Program to change filenames [SOLVED]

[Postman]

I'm trying to make a small program that will take files as arguments when starting the program then swap contents of two files (or swap filenames, either one is good..) but it also need to check if the two variables are indeed files before doing the swap. However since I'm extremely new with programming Linux I get stuck alot and this is one I haven't managed to solve and I would be glad if someone could point me in the right direction...

 

Here's the code

#! /bin/bash

if [ -f $1 && -f $2 ]
then
mv $1 temp
mv $2 $1
mv temp $2
rm temp
echo "Files has been swapped"
else
echo "One or both variables isn't a file"
fi

 

I have set exec-rights but when I execute I get an error saying

line 3: [: missing `] '

 

What is wrong with this picture?

Edited December 2, 2004 by Postman

[Steve Scrimpshire]

#! /bin/bash

if [ -f $1 ] && [ -f $2 ]
  then
    mv $1 temp.bak
    mv $2 $1
    mv temp.bak $2
    echo "Files has been swapped"
 else
   echo "One or both variables isn't a file"
fi

 

you also don't need the 'rm temp' because mv removes it.

Edited December 2, 2004 by Steve Scrimpshire

[Postman]

Many thanks!

[aru]

Just another way of doing the same.

See small differences just to show you other alternatives

 

 

#! /bin/bash

 

TMPFILE=$(mktemp -q ~/tmp/$0.XXXXXX)

 

[ $# != 2 ] && echo >&2 "Usage: $0 file1 file2" && exit 1

 

if [[ -f $1 && -f $2 ]]; then

    cat $1 > $TMPFILE

    cat $2 > $1

    cat $TMPFILE > $2

    echo "File contents have been swapped"

else

    echo "One or both parameter(s) isn't a file" && exit 1

fi

 

Edited December 2, 2004 by aru

[theYinYeti]

Very interesting aru! I did not know of this mktemp thing.

One small detail: it usually is better to quote filenames:

cat "$1" >"$TMPFILE"

 

For the record, I seem to remember another technique for not using more space on disk. It's not always better (not here for example if you use Steve's suggestion), but still good to know:

local HERETMP="$(dirname "$1")/.swap.tmp"
ln "$1" "$HERETMP"
mv -f "$2" "$1"
mv "$HERETMP" "$2"

This requires, however, that $1 and $2 be files, and on the same filesystem (two limitations of hard links).

 

Yves.

[aru]

One small detail: it usually is better to quote filenames:

cat "$1" >"$TMPFILE"

I agree 100%, it is just that I trust always my filenames :). The only 'but' is when any kind of 'shell expansions' is involved such tilde expansions, but since it doesn't apply here, using quotes is always safer than doesn't and should be taken as a general rule.

 

For the record, I seem to remember another technique for not using more space on disk. [...]

Sure, using hardlinks here is clever. And that has remember me a very old tip published in the old board about "ln":

 

how 'ln' can make a better understanding of Unix

 

It has been nice to read it again,thanks

 

 

Edited: corrected typos

Edited December 2, 2004 by aru

[aru]

#! /bin/bash

[ $# != 2 ] && echo >&2 "Usage: $0 file1 file2" && exit 1

if [[ -f "$1" && -f "$2" ]]; then

    HAM=$(< "$1"); cat "$2" > "$1"; echo "$HAM" > "$2"

    echo "File contents have been swapped"

else

    echo >&2 "One or both parameter(s) isnt a file" && exit 1

fi

Nothing new under the sun, not clever indeed, but yet another way, this time using a variable to hold temporary the contents of file1

...very bored this morning...