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redirect
cat
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🔍 See Original Answer on Ask Ubuntu ⧉ 🔗
URL:
https://askubuntu.com/q/1190733
Title:
What is the difference between "cat < filename" and "cat filename"?
ID:
/2019/11/22/What-is-the-difference-between-_cat-_-filename_-and-_cat-filename__
Created:
November 22, 2019
Edited: December 12, 2019
Upload:
April 8, 2024
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One Big Difference
One big difference is with the *, ?, or [ globbing characters (wildcards) or anything else the shell may expand into multiple filenames. Anything the shell expands into two or more items, rather than treating as a single filename, cannot be opened for redirection.
Without redirection (ie no <), the shell passes multiple filenames to cat, which outputs the files’ contents one after another. For example this works:
$ ls hello?.py
hello1.py hello2.py
$ cat hello?.py
# Output for two files 'hello1.py' and 'hello2.py' appear on your screen
But with redirection (<) an error message occurs:
$ ls < hello?.py
bash: hello?.py: ambiguous redirect
$ cat < hello?.py
bash: hello?.py: ambiguous redirect
One Tiny Difference
I thought with redirection it would be slower but there is no perceivable time difference:
$ time for f in * ; do cat "$f" > /dev/null ; done
real 0m3.399s
user 0m0.130s
sys 0m1.940s
$ time for f in * ; do cat < "$f" > /dev/null ; done
real 0m3.430s
user 0m0.100s
sys 0m2.043s
Notes:
- The difference is about 1/1000th (1 one thousandth) of a second in this test. In other tests it was 1/100th of a second which is still can’t be noticed.
- Alternate the tests a few times so data is cached into RAM as much as possible and more consistent comparison times are returned. Another option is to drop all caches before each test.