A shell script is a script written for the shell, or command line interface: Terminal. Typical operations performed by shell scripts include file manipulation, program execution, and printing text.
Examples of uses for shell scripts:
- Editing large text or data files.
- Automating tasks, such as backup operations.
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You can write a shell script to a text file to perform various tasks on your Mac.
Example:
#!/bin/sh
if [ -f ~/Desktop/hello.sh ]; then
say Hello!
fi
Save this code to a text file named hello.sh, on your desktop. It is best to use a program like Text Wrangler, or vi. Open the Terminal (in the Applications/Utilities/ folder) and type: chmod +x ~/Desktop/hello.sh, to make the file executable. To run the script type: ~/Desktop/hello.sh in a terminal window.
#!/bin/sh tells the shell to run the program in sh, a type of shell program.
if and fi are the beginning and end of an “if statement”. Allows execution of certain commands if certain arguments are true.
[ -f ~/Desktop/hello.sh ] is the argument. It means: if the file hello.sh exists on the desktop, do the commands that follow the “then” statement.
say Hello! is the command. This should cause your Mac to say “Hello!”.
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You will need learn some basic Unix commands to get around in the command line interface. Read a Mac Terminal tutorial (pdf). The basics can be learned quickly. Below are a few commands that you need to know:
ls, list the contents of the current directory.
pwd, print the path to the current directory to screen.
cd, change directory. Ex: cd ~/ will change directories to your user home directory.
chmod, changes permissions. Vital, but best to do a tutorial on this one.
mkdir, create a new directory.
Using the Terminal in OSX and Linux is similar.
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Utilities/commands useful for shell scripting.
awk A programming language that is designed for processing text-based data. awk video
grep Text search utility originally written for Unix. grep video grep video 2
cat Unix program used to concatenate and display files. cat video
sed is a Unix utility that (a) parses text files and (b) implements a programming language which can apply textual transformations to such files. Sed video
Example grep: this lists (ls) the items in the current directory, and recognizes directories in the list by using the -F option ( you will note the / after list items that are directories). The pipe (|) forwards the result to grep, which filters for directories because we used “/” as the argument.
ls -F | grep /
Example grep and awk: df lists the drives and partitions and disk space on the computer. -h makes it more (h)uman readable. Pipe (|) into grep is filtered for one of the disks, named disk0s2. Pipe (|) awk prints only the 5th column ($5). The result is just the disk space used ex: 39%.
df -h | grep disk0s2 | awk '{print $5}'
or just use awk in this format to get the same result
# more efficient
df -h | awk '/disk0s2/ {print $5}'
# prints it out in human readable formatting.
df -h | awk '/disk0s2/ {print $1 " is " $5 " used. "}'
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More info from shell scripting primer from Apple:
http://developer.apple.com/mac/library/documentation/opensource/conceptual/shellscripting/Introduction/Introduction.html
Advanced Bash-Scripting Guide
An example of a shell script in action, and a detailed explanation of its functionality.
