Mixing
cat and pipe vs. redirection [duplicate]
Clash Royale CLAN TAG#URR8PPP
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6
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This question already has an answer here:
How is this command legal ? “> file1 < file2 catâ€
1 answer
What's the difference between these two commands?
cat /proc/uptime | awk 'print $1'
< /proc/uptime awk 'print $1'
Specifically, how does the second command work? Doesn't the redirection operator < has to be accompanied by a command? What does it mean to redirect the contents of a file like that?
pipe io-redirection cat
edited Sep 2 at 23:27
Joseph Sible
676113
asked Sep 2 at 22:38
walksignison
435
marked as duplicate by Kusalananda, SivaPrasath, Thomas Dickey, Jeff Schaller, Archemar Sep 3 at 13:15
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
add a comment |Â
up vote
6
down vote
favorite
This question already has an answer here:
How is this command legal ? “> file1 < file2 catâ€
1 answer
What's the difference between these two commands?
cat /proc/uptime | awk 'print $1'
< /proc/uptime awk 'print $1'
Specifically, how does the second command work? Doesn't the redirection operator < has to be accompanied by a command? What does it mean to redirect the contents of a file like that?
pipe io-redirection cat
edited Sep 2 at 23:27
Joseph Sible
676113
asked Sep 2 at 22:38
walksignison
435
marked as duplicate by Kusalananda, SivaPrasath, Thomas Dickey, Jeff Schaller, Archemar Sep 3 at 13:15
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
I came across it online and wanted to know how it worked.. there was no explanation backing the usage.
– walksignison
Sep 2 at 22:41
unix.stackexchange.com/questions/131775/…
– walksignison
Sep 2 at 23:20
@Goro I don't see anything wrong with the second command. What's wrong about it?
– Joseph Sible
Sep 2 at 23:24
@JosephSible can you please explain how it works?
– walksignison
Sep 2 at 23:27
3
Reading this comment from Stéphane made me smarter; thought I’d include it here.
– Jeff Schaller
Sep 2 at 23:33
add a comment |Â
up vote
6
down vote
favorite
up vote
6
down vote
favorite
This question already has an answer here:
How is this command legal ? “> file1 < file2 catâ€
1 answer
What's the difference between these two commands?
cat /proc/uptime | awk 'print $1'
< /proc/uptime awk 'print $1'
Specifically, how does the second command work? Doesn't the redirection operator < has to be accompanied by a command? What does it mean to redirect the contents of a file like that?
pipe io-redirection cat
edited Sep 2 at 23:27
Joseph Sible
676113
asked Sep 2 at 22:38
walksignison
435
This question already has an answer here:
How is this command legal ? “> file1 < file2 catâ€
1 answer
What's the difference between these two commands?
cat /proc/uptime | awk 'print $1'
< /proc/uptime awk 'print $1'
Specifically, how does the second command work? Doesn't the redirection operator < has to be accompanied by a command? What does it mean to redirect the contents of a file like that?
This question already has an answer here:
How is this command legal ? “> file1 < file2 catâ€
1 answer
pipe io-redirection cat
edited Sep 2 at 23:27
Joseph Sible
676113
asked Sep 2 at 22:38
walksignison
435
edited Sep 2 at 23:27
Joseph Sible
676113
edited Sep 2 at 23:27
Joseph Sible
676113
edited Sep 2 at 23:27
Joseph Sible
676113
676113
asked Sep 2 at 22:38
walksignison
435
asked Sep 2 at 22:38
walksignison
435
asked Sep 2 at 22:38
walksignison
435
435
marked as duplicate by Kusalananda, SivaPrasath, Thomas Dickey, Jeff Schaller, Archemar Sep 3 at 13:15
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
marked as duplicate by Kusalananda, SivaPrasath, Thomas Dickey, Jeff Schaller, Archemar Sep 3 at 13:15
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
I came across it online and wanted to know how it worked.. there was no explanation backing the usage.
– walksignison
Sep 2 at 22:41
unix.stackexchange.com/questions/131775/…
– walksignison
Sep 2 at 23:20
@Goro I don't see anything wrong with the second command. What's wrong about it?
– Joseph Sible
Sep 2 at 23:24
@JosephSible can you please explain how it works?
– walksignison
Sep 2 at 23:27
3
Reading this comment from Stéphane made me smarter; thought I’d include it here.
– Jeff Schaller
Sep 2 at 23:33
add a comment |Â
I came across it online and wanted to know how it worked.. there was no explanation backing the usage.
– walksignison
Sep 2 at 22:41
unix.stackexchange.com/questions/131775/…
– walksignison
Sep 2 at 23:20
@Goro I don't see anything wrong with the second command. What's wrong about it?
– Joseph Sible
Sep 2 at 23:24
@JosephSible can you please explain how it works?
– walksignison
Sep 2 at 23:27
3
Reading this comment from Stéphane made me smarter; thought I’d include it here.
– Jeff Schaller
Sep 2 at 23:33
I came across it online and wanted to know how it worked.. there was no explanation backing the usage.
– walksignison
Sep 2 at 22:41
I came across it online and wanted to know how it worked.. there was no explanation backing the usage.
– walksignison
Sep 2 at 22:41
unix.stackexchange.com/questions/131775/…
– walksignison
Sep 2 at 23:20
unix.stackexchange.com/questions/131775/…
– walksignison
Sep 2 at 23:20
@Goro I don't see anything wrong with the second command. What's wrong about it?
– Joseph Sible
Sep 2 at 23:24
@Goro I don't see anything wrong with the second command. What's wrong about it?
– Joseph Sible
Sep 2 at 23:24
@JosephSible can you please explain how it works?
– walksignison
Sep 2 at 23:27
@JosephSible can you please explain how it works?
– walksignison
Sep 2 at 23:27
3
3
Reading this comment from Stéphane made me smarter; thought I’d include it here.
– Jeff Schaller
Sep 2 at 23:33
Reading this comment from Stéphane made me smarter; thought I’d include it here.
– Jeff Schaller
Sep 2 at 23:33
add a comment |Â
1 Answer
1
active
oldest
votes
up vote
6
down vote
accepted
In general, foo < bar and < bar foo are equivalent in bash scripting. Any time < filename is processed by the shell, it means that the command it's associated with will have its standard input come from that file. No extra command or process is involved with this; the shell does it itself.
Running cat filename reads the contents of the specified file and writes them to standard output. | between two commands means connect standard output of the left command to standard input of the right command.
Thus, both of your commands have the same effect of sending the contents of /proc/uptime to awk, but the first way starts an extra cat process to do so.
answered Sep 2 at 23:29
Joseph Sible
676113
1
(warning: snark): see also UUOC
– Anthony Sottile
Sep 3 at 3:59
add a comment |Â
1 Answer
1
active
oldest
votes
1 Answer
1
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
6
down vote
accepted
In general, foo < bar and < bar foo are equivalent in bash scripting. Any time < filename is processed by the shell, it means that the command it's associated with will have its standard input come from that file. No extra command or process is involved with this; the shell does it itself.
Running cat filename reads the contents of the specified file and writes them to standard output. | between two commands means connect standard output of the left command to standard input of the right command.
Thus, both of your commands have the same effect of sending the contents of /proc/uptime to awk, but the first way starts an extra cat process to do so.
answered Sep 2 at 23:29
Joseph Sible
676113
1
(warning: snark): see also UUOC
– Anthony Sottile
Sep 3 at 3:59
add a comment |Â
up vote
6
down vote
accepted
In general, foo < bar and < bar foo are equivalent in bash scripting. Any time < filename is processed by the shell, it means that the command it's associated with will have its standard input come from that file. No extra command or process is involved with this; the shell does it itself.
Running cat filename reads the contents of the specified file and writes them to standard output. | between two commands means connect standard output of the left command to standard input of the right command.
Thus, both of your commands have the same effect of sending the contents of /proc/uptime to awk, but the first way starts an extra cat process to do so.
answered Sep 2 at 23:29
Joseph Sible
676113
1
(warning: snark): see also UUOC
– Anthony Sottile
Sep 3 at 3:59
add a comment |Â
up vote
6
down vote
accepted
up vote
6
down vote
accepted
In general, foo < bar and < bar foo are equivalent in bash scripting. Any time < filename is processed by the shell, it means that the command it's associated with will have its standard input come from that file. No extra command or process is involved with this; the shell does it itself.
Running cat filename reads the contents of the specified file and writes them to standard output. | between two commands means connect standard output of the left command to standard input of the right command.
Thus, both of your commands have the same effect of sending the contents of /proc/uptime to awk, but the first way starts an extra cat process to do so.
answered Sep 2 at 23:29
Joseph Sible
676113
In general, foo < bar and < bar foo are equivalent in bash scripting. Any time < filename is processed by the shell, it means that the command it's associated with will have its standard input come from that file. No extra command or process is involved with this; the shell does it itself.
Running cat filename reads the contents of the specified file and writes them to standard output. | between two commands means connect standard output of the left command to standard input of the right command.
Thus, both of your commands have the same effect of sending the contents of /proc/uptime to awk, but the first way starts an extra cat process to do so.
answered Sep 2 at 23:29
Joseph Sible
676113
answered Sep 2 at 23:29
Joseph Sible
676113
answered Sep 2 at 23:29
Joseph Sible
676113
answered Sep 2 at 23:29
Joseph Sible
676113
676113
1
(warning: snark): see also UUOC
– Anthony Sottile
Sep 3 at 3:59
add a comment |Â
1
(warning: snark): see also UUOC
– Anthony Sottile
Sep 3 at 3:59
1
1
(warning: snark): see also UUOC
– Anthony Sottile
Sep 3 at 3:59
(warning: snark): see also UUOC
– Anthony Sottile
Sep 3 at 3:59
add a comment |Â
I came across it online and wanted to know how it worked.. there was no explanation backing the usage.
– walksignison
Sep 2 at 22:41
unix.stackexchange.com/questions/131775/…
– walksignison
Sep 2 at 23:20
@Goro I don't see anything wrong with the second command. What's wrong about it?
– Joseph Sible
Sep 2 at 23:24
@JosephSible can you please explain how it works?
– walksignison
Sep 2 at 23:27
3
Reading this comment from Stéphane made me smarter; thought I’d include it here.
– Jeff Schaller
Sep 2 at 23:33