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An Ignorant Wanderer
Asked: 2020-01-15 20:29:28 +0800 CST

How can I pipe the result of the wc command into an arithmetic expansion?

  • 772

I want to run wc -l and use the result in an arithmetic expansion. In other words, I want to do something like this:

wc -l (now somehow pipe/pass the result of this to) $((2 + result of wc command))

How can I do this?

Context

In my ~/Pictures directory, I have multiple files named "Screenshot from YYYY-MM-DD hh-mm-ss.png", as well as other files. I want to keep all files that are not Screenshots, and only a specified number of Screenshot pictures, based on how recent they were. I want to basically only keep the last 3 most recent Screenshots I have, and delete all the rest.

So here is what I have so far:

ls | grep "Screenshot *" | sort -r | wc -l

What I then wanted to do was to subtract 3 from wc -l, which would then allow me to use tail to list all the files that are not the first 3, and then just delete them.

command-line wc

2 Answers

  1. Best Answer

    For what you are asking (source):

    cat FILE | wc -l | xargs -n 1 bash -c 'echo $(($1 + 2))' args

    bash -c 'commands' run a new bash environment, where $1 is an argument given by xargs. Use man the_commmand to access manual pages and learn more about each command and their options.

    From man bash:

    SYNOPSIS
    xargs [options] [command [initial-arguments]]

    OPTIONS
    -c If the -c option is present, then commands are read from the first non-option argument command_string. If there are arguments after the command_string, the first argument is assigned to $0 and any remaining arguments are assigned to the positional parameters. The assignment to $0 sets the name of the shell, which is used in warning and error messages.

    $ bash -c 'echo $1' aaa bbb ccc
bbb
$ bash -c 'echo $0' aaa bbb ccc
aaa
$ echo $0
/bin/bash

    So in this case you could avoid using the placeholder args (note you can use any word) just using $0 instead. Also, for just wc -l you can avoid the -n 1:

    cat FILE | wc -l | xargs bash -c 'echo $(($0 + 2))'

    For what you need might be more suitable to remove interactively (-i) files older than eg 20 days.

    find ~/Pictures/ -type f -name "Screenshot*" -mtime +20 -exec rm -i {} +

    To get the first 3 newest files with that name pattern is more simple:

    ls -lt ~/Pictures/Screenshot* | head -n 3
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  2. Assuming FILE is a file that is located in the current directory:

    echo $((`wc -l < FILE`+2))

    wc -l < FILE returns the number of lines in FILE and echo $((`wc -l < FILE` +2)) returns the number of lines in FILE + 2.

    Translating a string into a numerical expression is relatively straightforward using arithmetic expansion of the form $((EXPRESSION)) where EXPRESSION is an arithmetic expression.

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