Use Variable for Input Instead of File in Bash Script

Question

I'd like to use "ed" to move lines up and down in linefeed-delimited text held in a variable. For example, to move line two down:.

input from variable:

one
two
three
four

output to variable:

one
three
two
four

The following one-liner works beautifully for a file, but I'd like to feed it a variable.

printf '%s\\n' '2,2m3' 'wq' | /bin/ed -s '/inputFile'

I've tried assorted variations on

printf '%s\\n' '2,2m0' 'wq' | /bin/ed -s <<<$inputVariable

which don't work, and my redirection expertise is clearly insufficient to the puzzle.

I'm running macOS 10.15.1.

Just curious, what is the output of echo $SHELL? I don't use Mac, but we did run into another question recently where MacOS was defaulting to zsh instead of bash which would explain why your herestring was not working with a variable. — David C. Rankin
– David C. Rankin, Commented Dec 7, 2019 at 3:47
The content of here-string is also written to ed's stdin, that might be the problem. I have no experience with ed though — oguz ismail
– oguz ismail, Commented Dec 7, 2019 at 4:34
macOS does indeed default to zsh, but my script is definitely #!/bash . — A Bowler Cat
– A Bowler Cat, Commented Dec 7, 2019 at 6:53

dash-o · Accepted Answer · 2019-12-07 06:38:26Z

3

The ed program process commands from STDIN only (and does not support passing commands via the command line). To make this work, you need to represent the input as a file name (not necessary a file).

The following "convert" the variable input to a filename (/dev/fd/3) using "here string",

#! /bin/bash

input=$(ls -l)
out=$(printf '%s\n' '2,2m3' '1,$p' | ed 3<<<"$input" /dev/fd/3)
echo "$out"

Alternatively, with explicit reference to /dev/fd, or to specific file descriptor.

#! /bin/bash

input=$(ls -l)
out=$(ed <(printf '%s', "$input") <<___
2,2m3
1,\$p
___

edited Dec 7, 2019 at 6:38

answered Dec 7, 2019 at 5:16

dash-o

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5 Comments

Gordon Davisson Over a year ago

/proc isn't universal; I think /dev/fd/3 is more portable (though it's not universal either). In bash, you could use ed <(echo "$input").

dash-o Over a year ago

@GordonDavisson as mentioned in answer, if STDIN is used for the input (variable), there is no way to pass the commands to ed.

dash-o Over a year ago

@GordonDavisson On Linux, the '/dev/fd' is symlinked to /proc/self/fd'. If /proc/ not available, than no '/dev/fd'. Also process substitution is (usually) implemented (on Linux) with /dev/fd - same dependency. I've modified the code to use the /dev/fds,

Gordon Davisson Over a year ago

That's just how Linux does it. OP mentioned that they're running macOS, which doesn't have /proc at all, but has /dev/fd/* (as direct character special devices, not symlinks). I checked a NetBSD system, and it's similar.

A Bowler Cat Over a year ago

dash-o's out=$(printf '%s\n' '2,2m3' '1,$p' | ed 3<<<"$input" /dev/fd/3) works perfectly under Darwin with the addition of -s to suppress diagnostics. out=$(printf '%s\n' '2,2m3' '1,$p' | ed -s 3<<<"$input" /dev/fd/3) Thanks.

Léa Gris · Accepted Answer · 2019-12-07 16:47:42Z

1

You can also use Bash's built-in index manipulation rather than spawning ed:

#!/usr/bin/env bash

inputVariable='one
two
three
four
five
six
'

IFS=$'\n' set -- $inputVariable

swp=3 # swap lines 3 and 4

printf '%s\n' "${@:1:swp-1}" "${@:swp+1:1}" "${@:swp:1}" "${@:swp+2}"

Output:

one
two
four
three
five
six

Detail of functions:

IFS=$'\n' set -- $inputVariable: Transfers $inputVariable's lines as arguments.
printf '%s\n': Prints arguments as lines.
"${@:1:swp-1}": Expands all arguments lines from 1^st index to last index before the swap.
"${@:swp+1:1}" "${@:swp:1}": Performs the actual arguments lines swap, with next index swp+1 followed by swp index.
"${@:swp+2}": Expands all remaining arguments after the swapped indexes swp+2.

edited Dec 7, 2019 at 16:47

answered Dec 7, 2019 at 12:21

Léa Gris

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