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--- unix_survival [2020/04/08 20:06]
joshd
+++ unix_survival [2020/04/08 23:09]
joshd
@@ Line 1: / Line 1: @@
 **UNIX Command Reference**
-The command prompt in UNIX is called the //shell//.  Here is a list of common UNIX commands which will work across many different versions of UNIX with discrepancies in Example/Notes.
+The command prompt in UNIX is called the //shell//, which typically uses a dollar sign ($) as its prompt.  Here is a list of common UNIX commands which will work across many different versions of UNIX with discrepancies in Example/Notes.
 ||Command||Description||Examples||Notes||
@@ Line 30: / Line 30: @@
 ** Useful UNIX References **
+[[http://www.bitsavers.org/pdf/stanford/stanford_4.2_BSD_manual/4.2_BSD_Vol_2C.pdf|The 4.2BSD Manual]] is a good guide to BSD systems (generally applicable to 4.3, 2.11 and V8 Research UNIX).
+[[https://www.livingcomputers.org/UI/UserDocs/UNIX-SVR3-(WE-3B2)/UNIX_System_V_Users_Guide_Second_Edition_1987.pdf|UNIX System V User's Guide (Second Edition)]] and [[https://livingcomputers.org/UI/UserDocs/UNIX-SVR3-(WE-3B2)/UNIX_System_V_Users_Reference_Manual_1987.pdf|UNIX System V User's Reference]] are a useful references for System V-based systems (or close relatives).
 [[https://www.livingcomputers.org/UI/UserDocs/Unix-v7-1/UNIX_Programmers_Manual_Seventh_Edition_Vol_2_1983.pdf|The UNIX Programmer's Manual (7th Edition)]] is an excellent overview for using UNIX for software development.
@@ Line 41: / Line 45: @@
 This wiki cannot provide an extensive tutorial for the C language, but we'll provide an example or two to get you started.  [[https://archive.org/details/TheCProgrammingLanguageFirstEdition/page/n13/mode/2up|The C Programming Language]] is the classic reference for this (and belongs on every UNIX hacker's bookshelf) and should apply to all of the UNIX systems we have online (although some may also support the later ANSI C specification(s) as well.
-Use the editor of your choice to create the source file.  We'll use //ed// here, as it's common to all UNIX systems, but //vi// may be a more friendly choice.
+Use the editor of your choice to create the source file.  We'll use //ed// here, as it's common to all UNIX systems, but //vi// may be a more friendly choice.  This program is a version of your average "Hello, world!" program.  It uses //printf// to output the //string// "Hello, world! to //stdout// and then exits.  This example uses [[https://en.wikipedia.org/wiki/C_(programming_language)#K&R_C|K&R]] syntax:
 <code>
@@ Line 98: / Line 102: @@
 </code>
+Here's a more complex program.  It makes use of //for// loops to print a small multiplication table.
+<code>
+$ ed table.c
+?table.c
+a
+int main(argc, argv)
+    int argc;
+    char** argv;
+{
+    int i,j;
+    /* Use nested 'for' loops to print a 4x4 multiplication table. */
+    for(i=1;i<5;i++)
+    {
+        for(j=1;j<5;j++)
+        {
+            /* '%d' means 'Substitute a numerical value here' */
+            printf("%d x %d = %d\t", i, j, i * j);
+        }
+        /* '\n' is the "newline" character -- this moves the cursor to the next line
+           for the next row of the table */
+        printf("\n");
+    }
+    return 0;
+}
+.
+w
+q
+$
+</code>
+Compiling this as in the above example and running it yields:
+<code>
+$ cc table.c
+$ ./a.out
+x 1 = 1       1 x 2 = 2       1 x 3 = 3       1 x 4 = 4
+x 1 = 2       2 x 2 = 4       2 x 3 = 6       2 x 4 = 8
+x 1 = 3       3 x 2 = 6       3 x 3 = 9       3 x 4 = 12
+x 1 = 4       4 x 2 = 8       4 x 3 = 12      4 x 4 = 16
+$
+</code>
+** Shell Programming **
+The shell itself is programmable via /shell scripts/.  As with C, a complete tutorial is beyond the scope of this page but we'll provide a few examples to get you started.  The below examples are for // /bin/sh//, the //Bourne shell//, as this is available on all online UNIX systems.  Other common shells are //ksh, csh, tcsh, and bash//.
+As with the C examples above, we'll use //ed// to create our source files.  (Short shell scripting can also be done interactively at the command line.)
+<code>
+$ ed hello.sh
+#!/bin/sh
+echo "Hello, world!"
+.
+w
+q
+$
+</code>
+The first line (//#!/bin/sh) is sometimes referred to as the "shebang" line (because UNIX programmers are weird) and tells the operating system what shell to invoke to execute the script file.
+Now we've created the script, and there it is on disk:
+<code>
+$ ls -l hello.sh
+-rw-rw-rw-   1 user       user             33 Apr  9 05:56 hello.sh
+</code>
+But if we try to run it, we get:
+<code>
+$ ./hello.sh
+sh: ./hello.sh: Execute permission denied.
+</code>
+(The exact message may vary depending on what UNIX you're on.)  This is telling us that the script we created doesn't have the //execute bit// set (only the //read// and //write// bits indicated by //rw//) -- so it cannot be run.  The //chmod// command can be used to set the execute bit on our new shell script:
+<code>
+$ chmod u+x hello.sh
+$ ls -l hello.sh
+-rwxrw-rw-   1 root       sys             33 Apr  9 05:56 hello.sh
+$
+</code>
+As you can see the hello.sh file now has the //execute bit// set (see the "x" in the file listing above).  Now it can be executed:
+<code>
+$ ./hello.sh
+Hello, world!
+$
+</code>

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