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I'm trying to write a simple Linux make command with bash scripting. here is what I have written so far:

#!/usr/bin/env bash

function make_cmd()
{
  read target colon sources
  for src in $sources; do
    if [ $src -nt $target ]; then
      while read cmd && [[ $(echo "$cmd" | grep "$(printf '\t')"*) ]]; do
        echo "executing $cmd";
        eval ${cmd#$(printf '\t')};
      done
      break;
    fi
  done
}

this is the format of input:

target.file : source.file
[tab]command

for example:

target.txt : source.txt
    ls
    cd

the script works well but it cannot find commands starting with tab. it always execute them. for example, the commands in this input is still executed.

target.txt : source.txt
ls
cd

how can I fix this?

Improve this question
2
  • The problem is the use of read, which splits on whitespace i.e. including tabs. Perhaps do IFS=' ' at the beginning to let only a space separate words. Commented Dec 5, 2019 at 8:06
  • 1
    See also: Understanding "IFS= read -r line" Commented Dec 5, 2019 at 8:32

1 Answer 1

Reset to default
1

The builtin read command splits words using the value of IFS, which by default contains a space, a tab and a newline. Hence when using read to get the input, the tab is removed.

Begin the function with:

IFS_SAVE="$IFS"
IFS=' '

Now only spaces will separate words. At the end of the function restore IFS to its original value:

IFS="$IFS_SAVE"

Note that you can use a literal tab if escaped by a backslash. Furthermore I would not use grep to match the tab, use builtins where possible as that's faster. My version of the function would be:

function make_cmd()
{
  SAVE_IFS="$IFS"
  IFS=' '
  read target colon sources
  for src in $sources; do
    if [ $src -nt $target ]; then
      while read cmd; do
        case "$cmd" in
          $'\t'*)  echo "executing $cmd"
                eval ${cmd# }
                ;;
          *)    ;;
        esac
      done
      break;
    fi
  done
  IFS="$SAVE_IFS"
}

The $'\t' substitutes a literal tab (thanks to Kusalananda for the tip).

Insert a literal tab after the # in the variable substitution. Using the printf might be more readable though.

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6
  • I added the lines you said to my code, still doesn't work. your code does not execute the cmd no matter how it is entered. Commented Dec 5, 2019 at 8:59
  • 1
    If you run cat -t filename on your script, does it show the tabs in the code as ^I? E.g. ^I\^I*)^Iecho "executing $cmd". If not, your editor isn't inserting the tabs as-is. Commented Dec 5, 2019 at 9:50
  • yes your right. it doesn't insert tabs. I will fix this and test everything again Commented Dec 5, 2019 at 10:08
  • @FatemehKarimi You may also be able to use $'\t'*) in the case statement. Commented Dec 5, 2019 at 10:54
  • @Kusalananda thanks my dear friend. I replaced it with $'\t'*) and it solved my problem. Commented Dec 6, 2019 at 18:49

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