What's the best way to edit a text file record using shell?

I have data file that stores records like this:

make=honda|model=civic|color=red
make=toyota|model=corolla|color=blue

What would be the best way to detect (based on the make field) whether a given make exists in th开发者_如何学Goe file and then replace it with a new record? (Using shell script)

For your purposes, the best solution is probably either the stream editor or awk.

Here is a script that uses a sed command that will do it:

filename='cars'
make='toyota'
replacement='make=nissan|model=sentra|color=green'
sed "s/^make=$make.*/$replacement/" $filename

There are a couple of problems with icarus127's answer which I have fixed and addressed here:

filename='cars'
saveIFS="$IFS"
IFS=$'\n'
# no need to call external cat, make the array at the same time the file is read
lines=($(<$filename))
# IMO, it's better and makes a difference to save and restore IFS instead of unsetting it
IFS="$saveIFS"

make='toyota'
replacement='make=nissan|model=sentra|color=green'

# the array variable MUST be quoted here or it won't work correctly
for line in "${lines[@]}"
do
    # you can use Bash's regex matching to avoid repeatedly calling
    # multiple external programs in a loop
    if [[ $line =~ ^make=$make ]]
    then
        echo "$replacement"
    else
        echo "$line"
    fi    
done

However, that (and the cat version) reads the whole file into an array, which could be a problem if it's large. It's better to use read in a while loop with a redirect:

filename='cars'

make='toyota'
replacement='make=nissan|model=sentra|color=green'

while read -r line
do
    # you can use Bash's regex matching to avoid repeatedly calling
    # multiple external programs in a loop
    if [[ $line =~ ^make=$make ]]
    then
        echo "$replacement"
    else
        echo "$line"
    fi    
done < "$filename"

This should do it in bash.

#get the file and create an array separated by new lines
lines=$(cat $filename)
IFS=$'\n'
lines=( $lines )
unset IFS


for line in ${lines[@]}
do
    #Get the value of make from the current line
    make=$(echo "$line" | cut -d "|" -f1 | cut -d "=" -f2)

    if [[ "$make" == "$makeImLookingFor" ]]
    then
        echo "modified line"
    else
        echo "$line"
    fi
done

by sed this is is a way

contents of temp.txt make=honda|model=civic|color=red make=toyota|model=corolla|color=blue

new_rec="make=honda|model=civic|color=blue"

$ sed -e "s/^make=honda.*/$new_rec/g" temp.txt make=honda|model=civic|color=blue make=toyota|model=corolla|color=blue

hope this helps