https://bobcopeland.com/blog/2012/10/goto-in-bash/

Skip to content
Insignificant Bits

Insignificant Bits

Posted on October 26, 2012May 21, 2019 by bob

goto in bash

If you are as old and nerdy as I, you may have spent your grade
school days hacking in the BASIC computer language. One of the
(mostly hated) features of the (mostly hated) language was that any
statement required a line number; this provided both the ability to
edit individual lines of the program without a screen editor, as well
as de facto labels for the (mostly hated) GOTO and GOSUB commands.
But you could also use line numbers to run your program starting from
any random point: "RUN 250" might start in the middle of a program,
typically after line 250 exited with some syntax error and was
subsequently fixed.

Today, in bash, we have no such facility. Why on earth would anyone
want it, with the presence of actual flow control constructs? Who
knows, but asking Google about "bash goto" shows that I am not the
first.

For my part, at $work, I have a particular script which takes several
days to run, each part of which may take many hours, and, due to moon
phases, may fail haphazardly. If a command fails, the state up to
that point is preserved, so I just need to continue where that left
off. Each major part of the job is already factored into individual
scripts, so I could cut-and-paste commands from the failure point
onward, but I'm lazy.

Thus, I present bash goto. It runs sed on itself to strip out any
parts of the script that shouldn't run, and then evals it all.
Prepare to cringe.

#!/bin/bash
# include this boilerplate
function jumpto
{
    label=$1
    cmd=$(sed -n "/$label:/{:a;n;p;ba};" $0 | grep -v ':$')
    eval "$cmd"
    exit
}

start=${1:-"start"}

jumpto $start

start:
# your script goes here...
x=100
jumpto foo

mid:
x=101
echo "This is not printed!"

foo:
x=${x:-10}
echo x is $x

results in:

$ ./test.sh
x is 100
$ ./test.sh foo
x is 10
$ ./test.sh mid
This is not printed!
x is 101

My quest to make bash look like assembly language draws ever nearer
to completion.

Update 2019/05/21: A reader pointed out that executing one of the
labels results in bash complaining "command not found" and suggested
putting the labels in a comment, which works just fine without any
other changes (but if you like, you can drop the "grep -v" in that
case). You might also be interested in the takes of folks on reddit,
or my own take of being discussed on reddit.

Categorieshacking Tagsbash, basic-hate

Post navigation

Previous PostPrevious Fake Thanksgiving
Next PostNext Olio
Search for: [                    ] Search
Recent Posts

  * Garden 2023
  * Blues For Alice
  * Four
  * Moose the Mooche
  * My One and Only Love

Tags

amp android arduino ath5k ath9k ath9k_htc ath10k bash basic-hate
c++-hate charts crosswords crypto fs fuse gardening git graphics
halloween hbase hostapd ipv6 java-hate javascript-hate karma kernel
keyboard logarithms mac-hate math memory mesh openwrt parenthood pcb
psa python python-hate sd8787 soldering vim wifi wl12xx wmediumd x86

Meta

  * Log in
  * Entries RSS
  * Comments RSS
  * WordPress.org

Proudly powered by WordPress