Essential System Administration, 3rd Edition

Basic Syntax

This section reviews some basic syntactic features of the Bourne shell, in a somewhat arbitrary order.

Lines in shell scripts beginning with number signs are comments:

# Start or stop the lp scheduler

In fact, comments can begin anywhere on a line:

grep ':00*:' /etc/passwd       # Check for UID=0 accounts

The first line of a shell script usually looks like this:

#!/bin/sh

This identifies the shell that should run the script, in this case, the Bourne shell. The path location can vary.

Note

The best practice is to begin every shell script with a line identifying the shell to be used to run it. If this line is not present, /bin/sh is assumed.

The Bourne shell offers some syntactic flexibility over other shells. For example, quotes remain in effect across physical lines, as in this example we looked at in Chapter 7:

echo "*** Non-root UID=0 or GID=0 accounts:"
grep ':00*:' /etc/passwd | \
  awk -F: 'BEGIN        {n=0}
           $1!="root"   {print $0 ; n=1}
           END          {if (n==0) print "None found."}'

Note that the arguments to the awk command extend across three lines, which is much more readable than forcing them onto a single line.

I/O Redirection

Another construct you’ll see quite often is this redirection of standard output to a file and of standard error to standard output (and thus to the same file):

/usr/lib/lpshut > /dev/null 2>&1

In this case the file is /dev/null, but the concept applies whether output goes to a real disk file, to /dev/console, or gets thrown out.

Note that standard output and error can also be ...