command line

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the useful command line on linux

the test command platform is ubuntu 20.11.3 of armbian

common

  • in bash , use the Tab to complete arguments or list all available commands and ctrl - r to search through command histor

  • in bash , use the ctrl-w to delete last wrd, and ctrl-u to delete the all line. ctrl-e to move cursor to end of line. ctrl-k to delete cursor to the end line, ctrl-l to clear the screen

  • use the history to see recent commands. follow with !n to excute again. !$ for last argument and !! for last command.

  • use the cd to get directory, access files relative to you home directory with the ~ prefix, in sh scripts refer to eht home dirctory as $HOME

  • use the cd - can to go back to previous working directory

  • pstree -p is a helpful display of the process tree

  • use the pgrep and pkill to find or signal processes by name (-f is helpful)

  • use nohup or disown to keep running a background process forever

  • check what processes are listening via netstat -lntp or ss -plat or lsof -iTCP -sTCP:LISTEN -p -n

  • use the lsof and fuser for open sockets and files

  • use the uptime or w to know how long time the system been running

  • use the alias to create shortcuts for commonly used commands. for example alias ll='ls -latr' create a new alias ll, save aliases, shell settings ,and functions you commonly use in ~/.bashrc

  • put the settings of enviroment variables as well as commands tha should be excuted when you login in ~/.bash_profile

  • in bash scripts, use set -x(or the variant set -v which logs raw input, includeing unexpanded variables and comments) for debugging out
    put , use stric model unless you hanve a good reason not to: use set -e to abort on error (nonzero exit code) . use set -u to detect unset variable usages. consider set -o popefail too, to abort on errors withon ppes. for more involved scripts also use trap on EXIT or ERR, a useful habit is start a script like this, which will make it detect an abort on common error and print a message:

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set -e pipefail
trap "echo 'error : Script failed: see faild comman above'" ERR
  • in bash scripts, subshells are convenient ways to group commands, A common example is to temporatily move to a different working directory
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# do somethin in current dir
(cd /root/script/dir && other-command)
# continue ub original dir
  • the output of a command can be treated like a file via <(some command) (know as process substitution) for example compare local /etc/hosts with a remote one:
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diff /etc/hosts <(ssh somehost cat /etc/hosts)

  • a simple web server for all files in the current directory python -m http.server 8888

  • for switching the shell to another user use su username or su - username the default username to root.

Processing files and date

  • to locate a file by name in the current directory find . -iname "*something*" to find a file anywhere by name, use locate somethin

  • user the grep -r to search throuth source or date file. including

  • to conver HTML to text: lynx -dump -stdin

  • for markdown , HTML and all kinds for document conversion, try pandoc, for example to convert a markdown document to word format: pandoc README.md --form markdown --to docx -o tmp.docx

  • use xmlstarlet to handle XML ,it is old but good.

  • for JSON use jq, for YAML use shyaml, for Excel or CSV files use scvkit.

  • you can set a specitic command;s environment by refixing its invocation with the enviroment var iable settings, as in TZ=Asia/Shanghai date

  • date and time : to get the current date and time in the helful ISO 8601 format. use date -u +"%Y-%m-%dT%H:%M:%SZ", to manipluate date and time expressions , ue dateadd, datadiff, strtime.

  • use zless zmore zcat and zgrep to operate on compressed fils.

System debugging

  • for web debugging curl and curl -I are handy, or their wget equivalents, or the more modern httpie

  • use the top or htop wo show current cpu/disk status, iostat and iotop use the iostat -mxz 15 for basic cpu and detiled per partion disk stas and performance insight

  • use netstat and ss can show network donnection details.

  • for a quick overview of what’s happening on a system. dstat is expecially useful, for broadest overview with details use glances

  • to know memory status, run and understand the output of free and vmstat , in particular be aware the cached values is memeory held by the linux kernel as file cache, so effectively counts toward the free value

  • for looking at why a disk is full ncdu saves time over the usual commands lie du -sh *

  • to find which socket or process is using band width try iftop or nethogs

  • the ab tool is helpful for quick-and-dirty checking of web server performance . for more complex load testing. try siege

  • for more serious network debugging wireshark tshark or ngrep