How to print the first line using grep command?

Question

I have a file called file.txt. How can I print the first line only using the grep command?

Answer 1

Although it's an unconventional application of grep, you can do it in GNU grep using

grep -m1 "" file.txt

It works because the empty expression matches anything, while -m1 causes grep to exit after the first match

-m NUM, --max-count=NUM
       Stop reading a file after NUM matching lines.

Answer 2

This is not something grep does. The name "grep" itself is an acronym for "globally search a regular expression and print", which is what the ed command g/re/p does (for a given regular expression re).

ed is an interactive line editor from 1969, but it's most likely installed on your system today nonetheless (it's a standard POSIX tool). We got grep from ed, and it can be seen as a shortcut or alias for a specific functionality of ed, and sed, which is "stream-ed", i.e. a (non-interactive) stream editor.

Let's use sed instead:

sed -n '1p' file.txt

The 1p string is a tiny sed "script" that prints (p) the line corresponding to the given address (1, the first line). The editing command 1p would (no surprise) do the same thing in the ed editor by the way.

The -n suppresses the output of anything not explicitly printed by the script, so all we get is the first line of the file file.txt.

Alternatively:

sed '1q' file.txt

This prints all lines of the file, but quits (q) at line 1 (after printing it). This is exactly equivalent to head -n 1 file.txt.

In the POSIX standard (in the rationale section for the head command) it says (paraphrasing) that head -n N is much the same as sed 'Nq', i.e. "print every line, but quit at line N". The reason head was included in the standard at all was due to symmetry with tail and backwards compatibility with existing Unix implementations.

Answer 3

Unless the first line has a unique string you cannot do this using only grep. head -n 1 file.txt would work in its place.

If you want to only print out the first line if it matches a pattern then pipe head into grep

head -n 1 * | grep [pattern]

Answer 4

Yet Another Unconventional Use of Grep -- a Schwartzian Transform that goes through several gyrations to number the lines, then uses grep to look for the line number, then strip the line number back off:

function grep1() (
  nlines=$(wc -l < "$1")
  nlw=$(printf "%d" "$nlines" | wc -c)
  nl -d '\n' -ba -n ln -w "$nlw" -s ' ' "$1" | grep '^1 ' | sed 's/^1 *//'
)

function greplast() (
  nlines=$(wc -l < "$1")
  nlines=$((nlines + 0))
  nlw=$(printf "%d" "$nlines" | wc -c)
  nl -d '\n' -ba -n ln -w "$nlw" -s ' ' "$1" | grep "^$nlines " | sed "s/^$nlines *//"
)

I'm putting this Answer here as an example of the idea that just because you can do something in (grep or bash or ... etc), doesn't mean that you should -- there's probably a better tool for the job. sed (sed 1q or sed -n 1p) and head (head -n 1) are the more appropriate tools for printing the first line from a file. For printing the last line of a file, you have tail -n 1 or sed -n '$p'. Not only are these tools a single command (instead of 3+ in the above functions), they are also much clearer for future readers -- perhaps yourself! -- of the scripts they're in. While I am not one of the (currently 3) down-voters of your question, it's likely that your insistence on an arbitrary tool for the job (without any supporting reasons) is the reason for the downvotes. It's extremely unlikely that a system that has grep does not also have head, tail, and sed.

Answer 5

Here is what I used to get a header on my df output. Basically two commands run in sequence with the ";". df -h | head -n1; df -h | grep sda1