Part 12: External Programs

External programs are often used within shell scripts; there are a few builtin commands (echo, which, and test are commonly built in to the shell), but many useful commands are actually Unix utilities, such as tr, grep, expr and cut.

The backtick (`) is also often associated with external commands. Because of this, we will discuss the backtick first.
The backtick is used to indicate that the enclosed text is to be executed as a command. This is quite simple to understand. First, use an interactive shell to read your full name from /etc/passwd:

$ grep "^${USER}:" /etc/passwd | cut -d: -f5
Steve Parker

Now we will grab this output into a variable which we can manipulate more easily:

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

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.

The $(command) Syntax

There is another syntax for the same purpose: Instead of using backticks, you can use $(..):

HTML_FILES=$(find / -name "*.html" -print)

The stated advantage of this is that it can be nested, which the backticks cannot.

I must admit that I have never come across a real-world situation where nested command substitution was necessary, and to be honest if I did, I would prefer to use a variable as intermediate storage rather than have such an unreadable mess. But that is the generally preferred solution by most people.