Simple shell script

Todd88 · 07-05-2008, 10:49 AM

I have a current code working(named subst1) having a user be able to type this line to substitute words using the sed command:

Code:

subst1 old-pattern new-pattern filename

Here is my shell script:

Code:

#!/bin/bash
# subst1

ARGS=3
E_BADARGS=65

if [ $# -ne "$ARGS" ]
then
  echo "Usage: `basename $0` old-pattern new-pattern filename"
  exit $E_BADARGS
fi

old_pattern=$1
new_pattern=$2

if [ -f "$3" ]
then
    file_name=$3
else
    echo "File \"$3\" does not exist."
    exit $E_BADARGS
fi

mv $file_name $file_name.bak
sed -e "s/$old_pattern/$new_pattern/g" $file_name.bak > $file_name

exit 0

The problem with this script is that it changes the date and creates a bak file. I need to alter this script so that when a user inputs the command line, the script checks if the given old-pattern is in the file and if it is, go ahead and do the substitution and create the bak file. If the old-pattern is not found, the script should do nothing. Any help would be appreciated!

colucix · 07-05-2008, 11:00 AM

You can previously look for the pattern using grep. The last part of the script could be:

Code:

if [ -f "$3" ]
then
    file_name=$3
else
    echo "File \"$3\" does not exist."
    exit $E_BADARGS
fi

if grep -q $old_pattern $file_name
then
    mv $file_name $file_name.bak
    sed -e "s/$old_pattern/$new_pattern/g" $file_name.bak > $file_name
else
    echo "Pattern $old_pattern not found in $file_name"
    echo "Nothing to do, here!
fi

Also consider the -i option of sed to edit the file in place: once you've safely created the backup file, you can simply do

Code:

cp -p $file_name $file_name.bak
sed -i "s/$old_pattern/$new_pattern/g" $file_name

Todd88 · 07-05-2008, 11:17 AM

Thank you so much for this! I knew it was an easy grep command but I couldn't seem to get why it was working. If you could help with just one more thing, I would love you for the rest of my life haha. I need to use this script and alter it so that it lets you change ANY number of files at once by just typing:

Code:

subst1 old-pattern new-pattern file file file file

where file can be any file and number of times.

colucix · 07-05-2008, 11:52 AM

In short, you can do something like this:

Code:

count=3
while [ $count -le $# ]
do
  eval sed -i "s/$1/$2/g" \$$count
  ((count++))
done

The eval before the sed command permits indirect reference to the positional parameter: $3, $4, $5 and so on... where 3, 4, 5... is the value of the variable count. Your script will become something like (again the last part only)

Code:

count=3
while [ $count -le $# ]
do
  eval file_name=\$$count
  if [ ! -f "$file_name" ]
  then
    echo "File \"$file_name\" does not exist."
    exit $E_BADARGS
  fi

  if grep -q $old_pattern $file_name
  then
    cp -p $file_name $file_name.bak
    sed -i "s/$old_pattern/$new_pattern/g" $file_name
  else
    echo "Pattern $old_pattern not found in $file_name"
  fi

  ((count++))
done

Quote:

I would love you for the rest of my life

Never mind. Save your love for a beautiful girl!

colucix · 07-05-2008, 12:00 PM

Just out of curiosity: why do you need to wrap the sed substitution in a shell script? Can't you instruct the users to do a sed command? It permits editing of multiple files as well:

Code:

sed -i 's/old_pattern/new_pattern/g' file1 file2 file3 file4 file5

Todd88 · 07-05-2008, 12:17 PM

When I tried this...I just get the output statement saying the way to use the shell script. This is what I see when testing it:

Code:

Usage: subst old-pattern new-pattern filename

Here is the code:

Code:

#!/bin/bash

# subst

ARGS=3
E_BADARGS=65

if [ $# -ne "$ARGS" ]
then
    echo "Usage: `basename $0` old-pattern new-pattern filename"
    exit $E_BADARGS
fi

old_pattern=$1
new_pattern=$2

if [ -f "$3" ]
then
    file_name=$3
else
    echo "File \"$3\" does not exist."
    exit $E_BADARGS
fi

count=3
while [ $count -le $# ]
do
  eval file_name=\$$count
  if [ ! -f "$file_name" ]
  then
    echo "File \"$file_name\" does not exist."
    exit $E_BADARGS
  fi

if grep -q $old_pattern $file_name
then
    mv $file_name $file_name.bak
    sed -e "s/$old_pattern/$new_pattern/g" $file_name.bak > $file_name
else
    echo "String \"$1\" does not exist."
    exit $E_BADARGS
fi

   ((count++))
done

Todd88 · 07-05-2008, 12:18 PM

Quote:

Originally Posted by colucix

Just out of curiosity: why do you need to wrap the sed substitution in a shell script? Can't you instruct the users to do a sed command? It permits editing of multiple files as well:

Code:

sed -i 's/old_pattern/new_pattern/g' file1 file2 file3 file4 file5

I know it would just be easier to have the users do the sed command but my colleague thinks that this command is used so often that it should be written into a shell script. bleh

Todd88 · 07-05-2008, 12:30 PM

also -i option for sed is invalid

colucix · 07-05-2008, 12:46 PM

Quote:

Originally Posted by Todd88

When I tried this...I just get the output statement saying the way to use the shell script.

You have to strip out the check for 3 arguments if you want an unlimited number of them.

Quote:

Originally Posted by Todd88

also -i option for sed is invalid

The -i option is for GNU sed only. Are you running on Solaris or something? Anyway, you can stick with your original version, editing the backup file and redirecting output to the original file name.

Todd88 · 07-05-2008, 12:53 PM

ah geez. Of course, thanks again. You saved me!

smoked kipper · 07-06-2008, 04:33 PM

It's generally easier to use a for loop rather than counting the command line arguments manually,

Code:

for file in $@; do
 ...
done

You'll need a

Code:

shift 2

after you assign the old/new patterns from $1 and $2, this will eat the patterns so all remaining arguments are filenames.

Also, it's a good idea to quote variables since most shells are retarded and will split them automatically if they contain whitespace.

Lastly, instead of exiting if a file or pattern doesn't exist, it would be better to continue after printing the error message (might be irritating if a user specified 200 files and the script exits at file #3 because of a typo).

Code:

#!/bin/bash

# subst

E_BADARGS=65

# need 2 patterns and at least 1 filename
if [ $# -lt 3 ]
then
    echo "Usage: `basename $0` old-pattern new-pattern filename [filename...]"
    exit $E_BADARGS
fi

old_pattern="$1"
new_pattern="$2"
shift 2

for file_name in "$@"; do
    if [ ! -f "$file_name" ]
    then
        echo "File \"$file_name\" does not exist."
        continue
    fi

    if grep -q "$old_pattern" "$file_name"
    then
        mv "$file_name" "${file_name}.bak"
        sed -e "s/$old_pattern/$new_pattern/g" "${file_name}.bak" > "$file_name"
    else
        echo "String \"$1\" does not exist in $filename."
    fi
done