Scripting from the Command Line

If you start a loop or conditional, such as a while, for, or if/then statement, while working at the shell prompt, the command line is extended until you have finished the steps in the c

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C H A P T E R 1 6

Scripting from the Command

Line

One of the advantages of working at the shell command line is that you’re working in a

shell That sounds sort of obvious and dumb, but please bear with me; pretty much

any-thing you can do in a shell script, you can also do from the command line I’ve coded

many ad hoc scripts right at the command line I wouldn’t recommend writing anything

significant that way, but for quickies it’s just the ticket

If you start a loop or conditional, such as a while, for, or if/then statement, while

working at the shell prompt, the command line is extended until you have finished the

steps in the code block In a traditional script, such code would customarily span several

lines in a file Here is a typical interaction with the shell when entering code directly from

the command line:

$ while : ; do

> clear

> ls -lrt

> sleep 3

> done

Note that after the first line is entered and the Enter key is pressed, the command line

returns a > prompt to continue the code block You can then keep adding lines until the

loop completes Once the last line has been entered, in this case the done line, the code

will begin to run In our case, it is an infinite loop, which can be stopped by a <ctrl>-c

The following examples are formatted using more traditional indentation for the sake

of readability If you were to enter these from the command line, you would see results

similar to those shown above None of the examples are particularly complex; they are

just representative of what can be done from the command line

A Few Examples

This while loop does nothing more than create a long listing of specific files over and over

while sleeping for three seconds between iterations It is part of a set of scripts I use for

104 C H A P T E R 1 6 ■ S C R I P T I N G F R O M T H E C O M M A N D L I N E

concurrent package installation on a large number of remote systems Each concurrent installation produces its own log file that documents its progress and any issues it encounters All log files are stored in a single directory

While watching the output of the installation loop, I can tell by the size of the log file when an installation of a specific node is complete This saves me from having to review each log individually Successful installations all have log files that end up being of a par-ticular size Files of a different size stand out and show me that I need to review that log Also, watching the growth rate of the files can convey information about how the install

is progressing A typical installation goes something like this: the install package is pushed out to the remote nodes; then the package is uncompressed on each of the remote sys-tems, where the install script is run until it completes By watching the file size of all log files increase, I can review the status of all installations at once and generally know at which point they have arrived, as well as note any problems, without actually viewing the contents of the log files themselves

This is a representative example of the miniscript I use for this task:

$ while : ; do

> clear

> ls -lrt install_log.*

> sleep 3

> done

Note that I use the -lrt switch with ls This sorts the output by modification time, with the newest files being listed last

The following miniscript is nothing more than a series of nested for loops:

$ for i in 1 2 3 4 5 6 7 8 9

> do

> for j in 1 2 3 4 5 6 7 8 9

> do

> for k in 1 2 3 4 5 6 7 8 9

> do

> touch $i$j$k

> done

> done

> done

I’ve used this type of script to create large numbers of empty files in a directory It is the result of an effort to test a monitor script that is supposed to send notifications in case the number of files in a directory exceeds a certain threshold Another use for it would be to perform a task a specific number of times In this case there are 729 files cre-ated by the 3 nested loops of 9 individual digits In the example, I could have replaced the touch command with something that didn’t reference any of the counter variables ($i, $j, or $k) It could have easily been an echo statement repeating 729 times, but that’s just boring

C H A P T E R 1 6 ■ S C R I P T I N G F R O M T H E C O M M A N D L I N E 105

The last example is something I do fairly regularly I often want to gather information

from each system named in a list of machines This example shows how to get the list of

node names by using a command call within back-ticks (` `) A command string enclosed

within back-ticks denotes not the given string, but the string obtained by evaluating the

command string and replacing it with what is returned

$ for node in `cat some_nodelist_file`

> do

> if [ "$node" = "cheese" ]

> then

> continue

> else

> ssh $node uname -a

> ssh $node uptime

> fi

> done

In this case, `cat some_nodelist_file` would be replaced with an actual list of nodes

originally contained in that file The file is assumed to contain the list of nodes that are

space-delimited or that appear individually on each line of the file, or a combination

of both This loop does not iterate through the file line-by-line.1The for loop iterates

through each of the nodes I’ve also included an if/then statement that will skip any node

named "cheese" to provide an example that includes a conditional

A final trick for using these kinds of scripts efficiently is command-line recall, which is

discussed in Chapter 15 If you make a mistake while typing (not unlikely when you’re

working with many lines at once), you can return to the previous (mistyped) command in

your history and then edit the command sequence using vi-style command-line editing

You can also recall a previously entered miniscript for easy modification and reuse

1 To read a file line-by-line where the lines contain more than a single “word,” refer to Chapter 10.