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--- unix_survival [2020/04/08 21:08]
joshd
+++ unix_survival [2020/04/08 23:16] (current)
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 ($) as its prompt.  Here is a list of common UNIX commands which will work across many different versions of UNIX with discrepancies in Example/Notes.
 ||Command||Description||Examples||Notes||
@@ Line 102: / Line 102: @@
 </code>
+Here's a more complex program.  It makes use of //for// loops to print a small multiplication table.
+<code>
+$ 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/.  A shell script can invoke any of the standard UNIX tools as well as other shell scripts, and can make use of control constructs, including loops and comparisons.  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 -- in this case the Bourne shell, in /bin/sh.
+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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