A "shell" is a program that listens to keyboard input from a user and performs actions as directed by the user. Android devices come with a simple shell program. This shell program is mostly undocumented. Since many people are curious about it I thought I'd write up some documentation for it.
Currently this documentation is incomplete, sorry!
The built-in shell has very limited error handling. When you type a command name incorrectly it will say "permission denied", even though the real problem is that it couldn't find the command:
dir: permission denied <---- this is a misleading error message, should say 'dir: not found'
... listing of current directory
The PATH variable
The Android shell will run any program it finds in its PATH. The PATH is a colon (':') seperated list of directories. You can find out what your shell's PATH is set to by using the built-in echo command:
$ echo $PATH
Depending upon your shell, you may see a different result.
Built in Commands
Every shell has a few built-in commands. Some common built-in commands are:
echo -- prints text to stdout.
set -- sets shell variables
export -- makes shell variables available to command-line programs
cd -- change the current directory.
pwd -- print name of the current directory.
To find out what commands you have available to you, use the "ls" command on each of the directories in the PATH variable.
Finding documentation for the Android commands.
Many of the Android commands are based on standard Linux (or bsd) commands. If you're curious about a command, you can sometimes learn how it works by using the "man" command on a desktop Linux or OSX (Apple Macintosh) computer. The Linux or OSX version of the command may be different in details, but much of the documentation will still apply to the Android version of the command.
Another source of documentation for people without a Linux or OSX machine handy is to use a web browser and use a web search engine to search for the text: "man Linux command-name".
List of commands
The following is a list of the commands that are present on a Nexus S phone running an Android 2.3.3 "user-debug" build. Many of these commands are not present on a "user" phone. (They are missing from a "user" phone because they are specific to developing or debugging the Android operating system.)
$ ls /data/local/bin
/data/local/bin: No such file or directory
Notice that by default there is no /data/local/bin directory. You can create this directory using the "mkdir" command if you like.
$ ls /sbin
opendir failed, Permission denied
The /sbin directory exists, but you don't have permission to access it. You need root access. If you have a developer phone, or otherwise have root access to your phone you can see what's in this directory.
# ls /sbin
Notice that the shell prompt changes from a '$' to a '#' to indicate that you have root access.
Notice also that neither of the /sbin commands are useful to the shell -- the adb and ueventd files are 'daemon' programs used to implement the Android Debugger "adb" program that is used by developers.
$ ls /vendor/bin
Vendor/bin is where device vendors can put device-specific executables. These files are from a Nexus S.
$ ls /system/sbin
/system/sbin: No such file or directory
This directory does not exist on a Nexus S.
$ ls /system/bin
am is the Android Activity Manager. It's used to start and stop Android activities (e.g. applications) from the command line. Type am by itself to get a list of options.
Command line audio file player.
Used to apply patches to android files.
Command line audio recorder.
Backup manager - type command by itself to get documentation.
Draws the boot animation. You may have to reset your phone to get out of this.
Copy the contents of a file to standard output.
Change the mode of a file (e.g. whether it can be read or written.)
Change the owner of a file.
Compare two files byte-by-byte
The dalvik virtual machine. (Used to run Android applications.)
Prints the current date and time
Convert and copy a file. By default copies standard in to standard out.
Shows how much space is free on different file systems on your device.
Shows the current configuration of network interfaces (IP, MAC address etc)
Shows the current processes using the network interfaces (top, but for networks)
Manage the firewall
Send signals to processes.
Used to set up a file system link.
Prints the Android runtime log.
Make a directory.
A program that sends random events, used to test applications. (Like having a monkey playing with the device.)
Move a file from one directory to another. (Only on the same file system. Use "cat a > b" to copy a file between file systems.
List active processes.
Reboot the device.
Remove a file.
Remove a directory.
Starts the Android runtime.
Stops the Android runtime.
Shows which processes are currently using the most CPU time.
Prints how long your device has been running since it was last booted.
$ ls /system/xbin
Secure copy program. (Used to copy files over the network.)
Used to administer SQLite databases.
System trace command - use to see what system calls a program makes.
Start a shell with root privileges.
Versions of the Android Shell
Android 1.0 used a shell that had no tab completion or history editing.
Android 2.3 added history editing. You can for example use the up/down arrows to edit previous commands.