External programs

External programs are often used within shell scripts; there are a few builtin commands (echo, which, and test are commonly builtin), but many useful commands are actually Unix utilities, such as tr, grep, expr and cut. The back-tick (`)is also often associated with external commands. The back-tick is used to indicate that the enclosed text is to be executed as a command.

$ MYNAME=`grep "^${USER}:" /etc/passwd | cut -d: -f5`
$ echo $MYNAME
Johan Tik

So we see that the backtick simply catches the standard output from any command or set of commands we choose to run. It can also improve performance if you want to run a slow command or set of commands and parse various bits of its output:

#!/bin/sh
find / -name "*.html" -print | grep "/index.html$"
find / -name "*.html" -print | grep "/contents.html$"

This code could take a long time to run, and we are doing it twice! A better solution is:

#!/bin/sh
HTML_FILES=`find / -name "*.html" -print`
echo "$HTML_FILES" | grep "/index.html$"
echo "$HTML_FILES" | grep "/contents.html$"

Note: the quotes around $HTML_FILES are essential to preserve the newlines between each file listed. Otherwise, grep will see one huge long line of text, and not one line per file. This way, we are only running the slow find once, roughly halving the execution time of the script.