Conditional execution in bash

1 minute read


Situation: You want to execute a script as soon as a particular file is created.

This is an easy one-liner:

# Usage: ./ script_to_be_run file_to_anticipate [time_delay, def. 10 mins] 
while [ ! -f $2 ]; do sleep ${3-10m}; done; bash $1

The while loop keeps checking the existence of the specified file: while the file does not exist the loop will run sleep on and on. When the file exist, loop will be exited and the specified script will be run. Could it get any simpler? It seems like a waste just to continually check, but I don’t see a better option for now. (If you are wu liao run bash -x to see how many sleeps your has to go through to finally run your desired script.)

One learning point – I was using this kind of thing before to make a positional argument optional:

[ "$3" = "" ] && time_delay="15m" || time_delay=$3

This is a “ternary” bash construction, which you can construe as a shorthand conditional: A && B || C means “if A is true; execute B; else execute C”.

Turns out the easier way is using shell parameter expansion magic:

time_delay=${3-15m}  # if $3 is unset, set $3 to 15m
time_delay=${3:-15m} # if $3 is unset or null, set $3 to 15m

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