The echo command will evaluate its arguments and display the result on the console. For example:

g! var1 = 'this is'
this is
g! var2 = ' a string'
a string
g! echo $var1$var2

this is a string

The grep command is used to search the input for lines that match a given pattern; it is very similar to the Unix grep tool. The input is either standard input, the output of a command piped to grep, or the contents of files.

The grep command will return false if there were no lines in the input that match the pattern. It will return true otherwise.

Its usage is as follows:

g! grep -?
grep - search for PATTERN in each FILE or standard input.
Usage: grep [OPTIONS] PATTERN [FILES]
-? --help show help
-i --ignore-case ignore case distinctions
-n --line-number prefix each line with line number within its input file
-q --quiet, --silent suppress all normal output
-v --invert-match select non-matching lines

true

The PATTERN argument is an encoded regular expression (regex) that defines the sequence of characters that are considered a match. It follows the regex pattern format, defined for the Java java.util.regex.Pattern class.

You can visit the following for more information on regex:

http://download-llnw.oracle.com/javase/1.5.0/docs/api/java/util/regex/Pattern.html

For example, to grep the output of the lb command for lines containing the string Apache:

g! lb | grep Apache
1|Active | 1|Apache Felix Bundle Repository (1.6.2)
2|Active | 1|Apache Felix Gogo Command (0.6.0)
3|Active | 1|Apache Felix Gogo Runtime (0.6.0)
4|Active | 1|Apache Felix Gogo Shell (0.6.0)

true

The -i (short for --ignore-case) option makes the pattern not case sensitive, thus matching both the lowercase and uppercase for a letter.

The -n (or --line-number) option requests that the command include the line number when printing the results. For example:

g! lb | grep -n Apache
4: 1|Active | 1|Apache Felix Bundle Repository (1.6.2)
5: 2|Active | 1|Apache Felix Gogo Command (0.6.0)
6: 3|Active | 1|Apache Felix Gogo Runtime (0.6.0)
7: 4|Active | 1|Apache Felix Gogo Shell (0.6.0)

true

The -q option (also --quiet or --silent) is used to suppress the grep command output. The command will only return true or false after it is finished with the input. This is especially useful when using the command for its returned value only (as a loop guard, for example).

The -v (or --invert-match) is used to show the lines that don't match the pattern (inverse match). For example:

g! lb | grep -v Apache
START LEVEL 1
ID|State |Level|Name
0|Active | 0|System Bundle (3.0.1)

true

The cat command is used to concatenate files and display their contents on the console. It takes one or more filenames relative to the current shell session directory (see cd and ls in the previous section) and displays them.

For example, to display the run.bat file we had created at the beginning of this chapter, use the following:

g! cat run.bat

java -jar bin/felix.jar

Separate the arguments with whitespace to display more than one file sequentially.

The tac command, in a way, is the opposite of the cat command. Here it takes the text from the standard input and either returns it as a string or as a list for use as input for another command or writes it to a file.

The syntax is as shown here:

g! tac -?
tac - capture stdin as String or List and optionally write to file.
Usage: tac [-al] [FILE]
-a --append append to FILE
-l --list return List<String>

-? --help show help

The following example makes a list of the input and then gets the second item in the resulting list.

g! var1 = tac -l ; $var1 get 1
a1
a2
a3
^Z

a2

Notice the use of Ctrl-Z (shows as ^Z on the console display) to terminate user input.

Passing a filename as an argument will create the file and write the input text to it. The -a or --append option can be used to append to an existing file. For example, this will create a file from the standard input and display its contents using the cat command:

g! tac test.out ; cat test.out
this is a test, line 1
last line
^Z
this is a test, line 1

last line

The input of the tac command can also be the piped output of another. For example, the next command will make a backup copy of our test.out file created previously:

g! cat test.out | tac test.out.bak
this is a test, line 1 last line
g!
g! cat test.out.bak
this is a test, line 1

last line

The set command is used to inspect session variable information, as well as turn session tracing on or off.

Help on the usage of the set command can be retrieved as follows:

g! set -?
set - show session variables
Usage: set [OPTIONS] [PREFIX]
-? --help show help
-a --all show all variables, including those starting with .
-x set xtrace option
+x unset xtrace option

If PREFIX given, then only show variable(s) starting with PREFIX

The -x option is used to turn execution traces on. For example, taking one of the samples used previously, without setting the xtrace option:

g! var = 'this is a string'
this is a string
g! echo $var

this is a string

When setting the xtrace option, the shell will output a trace message for each command it will execute and for each result of that command. In this case:

g! set -x
g!
g! var = 'this is a string'
+ var '=' 'this is a string'
this is a string
g! echo $var
+ echo $var

this is a string

Setting shell execution traces on is especially useful when working on a script (see the previous source).

Use set +x to turn traces off.

Let's test if you remember some of the basics of this chapter with a quick pop quiz.