Diff for "BashFAQ/089"

Differences between revisions 6 and 17 (spanning 11 versions)

I'm reading a file line by line and running ssh or ffmpeg, only the first line gets processed!

When reading a file line by line, if a command inside the loop also reads stdin, it can exhaust the input file. For example:

# Non-working example
while IFS= read -r file; do
  ffmpeg -i "$file" -vcodec libxvid -acodec libfaac -ar 32000 "${file%.avi}".mkv
done < <(find . -name '*.avi')

# Non-working example
while read host; do
  ssh "$host" some command
done <hostslist

What's happening here? Let's take the first example. read reads a line from standard input (FD 0), puts it in the file parameter, and then ffmpeg is executed. Like any program you execute from BASH, ffmpeg inherits standard input, which for some reason it reads. I don't know why. But in any case, when ffmpeg reads stdin, it sucks up all the input from the find command, starving the loop.

Here's one way to make it work:

while IFS= read -r file; do
  ffmpeg -i "$file" -vcodec libxvid -acodec libfaac -ar 32000 "${file%.avi}".mkv </dev/null
done < <(find . -name '*.avi')

Notice the redirection on the ffmpeg line: </dev/null. The ssh example can be fixed the same way, or with the -n switch (at least with OpenSSH).

Sometimes with large loops it might be difficult to work out what's reading from stdin, or a program might change its behaviour when you add </dev/null to it. In this case you can make read use a different FileDescriptor that a random program is less likely to read from:

while read -r line <&3; do
  ...
done 3<file

-  ⇤ ← Revision 6 as of 2009-03-16 20:31:13 → 
  Size: 1737
  Editor: localhost
  Comment:
+   ← Revision 17 as of 2015-04-08 22:24:06 → ⇥
  Size: 1708
  Editor: geirha
  Comment: read -r
-Deletions are marked like this.
+Additions are marked like this.
 Line 2:
-== I'm using a loop which runs once per line of input but it only seems to run once; everything after the first line is ignored? ==
Typically you'll see this behaviour in situations like these:
+== I'm reading a file line by line and running ssh or ffmpeg, only the first line gets processed! ==
When [[BashFAQ/001|reading a file line by line]], if a command inside the loop also reads stdin, it can exhaust the input file.  For example:
 Line 6:
-  while IFS= read -r file; do
    ffmpeg -i "$file" -vcodec libxvid -acodec libfaac -ar 32000 \
      "${file%.avi}".mkv
  done < <(find . -name '*.avi')
+# Non-working example
while IFS= read -r file; do
  ffmpeg -i "$file" -vcodec libxvid -acodec libfaac -ar 32000 "${file%.avi}".mkv
done < <(find . -name '*.avi')
 Line 13:
-  while read host; do
    ssh "$host" command
  done <hostslist
+# Non-working example
while read host; do
  ssh "$host" some command
done <hostslist
-Line 20:
+Line 21:
-Here's how you make it work:
+Here's one way to make it work:
-Line 22:
+Line 23:
-  while IFS= read -r file; do
    ffmpeg </dev/null -i "$file" -vcodec libxvid -acodec libfaac -ar 32000 \
      "${file%.avi}".mkv
  done < <(find . -name '*.avi')
+while IFS= read -r file; do
  ffmpeg -i "$file" -vcodec libxvid -acodec libfaac -ar 32000 "${file%.avi}".mkv </dev/null
done < <(find . -name '*.avi')
 Line 28:
-Notice the redirection on the ffmpeg line: `</dev/null`.  See the [[BashGuide/TheBasics/InputAndOutput#Redirection|redirection section]] of the BashGuide for more information on this.
+Notice the [[BashGuide/InputAndOutput#Redirection|redirection]] on the ffmpeg line: `</dev/null`. The ssh example can be fixed the same way, or with the `-n` switch (at least with [[http://www.openssh.org/|OpenSSH]]).
 Line 30:
-The ssh example can be fixed the same way, or with the `-n` switch (at least with [[http://www.openssh.org/|OpenSSH]]).

Sometimes with large loops it might be difficult to work out what's reading from stdin; or a program might change its behaviour when you add `</dev/null` to it.  In this case you can make read use a different file descriptor that a random program is less likely to read from:
+Sometimes with large loops it might be difficult to work out what's reading from stdin, or a program might change its behaviour when you add `</dev/null` to it.  In this case you can make read use a different FileDescriptor that a random program is less likely to read from:
-Line 34:
+Line 32:
-  while read <&3 line; do
    ......
  done 3<file
+while read -r line <&3; do
  ...
done 3<file