Alex's Slip-box

These are my org-mode notes in sort of Zettelkasten style

Shell tips

Created on May 13, 2022

Updated on August 8, 2023

Tags: shell

:ID: 3453ED9D-38E6-4EDA-9652-189BCABA429F

Just some various tips for writing shell scripts or doing cmd line stuff.

# Get the first n lines from STDOUT

We can use the head program for this. head will return the first n lines of a file, but can be used with STDOUT as well.

Example: get the docker image ID for the last created image. 

docker images --format "{{.ID}}" | head -n 1

See also man head

# Forward arguments

Forward all arguments passed to a script to some other program called by the script.

$@ Represents all arguments, so in a script that does something with npm, for example, we can forward all the arguments passed to the script to npm: 

npm $@

# Default arguments

If a parameter is unset or null, use a default value.

See also https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash.html#Shell-Parameter-Expansion 

npm run test ${FILE_PATH:-'tests/unit/**/*.spec.*(ts|js)'}

We can also set the parameter by using = instead of - above.

# List open files

Use lsof

It will include open sockets since anything IO is considered a file on a UNIX OS.

# Examples

# List Internet files

lsof -i

…on a specific port 

lsof -i :3000

…or list all processes that are listening on some port 

lsof -i -P -n | grep LISTEN

# Test if process is running

Here I wanted to test if reaper was already running before trying to start it. I don’t want two instances of the program. Note the & sends it to a subshell (child process) 

if ! [[ "`pidof -x reaper -o %PPID`" ]];
  then reaper &
fi

# Prevent scripts from going hog wild

Put set -euo pipefail as the first line of the script

  • -e : Fails the entire script if any command has a non-zero exit status (ie, prevent subsequent commands from being executed)
  • -o : Use the return code of any command that failed in a pipeline, not just the last successful one.
  • -u : Makes unbound variables throw an error.

# Repeat / for loops

I found this useful to test flaky RSpec tests 

for i in {1..5}
  do
  echo $i
done

# Get info about binary in your path

# whereis

Returns the location of the binary and the man page.

# Look up alias

Use the type or which utilities 

type grep
#=> grep is an alias for grep --color=auto --exclude-dir={.bzr,CVS,.git,.hg,.svn,.idea,.tox}

# Time how long a command takes

Use the time utility: 

time sleep 1
real    0m1.033s
user    0m0.001s
sys     0m0.000sp
  • real is actual clock time
  • user is CPU time on user processes
  • sys is CPU time on kernel processes

see also https://stackoverflow.com/a/556411

If the real time is much higher compared to CPU times then there is some I/O waiting happening.

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