Bash: variable in file path

Question

I am not understanding how this doesn't work:

ls /sys/class/net/wlan0
addr_assign_type  broadcast  device   flags    iflink     netdev_group  queues
subsystem  uevent  addr_len carrier    dormant  ifalias  link_mode  **operstate**
speed  tx_queue_len address  dev_id  duplex   ifindex  mtu  power statistics  


for f in $(ls /sys/class/net/); do $(cat /sys/class/net/${f}/operstate); done
cat: /sys/class/net/eth0/operstate: No such file or directory
cat: /sys/class/net/lo/operstate: No such file or directory
cat: /sys/class/net/wlan0/operstate: No such file or directory

I guess $f is not expanded in time for cat. I tried with quotes, like "$f", but still it didn't work.

This does work OK:

for f in $(ls /sys/class/net/); do echo /sys/class/net/${f}/operstate; done
/sys/class/net/eth0/operstate
/sys/class/net/lo/operstate
/sys/class/net/wlan0/operstate

And:

cat  /sys/class/net/eth0/operstate 
up

What have I missed? Why can't cat see the files in the loop?

echo doesn't care whether or not its argument exists in the filesystem. — Ignacio Vazquez-Abrams
– Ignacio Vazquez-Abrams, Commented Jul 1, 2013 at 2:04
The file link to other devices (hardware names), but cat does work? — Edouard
– Edouard, Commented Jul 1, 2013 at 2:09

John Kugelman · Accepted Answer · 2013-07-01 02:10:53Z

2

First, you should not be repeating /sys/class/net/ inside the loop since $f will already contain those directories.

for f in $(ls /sys/class/net/); do $(cat ${f}/operstate); done

Next, the $(...) inside the loop needs to go. There's no need to capture cat's output.

for f in $(ls /sys/class/net/); do cat ${f}/operstate; done

Now $(ls ...) can be done away with with a simple wildcard:

for f in /sys/class/net/*; do cat ${f}/operstate; done

Finally, the entire loop can be replaced with a single call to cat.

cat /sys/class/net/*/operstate

answered Jul 1, 2013 at 2:10

John Kugelman

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1 Comment

John Kugelman Over a year ago

@Edouard I honestly don't know. Is that the exact output from your shell? Those don't look like the right error messages. I would have expected cat: /sys/class/net//sys/class/net/eth0/operstate: No such file or directory

DevZer0 · Accepted Answer · 2013-07-01 02:05:57Z

0

Your bash script is trying to cat file called operstate in each subdirectory of /sys/class/net, and the file is not found. In the second example your simply echoing a string which prints out just fine.

answered Jul 1, 2013 at 2:05

DevZer0

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