Question

My assignment asks that:

Create a directory ~/UnixCourse/scriptAsst. Turn the two-line version,
above, of the substitution commands into a shell script, subst1 taking
three parameters: the string to be replaced the string with which to
replace it the name of the file in which to make the substitution.

For example,

`~/UnixCourse/scriptAsst/subst1 foo bar myFile.txt`

should replace all occurrences of foo in the file myFile.txt by bar, leaving the
original file as myFile.txt.bak.

Similarly,

`~/UnixCourse/scriptAsst/subst1 abc "" aardvark.dat`

should remove (replace by the empty string) all occurrences of abc in the file aardvark.dat with nothing, leaving the original file as aardvark.dat.bak.

My code that I came up with is:

#!/bin/bash

set p1 = "$1"
shift
set p2 = "$1"
shift
set p3 = "$*"

echo $p1
echo $p2
echo $p3

if grep "$p1" "$p3" > /dev/null; then
mv "$p3" "$p3.bak"
sed "s/$p1/$p2/g" "$p3.bak" > "$p3"
fi

When I try to run:

./subst1 foo bar myFile.txt

I keep getting:

grep: : No such file or directory

Please help!! What am I doing wrong??

Answer 1

or use the parameters directly…

#!/bin/bash

echo $1
echo $2
echo $3

if grep "$1" "$3" > /dev/null; then
  mv "$3" "$3.bak"
  sed "s/$1/$2/g" "$3.bak" > "$3"
fi