Sstrhsrtj

I was trying to replace the first word in a line with the last word in a line using a sed command i=but i can't seem to figure it out.

Using awk:

awk '{ t=$1; $1=$NF; $NF=t; print}'

This will:

• 
  t=$1 - set t to the first word


• 
  $1=$NF - set the first word to the last word


• 
  $NF=t - set the last word to the first word


• 
  print - print the new line.

$ echo 'one two three four five six' | awk '{ f=$1; l=$NF; $1=l; $NF=f; print}'

six two three four five one

Using Perl, and assuming whitespace delimited input and space-delimited output:

perl -ape '($F[0],$F[-1])=($F[-1],$F[0]);$_="@Fn"'

Testure:

$ printf 'Cleanse Fold and Manipulaten' | perl -ape '($F[0],$F[-1])=($F[-1],$F[0]);$_="@Fn"'

Manipulate Fold and Cleanse

The Perl code, using -a to split the input on whitespace into the array @F, simply swaps the two elements at the start and end of that array before joining the resulting list with spaces, adding a newline at the end.

A shorter Perl variant that matches the first and last words and swaps them in a substitution (this assumes that there are no flanking whitespace in the input though):

perl -pe 's/^(w*)(.*?)(w*)$/$3$2$1/'

The middle bit, .*?, matches the middle of the string non-greedily. We couldn't have done this this easily with sed as there is no non-greedy modifier like that ? after .*.

Semi-serious answer: it's not you, you are fine. The problem is totally in sed's s/// command's verbosity (compare this with the alternative answers):

$ echo "Hello some good world!" |

sed 's/(^[^[:space:]]+)([[:space:]].*[[:space:]]|[[:space:]]+)([^[:space:]]+$)/321/'

world! some good Hello

We may also want to swap the first and the last words even if we have space characters before the first and/or after the latter (thanks to comments and other answers):

$ echo "  Hello some   good world!  " |

sed 's/^([[:space:]]*)([^[:space:]]+)([[:space:]].*[[:space:]]|[[:space:]]+)([^[:space:]]+)([[:space:]]*)$/14325/'

  world! some   good Hello  

However, these commands use some non-POSIX GNU extensions to the BRE - Basic Regular Expression - syntax (namely, + and |).

A (more portable) command that satisfies the POSIX standard while keeping the convenience of alternation (|) would require Extended regular expressions. For example, using GNU sed with the --posix option (which disables GNU extensions):

$ echo "  Hello some   good world!  " |

sed --posix -E 's/^([[:space:]]*)([^[:space:]]{1,})([[:space:]].*[[:space:]]|[[:space:]]{1,})([^[:space:]]{1,})([[:space:]]*)$/14325/'

  world! some   good Hello