• Quick Introduction to ssh for Mac Admins (this post)
• SSH Keys, Part 1: Host Verification
• SSH Keys, Part 2: Client Verification
• Transferring files with ssh
• SSH Tunnels

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What is SSH?

SSH is short for ‘Secure SHell’. It is a protocol that allows you to open a shell (terminal session) on another remote computer over the network. On macOS you will usually initiate an SSH session with the Terminal application, though there are other third party applications for macOS and iOS that support the SSH protocol as well.

On macOS SSH access is sometimes called ‘Remote Login’.

As the name implies, connections over SSH are encrypted and secure. This may not seem unusual today, but it did replace many unsecured protocols such as telnet and rlogin.

SSH is supported on many platforms. You can use SSH to connect to Macs as well as any other computer which supports SSH and has it enabled. Of course, when you SSH to a different system, the environment you get through SSH might be very different (i.e. not bash, different commands, etc.), so be sure you understand the remote system.

Enable SSH access

By default SSH access is disabled on macOS. Before you can connect to a Mac with SSH you need to enable access. In the UI you can do this in System Preferences > Sharing, by enabling the ‘Remote Login’ option. Here you can also control whether all users on the Mac can get SSH access or just some of them.

The Sharing pane will also show the current hostname of this computer:

Computers on your local network can access your computer at:
client.example.com

You can also enable SSH access from the command line with

$ sudo systemsetup -setremotelogin on

and turn it off again with

$ sudo systemsetup -setremotelogin off

Turning SSH off with this command helpfully reminds you that if you are currently connected remotely, you will disconnect with this command and have to login locally to re-enable. To override this helpful notice, you can add the -f option:

$ sudo systemsetup -f -setremotelogin off

And if you want to know which hostname a Mac thinks it is, use the hostname command:

$ hostname
client.example.com

Note: If you want to control which users have access with a script, I have an example in this post. This post was written for Munki but should be fairly easy to adapt to other systems.

Connect with SSH

To connect to a machine from another Mac (the one you are working on) is simple enough. Open Terminal and enter:

$ ssh username@client.example.com

or alternatively (different syntax, same result, which one you prefer is a matter of taste)

$ ssh -l username client.example.com

If you do not give a username, ssh will use the short name you are currently logged in as. That is a useful shortcut if the username is the same on both systems.

When you connect to a remote machine for the first time you will get this prompt:

ssh client.example.com
The authenticity of host 'client.example.com (IP address)' can't be established.
ECDSA key fingerprint is SHA256:abcdefghhijklmonpqrstuvwxyz.
Are you sure you want to continue connecting (yes/no)? 

(Your actual fingerprint will look differently.) This prompt tells you that your ssh does not recognize this host and gives you a chance to not connect. For now, type yes to confirm. This will add the host to the list of known hosts, so the prompt will not return when you connect again. We will discuss keys and security in a later post.

Next ssh will prompt you for the password on the remote computer. Once you enter that you will get the prompt and have a secure shell to the remote computer. Any command you enter now, will be run on the remote computer.

You can connect with ssh when no user is logged in, i.e. the Mac is sitting at the login window. You can even connect with one user, while a different user is logged into the Mac with a UI session. In these cases, commands that interact with the UI, will fail, since the UI is either not running, or running as a different user. Most common examples are open and osascript (AppleScript).

$ open .
LSOpenURLsWithRole() failed with error -10810 for the file /Users/armin.

If you happen to be logged in on the same Mac with the same user, then the command will work, but open will open and display the Finder window on the remote Mac, not the Mac you are working on, which can be confusing.

Ending it

To end the remote session, just use the exit command.

$ exit
logout
Connection to client.example.com closed.

This will return you to the shell on your Mac where you executed the ssh command.

SSH Shortcut Files

If you frequently connect to certain hosts you may want to create an ssh short cut file, which you can then just double click or invoke with spotlight.

Execute Just One Command

Instead of opening a remote shell you can use ssh to just execute just a single command on the remote host:

$ ssh client.example.com sw_vers
Password:
ProductName:    Mac OS X
ProductVersion: 10.12.5
BuildVersion:   16F73

The ssh command will prompt for the password and the print the output of the command from the remote machine. This can be very useful. You can combine multiple commands with ;

$ ssh client.example.com hostname; sw_vers; system_profiler SPHardwareDataType

In some case the command you want to execute remotely can prompt for information, usually a password. You can add the -t option to make ssh use an interactive shell:

$ ssh -t client.example.com sudo installer -pkg ~/Downloads/myinstaller.pkg -tgt / -verbose

This will prompt twice for the password. The first time to establish the remote connection and the second time for sudo on the remote machine.

Sending single commands with ssh can be useful for automating workflows in scripts. However, the requirement to keep entering passwords will be very detrimental to automation. It will also be annoying when you frequently connect to specific remote Macs.

We will discuss how the key security works and how it can replace passwords in the following posts.

In the previous posts we talked about which files you could use to customize your bash environment.

There are (mainly) three customizations you can perform:

• environment variables
• aliases
• functions

This post will look at some useful aliases and functions.

bash aliases

Note: bash aliases are something completely different from Finder aliases. The closest shell equivalent for Finder alias files are symbolic links.

bash aliases are basically text substitutions. For example a common alias is to define:

alias ll="ls -l"

You can type this alias definition directly into bash but then the definition will only exist for that particular shell. When you open a new terminal window it will know nothing of this alias. If you want an alias to exist in all your shells (and usually you do), you need to add the above line to your .bash_profile or .bashrc. (Learn about the difference here.)

Whenever you modify the .bash_profile or .bashrc it will not automatically be loaded into any shell that is already open, you either have to close the Terminal window and open a new one or write

$ source ~/.bash_profile

(or ~/.bashrc) to load the new settings.

Once you have set the alias, anytime you type ll at the start of a command, bash will replace it with ls -l before executing. Since subsequent arguments are left alone they will just be picked up by the substituted ls command, so if you type ll -a bash will substitute that to ls -l -a and it will work just your would expect.

You can make the alias more complex if you want:

alias lll="ls -alhTOe@"

If you always want to use the long format of ls, you can alias the ls command itself:

alias ls="ls -l"

Then, when ever you type ls it will be replaced with ls -l. Note the lack of spaces around the ‘=’, as usual when assigning values in bash.

Uncovering the alias

You can unset or delete an alias with the unalias command:

$ unalias ls

This will return to the default ls command (for this bash instance only).

Since alias substitution only happens at the beginning of the command, you can also bypass it once by starting the command line with a different character:

$ "ls" -S
$ \ls -S

Either way of typing this will use the original ls command, avoiding substitution, just this once.

If you are unsure if a command has an alias or want to know what gets substituted, then you can have the alias definition printed with

$ alias ls
alias ls='ls -l'
$ alias lll
alias lll='ls -alhTOe@'

You can also list all defined aliases by running alias without any arguments:

$ alias
alias ll='ls -l'
alias lll='ls -alhTOe@'
alias ls='ls -l'

Some Alias examples

Some users like to alias the potentially dangerous commands rm, mv and cp with the -i option, which forces the user to confirm when a file is going to be deleted or overwritten:

alias rm="rm -i"
alias mv="mv -i"
alias cp="cp -i"

Then if you do something that could destroy an existing file you will be prompted to confirm:

$ rm my_important_file 
remove my_important_file? 

You can still use \rm to bypass the alias, if you believe you know what you are doing and want to avoid being prompted several times.

You can add short cuts to cd to parent folders:

alias ..="cd .."
alias ...="cd ../.."
alias cd..="cd .."

Since alias substitution only takes place at the beginning of the command, you can still use .. normally as an argument.

Note that the last alias cd.. is a substitution of a common typo. Since the output of the alias is not checked for further alias substitutions you cannot use alias cd..=.. to define it using the previous alias. Each alias must stand for itself.

You can also use aliases to make complex or hard to memorize commands easier to remember:

alias badge="tput bel"

The first command will play a system alert and set a ‘badge’ on the terminal application if it is in the background. This is useful to get notified of long running commands:

$ hdiutil convert image.sparseimage -format UDZO image.dmg; badge;

The substitution can be long and multiple commands can be separated by ;

alias dockspace="defaults write com.apple.dock persistent-apps -array-add '{\"tile-type\"=\"spacer-tile\";}'; killall Dock;"

The dockspace alias uses this hack to add a spacer item to the dock.

We have seen earlier with the ll alias that any text or arguments after the alias will just be added to the substituted command. We can use this to our advantage:

alias reveal='open -R'
alias xcode='open -a Xcode'
alias pacifist='open -a Pacifist'

All of these commands will require a file path as an ‘argument’ to work properly.

Functions: beyond the alias

While bash aliases are useful and easy to define, they have limitations. Aliases will only be replaced at the beginning of the command prompt. Also additional arguments will be appended after the replacement. If you need to insert additional arguments somewhere in the replacement, you cannot achieve that with aliases.

For example, to display the man page for ls in the Preview application you need the following commands:

$ man -t ls | open -f -a "Preview"

If you wanted to alias this you would have to insert the arguments after man -t and before the pipe. Since aliases, only substitute the beginning of the command, this will not work. we need to define a bash function:

function preman() {
    man -t $@ | open -f -a "Preview"
}

Sometimes you will see bash functions defined without the function keyword, but I prefer to use it since it makes the code more readable.

The $@ will be replaced with all the arguments passed into the function, so when use the function

$ preman ls

the argument ls will be substituted where the $@ is.

You can also use the positional argument variables $1, $2, etc:

function recipe-open() {
    open "$(autopkg info '$1' | grep 'Recipe file path' | cut -c 22-)"
}

Within functions the full power of bash scripting is at your service. You can even call other scripting languages.

# prints the path of the front Finder window. Desktop if no window open
function pwdf () {
    osascript <<EOS
        tell application "Finder"
            if (count of Finder windows) is 0 then
                set dir to (desktop as alias)
            else
                set dir to ((target of Finder window 1) as alias)
            end if
            return POSIX path of dir
        end tell
EOS
}

# changes directory to frontmost 
alias cdf='pwdf; cd "$(pwdf)"'

The last alias cdf has to be defined as an alias. Since a function or script could not change the directory for the current shell, you have to use alias substitution to get the cd.

In general functions (and scripts) are more powerful and versatile than aliases, but aliases provide and easy and comparatively safe way to customize the your shell environment.

Putting it all together

Here is a sample .bash_profile with many of the examples given here and in the previous article. You can use this as a starting point for your own bash configuration.

This is vacation time with a holiday in the Netherlands. Posting will be more sporadic than usual.

In the previous post we talked about which files you could use to customize your bash environment.

There are (mainly) three customizations you can perform:

• environment variables
• aliases
• functions

This post will look at some useful environment variables.

Choose your own PATH

The PATH environment variable contains a list of directories that bash will search through for commands. You can show your current path by echoing the PATH variable:

$ echo $PATH
/usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin:

This is the default PATH for a ‘clean’ macOS. Depending on what you have installed, there may be more elements in your PATH. The individual directories are separated by a colon :. When you enter a command, like for example systemsetup at the prompt, bash will search through the directories in PATH in the order until it finds a match.

Note: if there is ever any question on which command is actually executed, you can use the which command to get the effective path to a given command from the shell:

$ which python
/usr/bin/python
$ which systemsetup
/usr/sbin/systemsetup
$ which bash
/bin/bash

You might have installed other (newer versions) of certain tools (such as python or bash itself). Depending on how you install them they may be installed in a way that overrides the tool that is part of macOS. This may lead to problems when scripts assume the system version of a tool.

You can add your own directories by either appending (safe) or prepending (risky) your own directories to the PATH variable:

PATH=$PATH:~/bin
export PATH

This will add the ~/bin directory (which does not exist by default, you have to create and fill your own) to your PATH. The export command tells bash it should export this variable to child processes.

You can add more than one directory to the PATH, either at once, or one per line:

export PATH=$PATH:~/bin:~/Projects/scripts

or

PATH=$PATH:~/bin
PATH=$PATH:~/Projects/scripts
export PATH

Since bash stops looking when it finds a match, the order of the directories in the PATH is important. When you place your directories before the default directories you can override some of the system commands. This can be powerful, but also very dangerous. There are safer methods for overriding system commands. You should add your directory to the end of the PATH to be safe.

Note: the PATH (and other environment variables) are exported to child-shells and scripts executed from the shell. However, not every tool executes directly from a shell, or from a shell with your configuration file. So you should never assume PATH (or any other environment variable) is set to anything specific (or even set) when you write a script. You can either set the PATH at the beginning of a script, or use absolute paths for commands in scripts such as /usr/bin/defaults or /usr/bin/pkgbuild. You can determine the path to a command with the which command.

Prompt Me!

You can also configure the command prompt, i.e. the text that bash displays before you enter a command. By default the macOS prompt looks like this:

hostname:currentDir user$ 

The configuration for the prompt is stored in the PS1 environment variable. You can see the default value by echoing it:

$ echo $PS1
\h:\W \u\$

The letters with the backslash \ in the prompt variable are placeholders that will be replaced with a specific value when the prompt is printed out.

• \h will be replaced with the hostname of the computer up to the first . (\H would be the complete hostname)
• \W will be the basename of the current working directory
• \u is the current user
• \$ will be $ unless the current user is root or effectively elevated to root privileges, in which case it will be #. This is very useful as a warning when working with elevated privileges.

Since I am the only user on my Mac, and I also know which Mac I am sitting at, I have changed my prompt to something simpler:

export PS1="\W \$ "

Note the space after the \$. If you forget that the cursor will stick right on the $.

(When I log in to a different Mac with ssh the different prompt also reminds me that this shell is on a different Mac.)

You can find a list of escape sequences here.

You can also add color to the prompt data, but the escape sequences will look very messy very quickly. To have the directory name in gray, you use:

export PS1="\[\e[1;30m\]\W\[\e[m\] \\$ "

List in Color

You can also tell bash to color for some other commands, such as ls. To do that you just have to set the CLICOLOR variable:

export CLICOLOR=1

Change the Editor

Some terminal commands will open a text editor directly. For example git may open a text editor so you can enter a commit message. By default the system will open vi but you can change the editor that will be used. There are even some graphical editors that can be used this way. The environment variable for this behavior is EDITOR.

vi or vim:

export EDITOR=vi
export EDITOR=vim

emacs:

export  EDITOR=emacs

nano or pico:

export EDITOR=nano
export EDITOR=pico

TextEdit:

export EDITOR="open -f"

BBEdit:

export EDITOR="bbedit -w --resume"

TextMate:

export EDITOR="mate -w"

SublimeText:

export EDITOR="subl -n -w"

Atom:

export EDITOR="atom --wait"

Note: the -w or --wait argument tells the UI application that there is another process waiting for their output.

Conclusion

Putting all of this together, you can add these lines to your `.bash_profile` or `.bashrc`:

Up Next?

In the next post we will look at bash aliases and functions.

Note: bash_profile is completely different from configuration profiles. Learn more about Configuration Profiles in my book: ‘Property Lists, Preferences and Profiles for Apple Administrators’

You can learn more about using Terminal and the shell on macOS in my my book: “macOS Terminal and Shell” — Thank you!

In this spontaneous series on the macOS Terminal I have often mentioned adding something as an alias or function to your bash_profile or bashrc. Obviously, you may wonder: how do I do that? And which file should I use?

Why?

When you work with the command line and the bash shell frequently, you will want to customize the environment. This can mean changing environment variables, such as where the shell looks for commands or how the prompt looks, or adding customized commands.

For example, macOS sets the PATH environment variable to /usr/local/bin:/usr/bin:/bin:/usr/sbin:/sbin by default. This is a list of directories (separated by a colon ‘:’) that the system searches through in order for commands. I like to add a folder in my home directory ~/bin to that list, so that I can execute certain tools without needing to type out the full path. (e.g. munkipkg, quickpkg and ssh-installer).

In bash you append to existing PATH do this with:

export PATH="$PATH:~/bin"

You could type this command every time you open a new Terminal window (i.e. shell), or you can configure your shell to do this automatically.

Depending on which shell you use and how you start the shell, then certain script files will be executed which allow you to set up these customizations.

This article will talk about customizing bash on macOS. Other shells and other operating systems may have other files or rules.

So, which file?

Thanks to the rich and long history of bash the answer to which file you should put your configuration in, is surprisingly confusing.

There are (mainly) two user level files which bash may run when a bash shell starts. ~/.bash_profile or ~/.bashrc.

Both these files are on the first level of your home directory ~/. Since the file names start with a . Finder and normal ls will not show them. You need to use ls -a to see if they are present. Read more about invisible and hidden files here.

The usual convention is that .bash_profile will be executed at login shells, i.e. interactive shells where you login with your user name and password at the beginning. When you ssh into a remote host, it will ask you for user name and password (or some other authentication) to log in, so it is a login shell.

When you open a terminal application, it does not ask for login. You will just get a command prompt. In other versions of Unix or Linux, this will not run the .bash_profile but a different file .bashrc. The underlying idea is that the .bash_profile should be run only once when you login, and the .bashrc for every new interactive shell.

However, Terminal.app on macOS, does not follow this convention. When Terminal.app opens a new window, it will run .bash_profile. Not, as users familiar with other Unix systems would expect, .bashrc.

Note: The Xterm application installed as part of Xquartz runs .bashrc when a new window opens, not .bash_profile. Other third-party terminal applications on macOS may follow the precedent set by Terminal.app or not.

This is all very confusing.

There are two main approaches:

• When you are living mostly or exclusively on macOS and the Terminal.app, you can create a .bash_profile, ignore all the special cases and be happy.
• If you want to have an approach that is more resilient to other terminal applications and might work (at least partly) across Unix/Linux platforms, put your configuration code in .bashrc and source .bashrc from .bash_profile with the following code in .bash_profile:

if [ -r ~/.bashrc ]; then
   source ~/.bashrc
fi

The if [ -r ... ] tests wether a file exists and is readable and the source command reads and evaluates a file in place. Sometimes you see

[ -r ~/.bashrc ] && . ~/.bashrc

(mind the spaces) Which is a shorter way to do the same thing.

Since either file can drastically change your environment, you want to restrict access to just you:

$ chmod 700 ~/.bash_profile
$ chmod 700 ~/.bashrc

That was confusing. Is that all?

No. There are more files which may be executed when a shell is created.

When bash cannot find .bash_profile it will look for .bash_login and if that does not exist either .profile. If .bash_profile is present the succeeding files will be ignored. (though you can source them in your .bash_profile)

There is also a file /etc/profile that is run for interactive login shells (and Terminal.app). This provides a central location to configure the shells for all users on a system. On macOS /etc/profilesets the default PATH with the path_helper tool and then sources /etc/bashrc which (you guessed) would be the central file for all users that is executed for non-login interactive shells. For macOS Terminal.app /etc/bashrc sets the default prompt and then itself sources /etc/bashrc_Apple_Terminal which sets up the session persistence across logins.

So in macOS Terminal.app, before you even see a prompt, these scripts will be run:

• /etc/profile
  • /etc/bashrc
    • /etc/bashrc_Apple_Terminal
• if it exists: ~/.bash_profile
  • when ~/.bash_profile does not exists, ~/.bash_login
  • when neither ~/.bash_profile nor ~/.bash_login exist, ~/.profile
• ~/bash_profile can optionally source ~/.bashrc

There is also a file ~/.inputrc, where you can setup certain command line input options. One common example for this is to enable case-insensitive tab-completion. You can find a list of more options here.

Finally, there is ~/.bash_logout which is run when a shell exits or closes.

Ok, so I have the file, now what?

Whichever file you choose, (I went with option one and have everything in .bash_profile) now you want to put stuff in it.

Technically this is a script, so you can do anything you can code in bash. However, usually the contents of a .bash_profile or .bashrc fall into one of three categories:

• setting environment variables, usually ones that affect shell behavior (PATH) or look and feel (PS1) or set configuration for other commands or programs (CLICOLOR)
• aliases
• functions

I will show some examples for each in the next post!

There have been many posts on Terminal on macOS recently. To catch up, here is a list of all the posts, so far:

• Terminal–Finder Interaction
• The macOS open Command
• On Bash History Substitution
• Terminal and the Clipboard
• Terminal: The ‘[’ Marks the Spot
• Navigating the Terminal Prompt
• On Viewing man Pages

And here are a few older posts that also fit well in this topic:

• Make tab-completion in bash case-insensitive
• Randomize Window Backgrounds in Terminal
• On hidden Files, especially Library
• Mount a dmg off a web server
• Fun with find and git

I have a few more ideas and will continue this series as long as I can think of topics. If you have suggestions, please let me know on Twitter, the MacAdmins Slack or in the comments!

And for reading through all of this you get a reward! Download a two-page PDF with all the most important Terminal Keys and Commands! Print it and put them up next to your screen!

You can also download another two-page PDF reference for my book ‘Property Lists, Preferences and Profiles for Apple Administrators’.

I have talked a lot about the Terminal recently. One the most basic elements of a command shell is the prompt line, where you enter the command. Sometimes you have to move the cursor around the prompt line and there are more efficient ways of doing this than hitting the left arrow multiple times.

These key commands are for the default Terminal configuration with the bash shell.

Recalling previous commands

Often you want to retry or re-use a previous command. Hitting the up arrow will recall the previous command, leaving the cursor at the end of the line, so you can either hit return to execute the command or edit it. You can hit the up arrow multiple times to go back further in your command history. You can also hit the down arrow to move forward again.

Instead of hitting the up arrow several times, you can also hit ctrl-R and start typing a command you used before. This will search through the history backwards and recall the latest command you used starting with what you typed. So ctrl-R and then typing cd will recall the last cd command you typed. When you get ‘stuck’ in the search mode you can leave it with ctrl-G.

Moving the cursor

Once you have recalled (or typed) a command and want to edit it, you will have to move the cursor.

The left and right arrows move the cursor left and right. You can use (option) ⌥-left (or right) to move word by word. You can use ctrl-A to jump to the beginning of the line and ctrl-E to jump to the end.

ctrl-XX (hold ctrl and press ‘x’ twice) will jump to the beginning of the line and then back to the current position the second time you use it.

If you recalled a wrong command, then you can clear the entire part left of the cursor with ctrl-U and the part right of the cursor with ctrl-K. There does not seem to be a keystroke that just clears the entire current line, but most of the time your cursor will be at the end of the line, so ctrl-U will do the trick.

And finally, you can option-click with the mouse pointer on a character in the command line to move the cursor there.

Selecting text

When you hold the option key, the mouse pointer will turn from the text insertion pointer to a cross. This is because the option key also switches text selection to rectangular. This can be useful when copying text from the table like output like ls -l or df.

As in any Mac application, you can double-click to select a word in the Terminal. However, for macOS a ‘word’ ends at special characters, including the /. This is annoying when you want to select a file path. If you double-click while holding command-shift Terminal will select the entire file path. This works for URLs, too.

Usually, you select some text to copy and paste it into the command prompt. You can take a short cut here, too. Terminal has a ‘Paste Selection’ command in the ‘Edit’ menu (command-shift-V). This way you save the ‘copy’ step.

And (as a recap from the post on marks) command-shift-A will select the output of the previous command.

Tab completes me

When you are typing commands, file names or paths, then you can use the tab key ⇥ to save keystrokes and typos.

For example you can type cd ~/Doc⇥ and it will complete to cd ~/Documents/. You can hit tab over and over at different parts of the command:

$ cd ~/Li⇥
$ cd ~/Library/Appl⇥
$ cd ~/Library/Application\ Su⇥
$ cd ~/Library/Application\ Support/

When there are multiple options to complete, bash will play an alarm sound, if you then press tab for the second time, it will list all options:

$ cd ~/D⇥<beep>⇥
Desktop/   Documents/ Downloads/

Using tab-completion not only saves keystrokes, and time, but also reduces the potential for typos and errors. Tab-completion will also automatically escape problematic characters such as spaces:

$ cd /Library/Appl⇥
$ cd /Library/Application\ Support/

Note: usually bash is case-sensitive. However, since the macOS filesystem (HFS+) is (for now) case-insensitive, you may want to switch tab-completion to be case-insensitive, too. The case-sensitivity of the file system may change with the upcoming APFS file system.

There is another form of completion which you invoke with pressing the esc(ape) key twice. This will replace bash ‘globbing’ characters such as ?, * and ~. For example:

$ cd ~/Doc*[esc][esc]
$ cd /Users/armin/Documents

Note that this replaces the ~ and the *. When there are multiple possible expansions, there will be an alert sound. You can press esc twice again to see all possible expansions:

$ cd ~/D*[esc][esc]<beep>[esc][esc]
Desktop/   Documents/ Downloads/