Unix one-liner to swap/transpose two lines in multiple text files?

Question

I wish to swap or transpose pairs of lines according to their line-numbers (e.g., switching the positions of lines 10 and 15) in multiple text files using a UNIX tool such a

Answer 1

The use of the following helper script allows using the power of find ... -exec ./script '{}' l1 l2 \; to locate the target files and to swap lines l1 & l2 in each file in place. (it requires that there are no identical duplicate lines within the file that fall within the search range) The script uses sed to read the two swap lines from each file into an indexed array and passes the lines to sed to complete the swap by matching. The sed call uses its "matched first address" state to limit the second expression swap to the first occurrence. An example use of the helper script below to swap lines 5 & 15 in all matching files is:

find . -maxdepth 1 -type f -name "lnum*" -exec ../swaplines.sh '{}' 5 15 \;

For example, the find call above found files lnumorig.txt and lnumfile.txt in the present directory originally containing:

$ head -n20 lnumfile.txt.bak
 1  A simple line of test in a text file.
 2  A simple line of test in a text file.
 3  A simple line of test in a text file.
 4  A simple line of test in a text file.
 5  A simple line of test in a text file.
 6  A simple line of test in a text file.
<snip>
14  A simple line of test in a text file.
15  A simple line of test in a text file.
16  A simple line of test in a text file.
17  A simple line of test in a text file.
18  A simple line of test in a text file.
19  A simple line of test in a text file.
20  A simple line of test in a text file.

And swapped the lines 5 & 15 as intended:

$ head -n20 lnumfile.txt
 1  A simple line of test in a text file.
 2  A simple line of test in a text file.
 3  A simple line of test in a text file.
 4  A simple line of test in a text file.
15  A simple line of test in a text file.
 6  A simple line of test in a text file.
<snip>
14  A simple line of test in a text file.
 5  A simple line of test in a text file.
16  A simple line of test in a text file.
17  A simple line of test in a text file.
18  A simple line of test in a text file.
19  A simple line of test in a text file.
20  A simple line of test in a text file.

The helper script itself is:

#!/bin/bash

[ -z $1 ] && {              # validate requierd input (defaults set below)
    printf "error: insufficient input calling '%s'. usage: file [line1 line2]\n" "${0//*\//}" 1>&2
    exit 1
}

l1=${2:-10}                 # default/initialize line numbers to swap
l2=${3:-15}

while IFS=$'\n' read -r line; do  # read lines to swap into indexed array
    a+=( "$line" ); 
done <<<"$(sed -n $((l1))p "$1" && sed -n $((l2))p "$1")"

((${#a[@]} < 2)) && {       # validate 2 lines read
    printf "error: requested lines '%d & %d' not found in file '%s'\n" $l1 $l2 "$1"
    exit 1
}

                            # swap lines in place with sed (remove .bak for no backups)
sed -i.bak -e "s/${a[1]}/${a[0]}/" -e "0,/${a[0]}/s/${a[0]}/${a[1]}/" "$1"

exit 0

Even though I didn't manage to get it all done in a one-liner I decided it was worth posting in case you can make some use of it or take ideas from it. Note: if you do make use of it, test to your satisfaction before turning it loose on your system. The script currently uses sed -i.bak ... to create backups of the files changed for testing purposes. You can remove the .bak when you are satisfied it meets your needs.

If you have no use for setting default lines to swap in the helper script itself, then I would change the first validation check to [ -z $1 -o -z $2 -o $3 ] to insure all required arguments are given when the script is called.

While it does identify the lines to be swapped by number, it relies on the direct match of each line to accomplish the swap. This means that any identical duplicate lines up to the end of the swap range will cause an unintended match and failue to swap the intended lines. This is part of the limitation imposed by not storing each line within the range of lines to be swapped as discussed in the comments. It's a tradeoff. There are many, many ways to approach this, all will have their benefits and drawbacks. Let me know if you have any questions.

---

Brute Force Method

Per your comment, I revised the helper script to use the brute forth copy/swap method that would eliminate the problem of any duplicate lines in the search range. This helper obtains the lines via sed as in the original, but then reads all lines from file to tmpfile swapping the appropriately numbered lines when encountered. After the tmpfile is filled, it is copied to the original file and tmpfile is removed.

#!/bin/bash

[ -z $1 ] && {              # validate requierd input (defaults set below)
    printf "error: insufficient input calling '%s'. usage: file [line1 line2]\n" "${0//*\//}" 1>&2
    exit 1
}

l1=${2:-10}                 # default/initialize line numbers to swap
l2=${3:-15}

while IFS=$'\n' read -r line; do  # read lines to swap into indexed array
    a+=( "$line" ); 
done <<<"$(sed -n $((l1))p "$1" && sed -n $((l2))p "$1")"

((${#a[@]} < 2)) && {       # validate 2 lines read
    printf "error: requested lines '%d & %d' not found in file '%s'\n" $l1 $l2 "$1"
    exit 1
}

                            # create tmpfile, set trap, truncate
fn="$1"
rmtemp () { cp "$tmpfn" "$fn"; rm -f "$tmpfn"; }
trap rmtemp SIGTERM SIGINT EXIT

declare -i n=1
tmpfn="$(mktemp swap_XXX)"
:> "$tmpfn"

                            # swap lines in place with a tmpfile
while IFS=$'\n' read -r line; do

    if ((n == l1)); then
        printf "%s\n" "${a[1]}" >> "$tmpfn"
    elif ((n == l2)); then
        printf "%s\n" "${a[0]}" >> "$tmpfn"
    else
        printf "%s\n" "$line" >> "$tmpfn"
    fi
    ((n++))

done < "$fn"

exit 0

Answer 2

• If you want robust in-place updating of your input files, use gniourf_gniourf's excellent ed-based answer

• If you have GNU sed and want to in-place updating with multiple files at once, use
  @potong's excellent GNU sed-based answer (see below for a portable alternative, and the bottom for an explanation)

Note: ed truly updates the existing file, whereas sed's -i option creates a temporary file behind the scenes, which then replaces the original - while typically not an issue, this can have undesired side effects, most notably, replacing a symlink with a regular file (by contrast, file permissions are correctly preserved).

Below are POSIX-compliant shell functions that wrap both answers.

---

Stdin/stdout processing, based on @potong's excellent answer:

• POSIX sed doesn't support -i for in-place updating.
• It also doesn't support using \n inside a character class, so [^\n] must be replaced with a cumbersome workaround that positively defines all character except \n that can occur on a line - this is a achieved with a character class combining printable characters with all (ASCII) control characters other than \n included as literals (via a command substitution using printf).
• Also note the need to split the sed script into two -e options, because POSIX sed requires that a branching command (b, in this case) be terminated with either an actual newline or continuation in a separate -e option.

# SYNOPSIS
#   swapLines lineNum1 lineNum2
swapLines() {
  [ "$1" -ge 1 ] || { printf "ARGUMENT ERROR: Line numbers must be decimal integers >= 1.\n" >&2; return 2; }
  [ "$1" -le "$2" ] || { printf "ARGUMENT ERROR: The first line number ($1) must be <= the second ($2).\n" >&2; return 2; }
  sed -e "$1"','"$2"'!b' -e ''"$1"'h;'"$1"'!H;'"$2"'!d;x;s/^\([[:print:]'"$(printf '\001\002\003\004\005\006\007\010\011\013\014\015\016\017\020\021\022\023\024\025\026\027\030\031\032\033\034\035\036\037\177')"']*\)\(.*\n\)\(.*\)/\3\2\1/'
}

Example:

$ printf 'line 1\nline 2\nline 3\n' | swapLines 1 3 
line 3
line 2
line 1

---

In-place updating, based on gniourf_gniourf's excellent answer:

Small caveats:

• While ed is a POSIX utility, it doesn't come preinstalled on all platforms, notably not on Debian and the Cygwin and MSYS Unix-emulation environments for Windows.
• ed always reads the input file as a whole into memory.

# SYNOPSIS
#   swapFileLines lineNum1 lineNum2 file
swapFileLines() {
  [ "$1" -ge 1 ] || { printf "ARGUMENT ERROR: Line numbers must be decimal integers >= 1.\n" >&2; return 2; }
  [ "$1" -le "$2" ] || { printf "ARGUMENT ERROR: The first line number ($1) must be <= the second ($2).\n" >&2; return 2; }
  ed -s "$3" <<EOF
H
$1m$2
$2-m$1-
w
EOF
}

Example:

$ printf 'line 1\nline 2\nline 3\n' > file
$ swapFileLines 1 3 file
$ cat file
line 3
line 2
line 1

---

An explanation of @potong's GNU sed-based answer:

His command swaps lines 10 and 15:

sed -ri '10,15!b;10h;10!H;15!d;x;s/^([^\n]*)(.*\n)(.*)/\3\2\1/' f1 f2 fn

• -r activates support for extended regular expressions; here, notably, it allows use of unescaped parentheses to form capture groups.

• -i specifies that the files specified as operands (f1, f2, fn) be updated in place, without backup, since no optional suffix for a backup file is adjoined to the -i option.

• 10,15!b means that all lines that do not (!) fall into the range of lines 10 through 15 should branch (b) implicitly to the end of the script (given that no target-label name follows b), which means that the following commands are skipped for these lines. Effectively, they are simply printed as is.

• 10h copies (h) line number 10 (the start of the range) to the so-called hold space, which is an auxiliary buffer.
• 10!H appends (H) every line that is not line 10 - which in this case implies lines 11 through 15 - to the hold space.
• 15!d deletes (d) every line that is not line 15 (here, lines 10 through 14) and branches to the end of the script (skips remaining commands). By deleting these lines, they are not printed.
• x, which is executed only for line 15 (the end of the range), replaces the so-called pattern space with the contents of the hold space, which at that point holds all lines in the range (10 through 15); the pattern space is the buffer on which sed commands operate, and whose contents are printed by default (unless -n was specified).
• s/^([^\n]*)(.*\n)(.*)/\3\2\1/ then uses capture groups (parenthesized subexpressions of the regular expression that forms the first argument passed to function s) to partition the contents of the pattern space into the 1st line (^([^\n]*)), the middle lines ((.*\n)), and the last line ((.*)), and then, in the replacement string (the second argument passed to function s), uses backreferences to place the last line (\3) before the middle lines (\2), followed by the first line (\1), effectively swapping the first and last lines in the range. Finally, the modified pattern space is printed.

As you can see, only the range of lines spanning the two lines to swap is held in memory, whereas all other lines are passed through individually, which makes this approach memory-efficient.

Answer 3

With GNU awk:

awk '
FNR==NR {if(FNR==14) x=$0;if(FNR==26) y=$0;next} 
FNR==14 {$0=y} FNR==26 {$0=x} {print}
' file file > file_with_swap

Answer 4

if, you want swap two lines, to create script "swap.sh"

#!/bin/sh
sed -n "1,$((${2}-1))p" "$1"
sed -n "${3}p" "$1"
sed -n "$((${2}+1)),$((${3}-1))p" "$1"
sed -n "${2}p" "$1"
sed -n "$((${3}+1)),\$p" "$1"

next

sh swap.sh infile_name 14 26 > outfile_name

Answer 5

If you want to edit a file, you can use ed, the standard editor. Your task is rather easy in ed:

printf '%s\n' 14m26 26-m14- w q | ed -s file

How does it work?

• 14m26 tells ed to take line #14 and move it after line #26
• 26-m14- tells ed to take the line before line #26 (which is your original line #26) and move it after line preceding line #14 (which is where your line #14 originally was)
• w tells ed to write the file
• q tells ed to quit.

If your numbers are in a variable, you can do:

linea=14
lineb=26
{
    printf '%dm%d\n' "$linea" "$lineb"
    printf '%d-m%d-\n' "$lineb" "$linea"
    printf '%s\n' w q
} | ed -s file

or something similar. Make sure that linea<lineb.

Answer 6

If you want to swap two lines, you can send it through twice, you could make it loop in one sed script if you really wanted, but this works:

e.g.

test.txt: for a in {1..10}; do echo "this is line $a"; done >> test.txt

this is line 1
this is line 2
this is line 3
this is line 4
this is line 5
this is line 6
this is line 7
this is line 8
this is line 9
this is line 10

Then to swap lines 6 and 9:

sed ':a;6,8{6h;6!H;d;ba};9{p;x};' test.txt | sed '7{h;d};9{p;x}'

this is line 1
this is line 2
this is line 3
this is line 4
this is line 5
this is line 9
this is line 7
this is line 8
this is line 6
this is line 10

In the first sed it builds up the hold space with lines 6 through 8. At line 9 it prints line 9 then prints the hold space (lines 6 through 8) this accomplishes the first move of 9 to place 6. Note: 6h; 6!H avoids a new line at the top of the pattern space.

The second move occurs in the second sed script it saves line 7 to the hold space, then deletes it and prints it after line 9.

To make it quasi-generic you can use variables like this: A=3 && B=7 && sed ':a;'${A}','$((${B}-1))'{'${A}'h;'${A}'!H;d;ba};'${B}'{p;x};' test.txt | sed $(($A+1))'{h;d};'${B}'{p;x}'

Where A and B are the lines you want to swap, in this case lines 3 and 7.