bash—Better way to store variable between runs?

问题

I've made a bash script which I run every hour with crontab, and I need to store one variable so that I can access it the next time I run it. The script changes the variable every time it runs, so I can't hardcode it in. Right now I am writing it to a txt file and then reading it back. Is there a better way to do it than this? And the way I am reading the txt file is something I found on here, I don't understand it, and it's kinda clunky. Is there not a built in command for this? Anyway, here's the applicable code, with some of the variables changed to make it easier to read.

while read x; do
  var=$x
done < var.txt

# Do some stuff, change var to a new value

echo $var > var.txt

The variable is only a single integer, so the text file feels overkill.

回答1:

There's no need to use var; x will be in scope for the current shell. Alternately,

read var < var.txt
# do stuff with var
echo $var > var.txt

I recommend using a simple text file to store the variable. However, there is the (highly questionable) option of a self-modifying script. FOR ENTERTAINMENT PURPOSES ONLY!

#!/bin/bash

read val < <( tail -n 1 "$0" )

(( val++ ))
echo "$val"

tmp=$(mktemp /tmp/XXXXXXX)
sed '$s/.*/'$val'/' "$0" > "$tmp"
mv "$tmp" "$0"

exit
0

The key is to have the next-to-last line be the exit command, so nothing after it will execute. The last line is the variable value you want to persist. When the script runs, it reads from its own last line. Before it exits, it uses sed to write a copy of itself toa temp file, with the last line modified with the current value of the persistent value. Then we overwrite the current script with the temp file (assuming we will have permission to do so).

But seriously? Don't do this.

回答2:

I know this is an old question. But, I still decide to post my solution here in the hope that it might be helpful to others who come here in search of a way to serialize env vars between sessions.

The simple way is just write "var_name=var_value" into a file, say "./environ". And then "source ./envrion" in following sessions. For example:

echo "var1=$var1" > ./environ

A more comprehensive (and elegant?) way which persist all attributes of variables is to make use of "declare -p":

declare -p var1 var2 > ./environ
# NOTE: no '$' before var1, var2

Later on, after "source ./envrion" you can get var1 var2 with all attributes restored in addition to its value. This means it can handle arrays, integers etc.

One caveat for the "declare -p xx", though: if you wrap the "source ./environ" into a function, then all sourced variables are visible within the function only because "declare" by default declares variables as local ones. To circumvent this, you may either "source" out of any function (or in your "main" function) or modify the ./environ to add "-g" after declare (which makes corresponding variable global). For instance:

sed -i 's/^declare\( -g\)*/declare -g/' ./environ
# "\( -g\)?" ensure no duplication of "-g"

回答3:

1- You can simplify your script, as you only have one variable

var=`cat var.txt`
# Do some stuff, change var to a new value   
echo $var > var.txt

2- You can store your variable in the environment:

export var

# Do some stuff, change var to a new value

But you'll need to prompt it . script.ksh (dot at the beggining). But it shouldn't have 'exit' in it and i'm not sure this would work in cron...

回答4:

To store multiple variables between runs, a solution I considered is to save them under the format my_var=my_value in a separated file.

Then, I include two function to set and retrieve the variables

In the file storing the variables and their values:

Let's call this file context.dat

# Here I store the variables and their values
my_var_x=1
my_var_y=boo
my_var_z=0

In the actual script:

Let's call the file multiple_run.sh

context=./context.dat

function update_variables(){
    # update the variable context
    source $context
}

function set_variable(){
    # store variable
    variable=$1 #variable to be set
    value=$2    # value to give to the value
    # modify the file storing the value
    sed -i 's/'${variable}'.*/'${variable}'='${value}'/' $context
}

##################
# Test code
echo var_x
update_variables
echo var_x
# do something
set_variable var_x 2
echo $var_x

This is one approach among other. With such method, you need to create the storing file before and create each line for each variable. Besides, the context.dat is a priori accessible by any other script.

回答5:

I ended up doing the following. Would prefer the variables in one file, but this bloats the code slightly. How does this read thing work? You can store multiple variables in a seperate file, say variables.txt, and then have your main program in say main.sh. It might be better to write seperate scripts for loading and saving variables though.

For varibles.txt:

A=0
B=0
C=0

For main.sh:

#!/bin/bash

#reload variables
A=`cat ./variables.txt|grep "A="|cut -d"=" -f2`
B=`cat ./variables.txt|grep "B="|cut -d"=" -f2`
C=`cat ./variables.txt|grep "C="|cut -d"=" -f2`

#print variables
printf "$A\n"
printf "$B\n"
printf "$C\n"

#update variables
A=$((($A+1)))
B=$((($B+2)))
C=$((($C+3)))

#save variables to file
#for A
 #remove entry for A
 cat ./variables.txt|grep -v "A=">>./tmp.txt
 #save entry for A
 printf "A=$A\n">>./tmp.txt
 #move tmp.txt to variables.txt
mv ./tmp.txt ./variables.txt

#for B
 #remove entry for B
 cat ./variables.txt|grep -v "B=">>./tmp.txt
 #save entry for B
 printf "B=$B\n">>./tmp.txt
 #move tmp.txt to variables.txt
 mv ./tmp.txt ./variables.txt

#for C
 #remove entry for C
 cat ./variables.txt|grep -v "C=">>./tmp.txt
 #save entry for C
 printf "C=$C\n">>./tmp.txt
 #move tmp.txt to variables.txt
 mv ./tmp.txt ./variables.txt

来源：https://stackoverflow.com/questions/12334495/bash-better-way-to-store-variable-between-runs

标签

bash

variables

text

storage