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Command Line

All about command line on Linux.

References:

Softlinks & Hardlinks

Softlinks are just a reference to a file path existing somewhere. Editing it, will not affect the file it's linked to.

Hardlinks in turn is a direct reference to the target file path, thus, editing it will also edit the file it's linked to.

ln -s file-path.txt ~/soft-symlinked-file.txt

If target file is deleted, soft symlinked file will still exist but will be broken.

ln file-path.txt ~/hard-symlinked-file.txt

If target file is deleted, hard links will still be there with the latest content.

Open terminal commands in EDITOR

ctrl-x ctrl-e will open the current command in an EDITOR for multi-line editing

NOHUP (allow process to stay alive on terminal closing)

nohup command &

See also, jobs, disown, and command &

Run on command while staying on same DIR

# do something in current dir
(cd /some/other/dir && other-command)
# continue in original dir

Output of a command, can be treated as a file, within <()

diff ~/some_file.txt <(ssh host && cat ~/some_file.txt)

SSH Config

Few optimizations on SSH config. > settings contains config to avoid dropped connections in certain network environments, uses compression (which is helpful with scp over low-bandwidth connections), and multiplex channels to the same server with a local control file:

  TCPKeepAlive=yes
  ServerAliveInterval=15
  ServerAliveCountMax=6
  Compression=yes
  ControlMaster auto
  ControlPath /tmp/%r@%h:%p
  ControlPersist yes

128KB limit on command line

This "Argument list too long" error is common when wildcard matching large numbers of files. (When this happens alternatives like find and xargs may help.)

Tools for working with json, yaml, csv

JSON YAML CSV

ZCAT

Print data from gzip compressed files.

- Print the uncompressed contents of a gzipped file to the standard output:
  zcat file.txt.gz

- Print compression details of a gzipped file to the standard output:
  zcat -l file.txt.gz

See also zless, zmore, zgrep.

Best practices on shell scripts

Strict Mode

grep alternatives

ack ag ripgrep

xargs

takes whitespace separated strings from stdin and converts into command-line arguments

echo "home tmp" | xargs ls

will run ls home tmp

useful tips

xargs -n 1 will run a separate process per each input. Pass in -P n to set max number of parallel processes

awk

tiny language to manipulate columns of data.

Example:

ps | awk -F, '{print $3}

sed

most often used for replacements

sed s/match/replace/g file.txt
sed 5d # deletes the 5th line
sed /cat/d # deletes the line matching cat
sed -n 5,30p # prints lines between 5 to 30
sed 'i 17 panda' # insert panda on line 17
sed G # double space a file

bash tricks

  • Commands that start with space does not go to the history

  • !! run the last command ran as sudo

  • ctrl + r : search history

  • ctrl + z : suspend a process, fg to get back to it in foreground bg in background

  • fc : fix command, opens the last ran command in $EDITOR

disk usage

du # tells how much diskspace a /directory occupies
  -h # human readable format
  -s # summary

df # tells the free space over each partitions
  -h # human readable format

ncdu # interactive prompt going over files/directories showing the sizes
  -h # human readable format

watch

rerun commands intermittently (2 seconds by def)

cal

calendar

xsel/xclip

paste contents into terminal from system clipboard

comm

find common lines in two sorted files

file

figure out the mime details of the file

ts

add timestamps

kill

kill is not only used to kill processes. It can be used to send any signal to any program

kill -l

list all signals

pkill

kills a process based on pattern matched and pid found

pkill -f firefox

killall -SIGNAL name

signals all processes to be killed with NAME

useful flags:

  • -w : wait for process to die
  • -i : ask before signalling

zcat

cats a gzipped file

lsof

lists open files

lsof -i

lists open network sockets

lsof | grep deleted

lists deleted files

netstat

another way of showing open sockets