Last year, I wrote about Filezilla: figuring out the cause of “Connection timed out after 20 seconds of inactivity” about sftp connection problems.
The solution there was to exclude part of bashrc with an if [Wayback] statement so bash would skip it during sftp, but not during ssh login:
[WayBack] linux – Use .bashrc without breaking sftp – Server Fault
- From answer 1 (thanks [WayBack] Mike):
Try doing this instead
if [ "$SSH_TTY" ] then source .bashc_real fi- From Answer 2 (thanks [WayBack] Insyte):
A good trick for testing the cleanliness of your login environment is to
sshin with a command, which simulates the same wayscp/sftpconnect. For example:ssh myhost /bin/truewill show you exactly whatscp/sftpsees when they connect.
That caused some scripts not to be run when switching user, for instance by doing sudo su -.
The reason for that was that I forgot to put enough research in part of Answer 2, so I quote a few bits more of it (highlights and code markup mine):
… it’s worth pointing out that you can accomplish this carefully selecting which startup files to put the verbose stuff in. From the
bashman page:When
bashis invoked as an interactive login shell, or as a non-interactive shell with the--loginoption, it first reads and executes commands from the file/etc/profile, if that file exists. After reading that file, it looks for~/.bash_profile,~/.bash_login, and~/.profile, in that order, and reads and executes commands from the first one that exists and is readable. The--noprofileoption may be used when the shell is started to inhibit this behavior.When an interactive shell that is not a login shell is started,
bashreads and executes commands from~/.bashrc, if that file exists. This may be inhibited by using the--norcoption. The--rcfilefile option will force bash to read and execute commands from file instead of~/.bashrc.The
sftp/scptools start an interactive non-login shell, so.bashrcwill be sourced.
For further reading, there is the underlying bash manual as a PDF file [Wayback] and html document tree [Wayback]. Note it is large (the PDF is 190 pages).
I find the easiest way to navigate around bash documentation through these links:
- [Wayback] Top (Bash Reference Manual) (especially read the Bash Features section there)
- [Wayback] Indexes (Bash Reference Manual) (there are five of them, centered around various core topics)
Types of shell invocations: to login or to non-login and to interactive or to non-interactive
Basically, from the above answer there are [Archive.is] 4 types of shells (confirmed by these parts of the bash documentation: [Wayback] Section 6.1: Invoking-Bash and [Wayback] Section 6.2: Bash-Startup-Files):
- interactive login
- interactive non-login
- non-interactive login
- non-interactive non-login
And there are various means the shells can start (ssh, local console, …). The "$SSH_TTY" trick only checks interactive login via ssh, but fails to detect others.
So I did some digging for the correct information to log, which including the above are:
- [Wayback] Section 4.3.1: The-Set-Builtin
-hLocate and remember (hash) commands as they are looked up for execution. This option is enabled by default.-mJob control is enabled (see Job Control). All processes run in a separate process group. When a background job completes, the shell prints a line containing its exit status.-BThe shell will perform brace expansion (see Brace Expansion). This option is on by default.-HEnable ‘!’ style history substitution (see History Interaction). This option is on by default for interactive shells.
Note that in addition to this, there is the non-settable option
i: The current shell is interactive (see the-iin section 6.1 below). - [Wayback] Section 4.3.2: The-Shopt-Builtin
login_shellThe shell sets this option if it is started as a login shell (see Invoking Bash). The value may not be changed.
- [Wayback] Section 6.1: Invoking-Bash
There are several single-character options that may be supplied at invocation which are not available with the
setbuiltin.-iForce the shell to run interactively. Interactive shells are described in Interactive Shells.
…
A login shell is one whose first character of argument zero is ‘-’, or one invoked with the –login option.
- [Wayback] Section 6.2: Bash-Startup-Files explains about these shell invocation types:
- interactive login shell
- interactive non-login shell
- non-interactive shell
- [Wayback] Section 6.3.1: Is-this-Shell-Interactive
To determine within a startup script whether or not Bash is running interactively, test the value of the ‘-’ special parameter. It contains
iwhen the shell is interactive. For example:case "$-" in *i*) echo This shell is interactive ;; *) echo This shell is not interactive ;; esac
Alternatively, startup scripts may examine the variable
PS1; it is unset in non-interactive shells, and set in interactive shells. Thus:if [ -z "$PS1" ]; then echo This shell is not interactive else echo This shell is interactive fi
From theory to practice
After reading the above documentation links, I put the below code in the global .bashrc (which of course caused trouble with sftp, so I commented it out later):
echo "Option flags: '$-'" echo "PS1: '$PS1'" echo "shopt login_shell: '$(shopt login_shell)'" echo "Parameter zero: '$0'" [ "$SSH_TTY" ] ; echo "[ \"\$SSH_TTY\" ] outcome: $?"
And the output after these commands:
-
ssh user@hostOption flags: 'himBH' PS1: '\u@\h:\w> ' shopt login_shell: 'login_shell on' Parameter zero: '-bash' [ "$SSH_TTY" ] outcome: 0Verdict:
interactive,login -
ssh user@hostfollowed by
sudo su -Option flags: 'himBH' PS1: '\[\]\h:\w #\[\] ' shopt login_shell: 'login_shell on' Parameter zero: '-bash' [ "$SSH_TTY" ] outcome: 1Verdict:
interactive,login -
ssh user@hostfollowed by
bashOption flags: 'himBH' PS1: '\u@\h:\w> ' shopt login_shell: 'login_shell off' Parameter zero: 'bash' [ "$SSH_TTY" ] outcome: 0Verdict:
interactive,non-login -
ssh user@hostfollowed by
sudo su -then by
bashOption flags: 'himBH' PS1: '\[\]\h:\w #\[\] ' shopt login_shell: 'login_shell off' Parameter zero: 'bash' [ "$SSH_TTY" ] outcome: 1Verdict:
interactive,non-login -
ssh user@host /bin/trueOption flags: 'hBc' PS1: '' shopt login_shell: 'login_shell off' Parameter zero: 'bash' [ "$SSH_TTY" ] outcome: 1Verdict:
non-interactive,non-login
The final one is what for instance sftp will see. It excludes the non-interactive mark in the shopt option flags.
Modifications to my .bashrc file
Since the [Wayback] test for "$SSH_TTY" is inconsistent with the login being interactive, I modified the .bashrc section
if [ "$SSH_TTY" ] then source .bashc_real fi
to become
if [[ $- =~ i ]] then # only during interactive login shells source .bashc_real fi
I know the [[...]] over test shorthand [...] is a bashism, see [Wayback] if statement – Is double square brackets [[ ]] preferable over single square brackets [ ] in Bash? – Stack Overflow for why I like it.
More relevant documentation
I based the above changes not only on the mentioned StackOverflow post, but also doing some more Googling revealing these useful documentation and question/answer links:
- [Wayback] Section 3.2.5.2: Conditional Constructs;
[[ expression ]]
[[…]][[ expression ]]Return a status of
0or1depending on the evaluation of the conditional expression expression. Expressions are composed of the primaries described below in Bash Conditional Expressions. Word splitting and filename expansion are not performed on the words between the[[and]]; tilde expansion, parameter and variable expansion, arithmetic expansion, command substitution, process substitution, and quote removal are performed. Conditional operators such as ‘-f’ must be unquoted to be recognized as primaries.…
An additional binary operator, ‘
=~’, is available, with the same precedence as ‘==’ and ‘!=’. When it is used, the string to the right of the operator is considered a POSIX extended regular expression and matched accordingly (using the POSIXregcompandregexecinterfaces usually described inregex(3)). The return value is0if the string matches the pattern, and1otherwise. If the regular expression is syntactically incorrect, the conditional expression’s return value is2. - [Wayback] Section 4.1: Bourne Shell Builtins;
testor[...](Bash Reference Manual)
test expr
Evaluate a conditional expression expr and return a status of
0(true) or1(false). Each operator and operand must be a separate argument. Expressions are composed of the primaries described below in Bash Conditional Expressions.testdoes not accept any options, nor does it accept and ignore an argument of--as signifying the end of options.When the
[form is used, the last argument to the command must be a]. - [Wayback] bash – What are the special dollar sign shell variables? – Stack Overflow (thanks [Wayback] kojiro!):
$1,$2,$3, … are the positional parameters."$@"is an array-like construct of all positional parameters,{$1, $2, $3 ...}."$*"is the IFS expansion of all positional parameters,$1 $2 $3 ....$#is the number of positional parameters.$-current options set for the shell.$$pid of the current shell (not subshell).$_most recent parameter (or the abs path of the command to start the current shell immediately after startup).$IFSis the (input) field separator.$?is the most recent foreground pipeline exit status.$!is the PID of the most recent background command.$0is the name of the shell or shell script.
Most of the above can be found under Special Parameters in the Bash Reference Manual. There are all the environment variables set by the shell.
For a comprehensive index, please see the Reference Manual Variable Index.
- [Wayback] bash – Differentiate Interactive login and non-interactive non-login shell – Ask Ubuntu (thanks [Wayback] terdon)
Briefly (see here for more details), with examples:
- interactive login shell: You log into a remote computer via, for example
ssh. Alternatively, you drop to a tty on your local machine (Ctrl+Alt+F1) and log in there. - interactive non-login shell: Open a new terminal.
- non-interactive non-login shell: Run a script. All scripts run in their own subshell and this shell is not interactive. It only opens to execute the script and closes immediately once the script is finished.
- non-interactive login shell: This is extremely rare, and you’re unlikey to encounter it. One way of launching one is
echo command | ssh server. Whensshis launched without a command (sosshinstead ofssh commandwhich will runcommandon the remote shell) it starts a login shell. If thestdinof thesshis not a tty, it starts a non-interactive shell. This is whyecho command | ssh serverwill launch a non-interactive login shell. You can also start one withbash -l -c command.
If you want to play around with this, you can test for the various types of shell as follows:
- Is this shell interactive?Check the contents of the
$-variable. For interactive shells, it will includei:## Normal shell, just running a command in a terminal: interacive $ echo $- himBHs ## Non interactive shell $ bash -c 'echo $-' hBc - Is this a login shell?There is no portable way of checking this but, for bash, you can check if the
login_shelloption is set:## Normal shell, just running a command in a terminal: interacive $ shopt login_shell login_shell off ## Login shell; $ ssh localhost $ shopt login_shell login_shell on
Putting all this together, here’s one of each possible type of shell:
## Interactive, non-login shell. Regular terminal $ echo $-; shopt login_shell himBHs login_shell off ## Interactive login shell $ bash -l $ echo $-; shopt login_shell himBHs login_shell on ## Non-interactive, non-login shell $ bash -c 'echo $-; shopt login_shell' hBc login_shell off ## Non-interactive login shell $ echo 'echo $-; shopt login_shell' | ssh localhost Pseudo-terminal will not be allocated because stdin is not a terminal. hBs login_shell on - interactive login shell: You log into a remote computer via, for example
- [Wayback] Difference between Login Shell and Non-Login Shell? – Unix & Linux Stack Exchange
A login shell is the first process that executes under your user ID when you log in for an interactive session. The login process tells the shell to behave as a login shell with a convention: passing argument 0, which is normally the name of the shell executable, with a
-character prepended (e.g.-bashwhereas it would normally bebash. Login shells typically read a file that does things like setting environment variables:/etc/profileand~/.profilefor the traditional Bourne shell,~/.bash_profileadditionally for bash†,/etc/zprofileand~/.zprofilefor zsh†,/etc/csh.loginand~/.loginfor csh, etc.When you log in on a text console, or through SSH, or with
su -, you get an interactive login shell. When you log in in graphical mode (on an X display manager), you don’t get a login shell, instead you get a session manager or a window manager.It’s rare to run a non-interactive login shell, but some X settings do that when you log in with a display manager, so as to arrange to read the profile files. Other settings (this depends on the distribution and on the display manager) read
/etc/profileand~/.profileexplicitly, or don’t read them. Another way to get a non-interactive login shell is to log in remotely with a command passed through standard input which is not a terminal, e.g.ssh example.com <my-script-which-is-stored-locally(as opposed tossh example.com my-script-which-is-on-the-remote-machine, which runs a non-interactive, non-login shell).When you start a shell in a terminal in an existing session (screen, X terminal, Emacs terminal buffer, a shell inside another, etc.), you get an interactive, non-login shell. That shell might read a shell configuration file (
~/.bashrcfor bash invoked asbash,/etc/zshrcand~/.zshrcfor zsh,/etc/csh.cshrcand~/.cshrcfor csh, the file indicated by theENVvariable for POSIX/XSI-compliant shells such as dash, ksh, and bash when invoked assh,$ENVif set and~/.mkshrcfor mksh, etc.).When a shell runs a script or a command passed on its command line, it’s a non-interactive, non-login shell. Such shells run all the time: it’s very common that when a program calls another program, it really runs a tiny script in a shell to invoke that other program. Some shells read a startup file in this case (bash runs the file indicated by the
BASH_ENVvariable, zsh runs/etc/zshenvand~/.zshenv), but this is risky: the shell can be invoked in all sorts of contexts, and there’s hardly anything you can do that might not break something.† I’m simplifying a little, see the manual for the gory details.
If you want to avoid the [[...]] bashishm, then read [Wayback] Bashism: How to make bash scripts work in dash – Greg’s Wiki.
–jeroen
The Wiert Corner - irregular stream of stuff 