Shell function to show line in file with context

Today I wrote a shell function to display lines in a file, with optionally some surrounding lines for context. This can come in handy when looking at output like stack traces including files and line numbers and you want to see what’s in that file on that line.

Built-in IDE terminals often feature clickable links, this line function is for when you don’t have that luxury.

Usage

$ line
Usage: line <file> <line_number> [lines_around=0]
       line <file:line_number[:column]>
       cat <file> | line <line_number> [lines_around=0]

The following examples are all equivalent and show line number 10 of file.txt with 2 extra lines before and after:

$ line file.txt 10 2
$ line file.txt:10:5 2
$ cat file.txt | line 10 2

The default context is 0 lines.

Script

Add this function somewhere in your $HOME/.bash_profile (or equivalent):

line() {
  local FILE LINE_NUMBER LINES_AROUND=0
  local NAME="${FUNCNAME[0]}"

  if [[ ! -t 0 ]]; then
    LINE_NUMBER=$1
    LINES_AROUND=${2:-$LINES_AROUND}
  elif [[ $1 =~ ^([^:]+):([0-9]+)(:[0-9]+)?$ ]]; then
    FILE="${BASH_REMATCH[1]}"
    LINE_NUMBER="${BASH_REMATCH[2]}"
    LINES_AROUND=${2:-$LINES_AROUND}
  else
    FILE=$1
    LINE_NUMBER=$2
    LINES_AROUND=${3:-$LINES_AROUND}
  fi

  if [[ -t 0 && -z "$FILE" || -z "$LINE_NUMBER" ]]; then
    echo "Usage: ${NAME} <file> <line_number> [lines_around=0]
       ${NAME} <file:line_number[:column]>
       cat <file> | ${NAME} <line_number> [lines_around=0]"
    return 1
  fi

  if [[ -t 0 && -n "$FILE" && ! -f "$FILE" ]]; then
    echo "${NAME}: $FILE: No such file or directory"
    return 1
  fi

  sed -n "`expr $LINE_NUMBER - $LINES_AROUND`,`expr $LINE_NUMBER + $LINES_AROUND`p" ${FILE}
}

TIL

What I found interesting:

1. Use -t 0 to check if stdin (file descriptor 0) is connected to a terminal, and if negative it means non-interactive and thus piped (or redirected, tbh I haven’t dug into that)

2. BASH_REMATCH is the equivalent of, for example, this in JavaScript:

   const BASH_REMATCH = str.match(regex);

3. FUNCNAME[0] holds the function name of itself

4. That sed always seems to have a new trick up its sleeve. The result in the script with sed + expr above is, for example, sed -n "8,12p" which shows lines 8-12 of the input.

In closing

Obviously I’ve added line to my dotfiles!