Bash – a practical example of using the suspend command in Bash

bashjob-controlprocess-management

suspend is a builtin command in Bash. When would you naturally use this command and find it useful?

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Let's say you lack both GNU screen and tmux (and X11, and virtual consoles) but want to switch between a login shell and another interactive shell.

You would first login on the console, and then start a new shell, temporarily blocking the login shell. To get the login shell back to do some work there, you'd do suspend. Then you would fg to get the interactive shell back to continue with whatever it was you did there.

In fact, with job control, the login shell could spawn a number of interactive shells as background jobs that you could switch to with fg %1, fg %2 etc., but to get back to the login shell, you would need to use suspend unless you wanted to manually kill -s STOP $$.

Also note that Ctrl+Z at the prompt in an interactive shell won't suspend it.

EDIT: I initially had a long hypothetical section about the use of suspend in a script, but since the command requires job control and since non-interactive shells usually don't have job control, I deleted that section.

Deleted section with suspend replaced by kill -s STOP $$ (this really doesn't belong to the answer any more, but it may be interesting to others anyway):

Let's say you have a background process (a script) in a script, and that this background process at some stage needs to stop and wait for the parent process to tell it to go on. This could be so that the parent has time to extract and move files into place or something like that.

The child script would suspend (kill -s STOP $$), and the parent script would send a CONT signal to it when it was okay to continue.

It gives you the opportunity to implement a sort of synchronisation between a parent process and a child process (although very basic as the parent shell process more or less needs to guess that the child process is suspended, although this can be fixed by having the child trap CONT and not suspend if that signal is received too early).

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How to Suspend and Resume Processes Like Bash

Shell jobs live in "process groups"; look at the PGRP column in extended ps output. These are used both for job control and to determine who "owns" a terminal (real or pty).

POSIX (taken from System V) uses a negative process ID to indicate a process group, since the process group is identified by the first process in the group ("process group leader"). So you would use ps to determine the process group, then kill -s TSTP "-$pgrp". (Try ps -u"$USER" -opid,ppid,pgrp,cmd.)

In your process tree, the process group starts with the firefox script launched by bash, so the process group would be 27980 and the command would be kill -s TSTP -27980.

Naturally, to resume the process group, issue kill -s CONT -- -27980.

Bash – What is the ‘eval’ Command?

eval is part of POSIX. Its an interface which can be a shell built-in.

Its described in the "POSIX Programmer's Manual": http://www.unix.com/man-page/posix/1posix/eval/

eval - construct command by concatenating arguments

It will take an argument and construct a command of it, which will be executed by the shell. This is the example of the manpage:

1) foo=10 x=foo
2) y='$'$x
3) echo $y
4) $foo
5) eval y='$'$x
6) echo $y
7) 10

In the first line you define $foo with the value '10' and $x with the value 'foo'.
Now define $y, which consists of the string '$foo'. The dollar sign must be escaped with '$'.
To check the result, echo $y.
The result will be the string '$foo'
Now we repeat the assignment with eval. It will first evaluate $x to the string 'foo'. Now we have the statement y=$foo which will get evaluated to y=10.
The result of echo $y is now the value '10'.

This is a common function in many languages, e.g. Perl and JavaScript. Have a look at perldoc eval for more examples: http://perldoc.perl.org/functions/eval.html

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Related Solutions

How to Suspend and Resume Processes Like Bash

Bash – What is the ‘eval’ Command?

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