Grep lines but let the first line through

Question

So,

I'm trying Facebook.api("/me/friends?fields=installed",handleFunc); with no joy, I get a bad request error in Firebug.

However, I have no problem getting just a list of friends: Facebook.api("/me/friends", handleFriendsLoad);

I know there is a FQL way of doing this. But, why does the above not work for me? If FQL is the only way of doing this, what is the code needed?

Many thanks

Solution 1

I would use a slightly more sophisticated approach than simple grep:

1. awk

   df -h | awk 'NR==1 || /^\/dev/'
   

   NR is the current line number so the awk scriptlet above will print if this is the first line or if the current line begins with /dev. And after posting this I see it is the same as @1_CR's answer. Oh well...

2. Perl

   df -h | perl -ne 'print if (/^\/dev/ || $.==1)'
   

   The same idea here, in Perl the special variable $. is the current line number. An alternative way would be

   df -h | perl -pe '$_="" unless /^\/dev/ || $.==1'
   

   The -p switch will print all lines of the input file. The value of the current line is held in $_ so we set $_ to empty unless we want the current line.

3. sed

   df -h | sed -n '1p; /^\/dev/p'
   

   The -n suppresses normal output so no lines are printed. The 1p means print the first line and the /^\/dev/p means print any line that starts with /dev.

   As pointed out in the comments below, in the unlikely case where the locale on your current system causes the header line to start with /dev, the command above will print it twice. Stephane Chazelas points out that this one will not have that problem:

   df -h | sed  -e 1b -e '/^\/dev/!d'
   

4. grep

   df -h | grep -E '^(/dev|File)'
   

   This might not be portable because of LOCALE problems as you said. However, I am reasonably certain that no locale or df version will give a path in the first line, so searching for lines that contain no / should also work:

   df -h | grep -E '^[^/]*$|^/dev'
   

Solution 2

You are going to run into issues attempting to parse df output, however for simple cases, the following may work

LC_ALL=C df -P | awk 'NR == 1 || /^\/dev/' 

Solution 3

This should work as well (not tested, though):

df | (read a; echo "$a"; grep /dev)

or

df | (head -n 1; grep ^/dev)

Solution 4

df -h | tee >(head -1) >(sleep 0.5;grep ^/dev) > /dev/null;sleep 1.0

Solution 5

df | head -n 1; df | grep ^/dev

View more solutions

Comments

[Totor]

locale dependant maybe, but your can force the language with LANG=C df.

[terdon]

@Totor see 1_CR's answer, that's exactly what he suggested.

[Mikel]

This has been answered several times previously. See e.g. body from unix.stackexchange.com/a/11859/3169

[Jashwant]

Mark as answer if it helps you :)

[Stéphane Chazelas]

Your sed one would print the 1st line twice if it started with /dev. Unlikely, but sed -e 1b -e '/^\/dev/!d' wouldn't have the problem.

[Keith Thompson]

I just tried that; it printed the header line first.

[Keith Thompson]

Not bad, but it has the disadvantage that it invokes df twice.

[Keith Thompson]

Not particularly, but there are solutions like terdon's df -h | sed -e 1b -e '/^\/dev/!d' that only invoke df once. The df command can hang in some circumstances; in that case, invoking it just once is probably better.

[l0b0]

@KeithThompson Why is that a problem? The accepted answers also print the header line first.

[Keith Thompson]

@l0b0: Sorry, I mistyped; it printed the header line last.

[terdon]

Nice trick but it will not work as expected if you are mounting by UUID instead of device name. The OP wants df style output which means /dev/foo your command will return /dev/disk/by-uuid/ format output unless fstab contains the /dev name.

[mahendra yadav]

Worked correcly on my machine. Try df -h | tee >(head -1) >(sleep 0.5;grep ^/dev) > /dev/null;sleep 1.0

[Chaim Geretz]

very nice. ^/dev will limit this to lines that start with /dev. Wasn't able to do that as an an edit since I was short 9 characters

[derobert]

OP has said the heading is locale-dependent, so you can't match it like that. And what is the sda1 doing at the end?

[LovelyI]

The mount command uses /etc/mtab, which is updated dynamically, and while I am not sure where df gets its data from, I suspect that on Linux it reads /proc/mounts. Either way I don't understand how fstab is relevant and could not reproduce the problem with UUID-mounted disks. Can you elaborate on it?

[derobert]

@syntaxerror the heading is locale-dependent, as is sorting. So that won't work either, for the same reason. Try LANGUAGE=ja df -h, for example. Or Spanish, or French, or many others.

[mahendra yadav]

Anonymous edited my post and it does not work for me. df -h | tee >(head -n 1) | grep ^/dev

[steveayre]

First one works, but you should echo "$a" (note quotes) or any spacing in the header will shrink down to single spaces (eg with column aligned output). Second one does not.

[bnikhil]

@steveayre Thanks for the suggestion. Integrated it into answer.

[pabouk - Ukraine stay strong]

@steveayre even echo "$a" is not completely safe (possible different interpretation of - or \ ). It is always better to use printf %s\\n "$a" to print a string which could possibly contain the mentioned characters.