bash: How can I assemble the string: `“filename=output_0.csv”`

问题

I am using a bash script to execute a program. The program must take the following argument. (The program is gnuplot.)

gnuplot -e "filename='output_0.csv'" 'plot.p'

I need to be able to assemble the following string: "filename='output_0.csv'"

My plan is to assemble the string STRING=filename='output_0.csv' and then do the following: gnuplot -r "$STRING" 'plot.p'. Note I left the words STRING without stackoverflow syntax style highlighting to emphasise the string I want to produce.

I'm not particularly proficient at bash, and so I have no idea how to do this.

I think that strings can be concatenated by using STRING="$STRING"stuff to append to string? I think that may be required?

As an extra layer of complication the value 0 is actually an integer which should increment by 1 each time the program is run. (Done by a for loop.) If I have n=1 in my program, how can I replace the 0 in the string by the "string value" or text version of the integer n?

回答1:

A safest way to append something to an existing string would be to include squiggly brackets and quotes:

STRING="something"
STRING="${STRING}else"

You can create the "dynamic" portion of your command line with something like this:

somevalue=0
STRING="filename='output_${somevalue}.csv'"

There are other tools like printf which can handle more complex formatting.

somevalue=1
fmt="filename='output_%s.csv'"

STRING="$(printf "$fmt" "$somevalue")"

Regarding your "extra layer of complication", I gather that this increment has to happen in such a way as to store the value somewhere outside the program, or you'd be able to use a for loop to handle things. You can use temporary files for this:

#!/usr/bin/env bash

# Specify our counter file
counter=/tmp/my_counter

# If it doesn't exist, "prime" it with zero
if [ ! -f "$counter" ]; then
  echo "0" > $counter
fi

# And if it STILL doesn't exist, fail.
if [ ! -f "$counter" ]; then
  echo "ERROR: can't create counter." >&2
fi

# Read the last value...
read value < "$counter"

# and set up our string, per your question.
STRING="$(printf "filename='output_%d.csv'" "${value}")"

# Last, run your command, and if it succeeds, update the stored counter.
gnuplot -e "$STRING" 'plot.p' && echo "$((value + 1))" > $counter

As always, there's more than one way to solve this problem. With luck, this will give you a head start on your reading of the bash man page and other StackOverflow questions which will help you learn what you need!

回答2:

An answer was posted, which I thought I had accepted already, but for some reason it has been deleted, possibly because it didn't quite answer the question.

I posted another similar question, and the answer to that helped me also answer this question. You can find said question and answer here: bash: Execute a string as a command

来源：https://stackoverflow.com/questions/30868964/bash-how-can-i-assemble-the-string-filename-output-0-csv

标签

string

bash

concatenation