Linux

Linux commands. From the book of A Practical Guild to Linux Commands, Editors and Shells.

stdio

File descriptor 0 (stdin), 1 (stdout), 2 (stderr).

• > is short for 1>, meaning redirecting standard output.
• < is short for 0< redirects standard input.
• 2> redirects stardard error.

Example

$ cat y
This is y

$ cat x
cat: x: No such file or directory

$ cat x y
This is y
cat: x: No such file or directory

pipe only sends stdout not stderr

$ cat x y | tr "[a-z]" "[A-Z]"
cat: x: No such file or directory
THIS IS Y

In the next example, 1> redirects stdout to hold, then 2>&1 declares file descriptor 2 to be a duplicate of file descriptor 1, so both stdout and stderr are redirected to hold. Notice that 1> should happen first.

$ cat x y 1> hold 2>&1
$ cat hold
cat: x: No such file or directory
This is y

In the next example, 2>&1 means also send stderr to stdout. Then pipes stdout to tr

$ cat x y 2>&1 | tr "[a-z]" "[A-Z]"
CAT: X: NO SUCH FILE OR DIRECTORY
THIS IS Y

2>&1 | can be shortened as |&

pipe | - tunnels the output (stdout only) of left to right.

Example:

$ cat abstract
cab

$ cat abstract | tr abc ABC
CAB

$ tr abc ABC < abstract
CAB

&& and ||, cmd1 && cmd2 means run cmd2 if cmd1 succeeds. cmd1 || cmd2 means run cmd2 if cmd1 fails.

adding & means running the command in the background.

kill can kill a job with PID or job number.

$ ps

11829 pts/10

$ kill 11829

$ jobs
[1]

$ kill %1

Bash

./whoson the ./ tells the shell to look for an executable file in the working dir.

#!/bin/bash tells the system which shell to use. #! is called hashbang or shebang

-e will cause bash to exit when command fails. -u will cause bash to display a message and exit when it tries to expand an unset variable.

d & e & f will run d and e in the background and run f in the foreground.

[1]- Done  d
[2]+ Done  e

The + means it's the last job. - means it's the job before the last one.