Tag Archives: bash

Bash trick of the week – call stacks

For someone that used to be very vocal about hating shell scripting, I seem to be building more and more tools related to it every day. The latest is caller (from “man bash”):

caller [expr]
Returns the context of any active subroutine call (a shell function or a script executed with the . or source builtins). Without expr, caller displays the line number and source filename of the current subroutine call. If a non-negative inte‐ ger is supplied as expr, caller displays the line number, subroutine name, and source file corresponding to that position in the current execution call stack. This extra information may be used, for example, to print a stack trace. The current frame is frame 0. The return value is 0 unless the shell is not executing a subroutine call or expr does not correspond to a valid position in the call stack.

This means that if your bash code makes heavy use of functions, you can get the call stack back out. This turns out to be really handy for things like writing testing scripts. I recently added some more unit testing to devstack-gate, and used this to make it easy to see what was going on:

# Utility function for tests
function assert_list_equal {
    local source=$(echo $1 | awk 'BEGIN{RS=",";} {print $1}' | sort -V | xargs echo)
    local target=$(echo $2 | awk 'BEGIN{RS=",";} {print $1}' | sort -V | xargs echo)
    if [[ "$target" != "$source" ]]; then
        echo -n `caller 0 | awk '{print $2}'`
        echo -e " - ERROR\n $target \n != $source"
        ERRORS=1
    else
    # simple backtrace progress detector
        echo -n `caller 0 | awk '{print $2}'`
        echo " - ok"
    fi
}

The output ends up looking like this:

ribos:~/code/openstack/devstack-gate(master)> ./test-features.sh 
test_full_master - ok
test_full_feature_ec - ok
test_neutron_master - ok
test_heat_slow_master - ok
test_grenade_new_master - ok
test_grenade_old_master - ok
test_full_havana - ok

I never thought I’d know this much bash, and I still think data structure manipulation is bash is craziness, but for imperative programming that’s largely a lot of command calls, this works pretty well.

Robert Muth: Better Bash Scripting in 15 Minutes

Better Bash Scripting in 15 Minutes. The tips and tricks below originally appeared as one of Google’s “Testing on the Toilet” TOTT episodes. This is a revised and augmented version.

via Robert Muth: Better Bash Scripting in 15 Minutes.

Some good bits in here. We’ve implemented some of them in devstack, and I think a few more (like uninitialized and enforcing double brackets on all conditionals) would be helpful. It also makes me think about things to enforce in bash8.

Unifying Bash History

A question came up in our Linux User Group IRC channel about the seemingly random mess that is your bash command history. As I googled around for the answer to what was going on I came across this snippet of pure joy.

shopt -s histappend
export PROMPT_COMMAND="history -a; history -n; $PROMPT_COMMAND"

If you add those 2 lines to your .bashrc what you get is a unified bash history. All commands from all terminals get saved in real time and are made available to all other terminals.

It’s pretty great, though it does make you realize that some times you really did rely on the old behavior. The up key no longer does exactly what you are used to.