally read one command at a time from your terminal, they can also read commands from a file. Such a file is called a shell script, and may include a variety of control structures, such as loops and if-then-else blocks. Thus, in addition to a user interface, the shell also provides a simple programming language. For more information about the programming features of a particular shell, see the appropriate man page.
Unless otherwise noted, the syntax and features of tcsh will be assumed throughout the remainder of this chapter.
Shell programs provide methods to initialize your environment when they detect that they are being invoked as login shells, but these methods differ. The only shells supported by SCS as login shells are tcsh and csh; the former is recommended, and the latter is supported primarily for backward compatibility. To change your login shell you must run the chsh command on host unixhub (the change may take an hour or longer to actually go into effect):
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ilse@janus $ rlogin unixhub Log in to UNIX host unixhub.
Password: Type your password.
Last login: Tue Aug 24 08:44:25 from JANUS.SLAC.Stanf
SunOS Release 4.1.2 (SERVER) #1: Tue Feb 23 18:52:48 PST 1993
WARNING: Unauthorized access to this computer system is prohibited.
Violators are subject to criminal and civil penalties.
ilse@unixhub $ chsh Change shell command.
Changing NIS login shell for ilse on unixhub.
Old shell: /bin/csh
New shell: /bin/tcsh Type full pathname of new shell.
Password: Type your password.
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Assuming you're switching between the csh and tcsh shells, you must also modify the .cshrc file in your home directory once the above change has gone into effect (See the next section for more information about the .cshrc file). Open this file in a text editor (see "Editors" on page 39 for information about text editors) and look for the invocation of SLAC's environ shell script, i.e., a line near the beginning of the file that looks something like this:eval `/usr/local/bin/environ /bin/csh -i${?prompt} -e emacs:vi`
The string "/bin/csh" must be changed to correspond to your new login shell; e.g., if you've just changed to the tcsh shell, change this line to:
eval `/usr/local/bin/environ /bin/tcsh -i${?prompt} -e emacs:vi`
Save the file and exit the editor, then logout and login again.
Similar to program variables (place holders of information), shell and environment variables store values that the shell uses in executing your commands. Shell variables help determine how the shell program interacts with you. You can create your own shell variables and assign them values with the set command, and you can insert the variable value in a command line by prefixing the variable name with $.
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$ set docdir=/usr/local/doc
Sets the variable docdir to /usr/local/doc. That is, docdir
becomes an abbreviation for /usr/local/doc.
$ echo $docdir Shows the value assigned to docdir.
/usr/local/doc
$ cd $docdir The current working directory changes to
/usr/local/doc.
$ unset docdir Removes the shell variable docdir. Now, you can no longer use
$docdir to refer to the directory
/usr/local/doc.
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The procedure shown in the preceding sample session sets the shell variable for use during your present interaction with the current shell. For example, if you used the script command, a new shell would begin, and this shell variable would no longer be set. Even if you used only the login shell, the next time you login you would have to repeat the procedure of setting the shell variable.If you want to use the shell variable every time you interact with the C shell, use a text editor to add the command line to the .cshrc file. (See "Editors" on page 39 for more information on editors.) The .cshrc is the C Shell Run Commands file, a dot file in your home directory. (See "Working With Files" on page 15 for more information on dot files.) Every new UNIX account gets a .cshrc file that sets a number of defaults such as the architecture of the host machine and via SLAC's environ shell script. The environ script is invoked via an eval command that looks like this:
eval `/usr/local/bin/environ /bin/csh -i${?prompt} -e emacs:vi`
Changes should normally be added after the invocation of the environ script.
Use the set command to see shell variables and their values.
------------------------------------------------------------------------------------ $ set Without argument, set displays shell variables and their values. argv () cwd /u/sf/ilse term vt100 history 50 ------------------------------------------------------------------------------------A few shell variables have special meaning to the shell program. By assigning values to these variables, you customize the manner in which the shell executes your commands. The one covered here is the path variable. path, as well as others such as prompt, history, and term, are set in your default .cshrc file, and, of course, you can change them if you like.
/u/<gg>/<userid>/bin /usr/local/bin /usr/afsws/bin /usr/ucb /bin /usr/bin
where"/u/<gg>/<userid>/bin" identifies your personal bin directory. Whenever you type a program name or command, the shell looks first in your bin directory, second in the directory /usr/local/bin, third in the directory /usr/afsws/bin, and so on. It keeps looking until it either finds the program or command with that name or finishes looking through all the directories in your search path.
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$ joes_program Let's say you execute
joes_program frequently.
joes_program: command not found The command is not in the search path.
$ set path=($path /joe/bin) Adds the directory where
joes_program is located to the search
path for that particular session only.
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The procedure in the sample session above adds a directory to your search path for your present session with the shell. Notice the following features of the above set path command:
Your search path may include a period to represent your current working directory. Though this is sometimes convenient, it presents a significant security risk via so-called "trojan horse" attacks, particularly when the period is at the beginning of the search path.
If there is a period early in your search path, please consider moving it to the very end or, better yet, removing it altogether. It is always possible to execute a command in your current directory simply by prefixing it with "./".
Removing a directory from your path shell variable is a little less straightforward than adding one. Here's an example that will safely remove the period representing your current directory from anywhere in your search path (note that the blanks in this example are significant):
set path=( `echo " $path " | /bin/sed -e "s% . % %g"` )
If you find that you frequently use a program or command not included in your default search path, you can edit or add a "set path=" command in your .cshrc file.
After you add or modify command lines in the .cshrc file, the changes will be in effect for all subsequently started shells. However, neither your currently running shell, nor any of its ancestors will automatically see the changes. To invoke the commands added to your .cshrc file in your current shell, use the source command, as shown in the following sample session.
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$ source .cshrc Now the commands in the .cshrc file run, so the shell vari
ables are set as you specified in the .cshrc file.
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Environment variables are similar to shell variables, but other programs besides the shell use their values. Typically, these environment variable names are all uppercase. An example of an environment variable is PRINTER. The value associated with this environment variable determines the default printer. At SLAC a variety of printers are available to UNIX users. They are listed in the file /etc/qconfig (RS/6000 users only) or /etc/printcap (all other UNIX users). The following sample printer description from the former file shows an HP printer located in the computer building:
...
* Computer Building, 1st Floor, George Maclin
hpcgb1a:
host = lpd01
s_statfilter = /usr/lpd/bsdshort
l_statfilter = /usr/lpd/bsdlong
rq = hpcgb1a
device = rlp218
...
The same printer in the latter file appears as:
...
hpcgb1a|Computer Building, 1st Floor, George Maclin:\
:sh:\
:mx#0:\
:rm=lpd01:\
:lp=:\
:sd=/usr/spool/lp/hpcgb1a:\
:rp=hpcgb1a:
...
For more information about printing, see "Printing" on page 43.
The commands to set, unset, and look at environment variables are slightly different than those used with shell variables. As for shell variables, command lines to set environment variables can be added to your .cshrc files.
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$ setenv PRINTER hpcgb1a
Sets environment variable PRINTER to hpcgb1a.
$ unsetenv PRINTER The environment variable PRINTER is no longer set.
$ printenv Prints environment variables and their values.
HOME=/u/sf/ilse
SHELL=/bin/tcsh
TERM=vt100
USER=ilse
...
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appears in the command line that the shell reads, its text is replaced by the definition of the alias. Use the alias command to create aliases and the unalias command to delete aliases.
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$ alias prt lpr -Phpcgb1a prt will be shorthand for printing on the hpcgb1a
printer.
$ alias prt Shows commands associated with prt.
lpr -Phpcgb1a
$ alias By itself, alias displays the aliases you set up.
ll ls -l !*
ls (ls -F)
prt lpr -Phpcgb1a
...
$ prt unix.doc Prints file unix.doc on the hpcgb1a printer.
$ unalias prt Removes the alias prt.
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This sample session sets up the alias for your use during the present interaction with the current shell. However, if you know you want to use the alias in later interactions with the shell program, you should add the command line to the .cshrc file in your home directory.
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$ who who shows who is logged on to the system
raines ttyp0 Aug 20 09:08 (CGIBM1.SLAC.Stan)
ohnishi ttyp1 Aug 20 09:17 (134.79.128.89:0.)
meb ttyp3 Aug 20 08:15 (sisyphus:0.0)
$ who > who.doc Places the output of the who command into
the file who.doc. If the file already exists, its
contents are replaced by the output of the who
command. If the file does not already exist, it
is created.
$ sort who.doc sort re-orders the contents of file who.doc
alphabetically, and writes the result to standard
output.
meb ttyp3 Aug 20 08:15 (sisyphus:0.0)
ohnishi ttyp1 Aug 20 09:17 (134.79.128.89:0.)
raines ttyp0 Aug 20 09:08 (CGIBM1.SLAC.Stan)
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$ who >> info.doc Append the output of the who command to the
end of file info.doc. The contents of this
file remain intact with the output of the who
command attached at the end.
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The tr command in the following example only reads standard input and writes standard output unlike some commands which allow you to specify an alternative input and/or output file.
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$ tr a-z A-Z < Text > Text.upper Translates all lowercase characters in the file
Text to uppercase in the file Text.upper.
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The cat command allows you to concatenate files together. It simply copies the files you specify as arguments to its standard output, which defaults to your terminal screen. With the > character, however, you can redirect the output from the screen to another file. Note the warning in the paragraph above.
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$ cat sec1.doc sec2.doc > unixbook Strings together sec1.doc and sec2.doc.
Redirects the output of the cat command to the
file unixbook.
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Often it is convenient to make the output of one command become the input of another command. The UNIX shell provides a means of connecting the output and input of two commands via a mechanism called a pipe. The pipe character (|) is placed between two commands when the first command's output should be the input of the second.
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$ ls -l /bin | more Lists the files in directory /bin (the com
mand ls -l /bin), one screenful at a time
(the command more).
total 2722
-rwxr-xr-x 2 root 4008 Jul 21 1992 [*
-r-xr-xr-x 1 root 9080 Jul 21 1992 ar*
-r-xr-xr-x 1 root 122880 Jul 21 1992 as*
...
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The characters in the command line that redirect input/output (<, >, and >>) and pipe commands (|) are not part of the commands themselves. Rather, they are special characters, which hold special meaning to the shell. That is, the commands themselves know nothing about the <, >, and | characters, but the shell recognizes them and provides the mechanism by which this redirection takes place.
TABLE 4. Special Characters Used with Filenames
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The Character What It Means
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* Refers to any string of zero or more characters.
? Refers to any single character.
~ Has special meaning when associated with a username as in ~smith. In these
cases, this refers to the user smith's home directory. Thus, ~smith/.login
refers to the file .login in smith's home directory. By itself, ~ refers to your home
directory.
[ ] Indicates any single character of the ones in the brackets.
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$ ls Lists all files in the working directory.
Makefile main.c queues.h sub.c
globals.h queues.c sim.h test1
$ ls *.c Lists all files ending with .c.
main.c queues.c sub.c
$ ls s*.c Lists all files beginning with s and ending with .c.
sub.c
$ ls test? Lists all files beginning with test, ending in any
single character.
test1
$ ls *.[ch] Lists all the files, ending in .c or .h.
globals.h queues.c sim.h
main.c queues.h sub.c
$ ls ~smith Lists all files in the home directory for smith.
Work mbox bin
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The C shell has a similar but more limited feature. It only works with filenames and the ~username convention, is triggered by the ESC key rather than the TAB key, and is disabled by default (to enable it, define the special shell variable "filec" to any value in your .cshrc file, e.g., by adding a line like "set filec=on").
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$ set history=5 Sets number of commands for UNIX to save at n (in this case, 5).
$ history Displays the previous 5 commands, as set in the step above.
15 who
16 finger smith
17 talk smith
18 cd ~
19 rm who.doc * [.1in]
$ !! Repeats the immediately preceding command.
history
16 finger smith
17 ...
:
$ !finger Repeats most recent command starting with string finger.
finger smith
$ !16 Repeats command 16.
finger smith
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When using tcsh (but not csh) you can scroll through your command history using the up- and down-arrow keys, position the cursor within a command using the left- and right-arrow keys, and edit the command before reexecuting it.Sometimes you may want to save your commands and the shell's response in a file. That way you can see or print the information later. You may want to record the running of a specific program and inspect this session later. The script command saves the record of your interaction with the shell in a file named typescript in the working directory. You must give the script command before you start the process you wish to record. You can specify different file names if desired by typing script filename; type man script at the shell prompt for more infor
mation. To signal the end of the recording, type the command exit. At that point, you can look at the typescript file by typing more typescript at the prompt.
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$ script Saves all commands and shell
responses that follow.
Script started, file is typescript
csh> testprog
What is the result of 3+4? 7
Correct!
csh> exit Ends the recording.
csh> Script done, file is typescript
$ more typescript Displays the commands and shell
responses just saved.
Script started on Tue Nov 06 23:57:06 1990
csh> testprog
What is the result of 3+4? 7
Correct!
csh> exit
script done on Tue Nov 06 23:57:06 1990
$
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