Error when running script in bash

Question

I wrote a script in bash to basically create aliases, but for any reason, it doesn't work in some cases. Here it's the code:

#!/bin/bash
#Script to create unique and permanent aliases

_add_alias()
{
    echo "alias $1='$2'" | sudo tee -a $HOME/.bash_aliases
    echo "Alias successfully created"
}

if [ `cat $HOME/.bash_aliases | grep "alias $1=" | wc -l` == 0 ]
    then
    if [ $# -eq 2 ]
        then
        if [ -f "$2" ] #This condition is always false, don't know why.
            then
            _add_alias $1 "$2"
        elif $2 > tmp.txt 2> tmp.txt
            then
            _add_alias $1 "$2"
        else
            echo "Wrong command"
        fi
        rm tmp.txt
    elif [ $# -gt 2 ]
        then
    echo "Error: Usage: addal alias_name 'command' (comand between quotes)"
    elif [ $# -lt 2 ]
        then
        echo "Wrong number of parameters"
    fi
else
    echo "Alias name already exists"
fi

I hope you could help me fix the script.

Answer 1

Here you're a corrected and improved script:

#!/bin/bash

###################################################
## Script to create permanent and unique aliases ##
###################################################

## alias is equal to the first argument.
alias="$1"

## command is equal to an array holding all
## the positional parameters minus the first.
## (i.e. for "ls -la" it would be ["ls, "-la"]).
command=("${@:2}")

## If .bash_aliases exists, load it. Now the alias check
## will take in account any alias not sourced yet on the current shell.
test -f ~/.bash_aliases && source $_

## If there are less than 2 parameters, prints the usage and exits.
if [ $# -lt 2 ]; then
    echo "Usage: $0 <alias_name> <command>"
    exit 1
fi

## If the command is not valid, prints an error and exits.
if ! type "${command[0]}" &> /dev/null; then
    echo "Wrong command"
    exit 3
fi

## If the current alias already exists, prints a error message and then exits.
if [[ "$(type -t '$alias')" == "alias" ]]; then
    echo "This alias alredy exists"
    exit 2
fi

## Append the alias to the .bash_aliases file.
echo "alias $alias='${command[@]}'" >> ~/.bash_aliases

## Print a sucess message.
echo "Alias '$alias' linked to the command '${command[@]}'!" 

Things that I changed:

• Commented code.
• Used type to detemine if the alias already exists.
• Removed the function because I think that is not necessary.
• Used type to detemine if a command is valid or not.
• Removed some unnecessary elif.
• Improved the conditionals.
• command may be unquoted (i.e. myscript pingoogle ping google.com)

Answer 2

Here are your errors (source):

• SC2002 Useless cat. Consider 'cmd < file | ..' or 'cmd file | ..' instead.
• SC2006 Use $(..) instead of legacy `..`.
• SC2046 Quote this to prevent word splitting.
• SC2086 Double quote to prevent globbing and word splitting.
• SC2126 Consider using grep -c instead of grep|wc.

---

   1  #!/bin/bash
   2  #Script to create unique and permanent aliases
   3  
   4  _add_alias()
   5  {
   6      echo "alias $1='$2'" | sudo tee -a $HOME/.bash_aliases
                                             ^––SC2086 Double quote to prevent globbing and word splitting.
   7      echo "Alias successfully created"
   8  }
   9  
  10  if [ `cat $HOME/.bash_aliases | grep "alias $1=" | wc -l` == 0 ]
           ^––SC2046 Quote this to prevent word splitting.
           ^––SC2006 Use $(..) instead of legacy `..`.
                ^––SC2086 Double quote to prevent globbing and word splitting.
                ^––SC2002 Useless cat. Consider 'cmd < file | ..' or 'cmd file | ..' instead.
                                      ^––SC2126 Consider using grep -c instead of grep|wc.
  11      then
  12      if [ $# -eq 2 ]
  13          then
  14          if [ -f "$2" ] #This condition is always false, don't know why.
  15              then
  16              _add_alias $1 "$2"
                             ^––SC2086 Double quote to prevent globbing and word splitting.
  17          elif $2 > tmp.txt 2> tmp.txt
  18              then
  19              _add_alias $1 "$2"
                             ^––SC2086 Double quote to prevent globbing and word splitting.
  20          else
  21              echo "Wrong command"
  22          fi
  23          rm tmp.txt
  24      elif [ $# -gt 2 ]
  25          then
  26      echo "Error: Usage: addal alias_name 'command' (comand between quotes)"
  27      elif [ $# -lt 2 ]
  28          then
  29          echo "Wrong number of parameters"
  30      fi
  31  else
  32      echo "Alias name already exists"
  33  fi

Answer 3

Some thoughts:

myscript alias1 "$HOME/dir/ascript"

will evaluate in test part to

[ -f '/home/me/dir/ascript' ]

which may be true or false.

myscript alias2 '$HOME/dir/anotherscript'

will evaluate in test part to

[ -f '$HOME/dir/ascript' ]

which is always false (unless you have a dir named $HOME in local dir).

and

myscript alias3 "$HOME/dir/sample.sh foo bar"

will evaluate in test part to

[ -f '/home/me/dir/sample.sh foo bar' ]

which is false, unless there is a 'sample.sh foo bar' file in /home/me/dir.

and you should fix .bash_aliases and use

echo "alias $1='$2'" >> $HOME/.bash_aliases