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Fluency on the command line is a skill often neglected or considered arcane, but it improves your flexibility and productivity as an engineer in both obvious and subtle ways. This is a selection of notes and tips on using the command-line that I've found useful when working on Linux. Some tips are elementary, and some are fairly specific, sophisticated, or obscure. This page is not long, but if you can use and recall all the items here, you know a lot.
but given the interest there, it seems it's worth using Github, where people more talented than I can readily suggest improvements. If you see an error or something that could be better, please submit an issue or PR! (Of course please review the meta section and existing PRs/issues first.)
- This guide is both for beginners and the experienced. The goals are *breadth* (everything important), *specificity* (give concrete examples of the most common case), and *brevity* (avoid things that aren't essential or digressions you can easily look up elsewhere). Every tip is essential in some situation or significantly saves time over alternatives.
- This is written for Linux, with the exception of the "[MacOS only](#macos-only)" section. Many of the other items apply or can be installed on other Unices or MacOS (or even Cygwin).
- It includes both "standard" Unix commands as well as ones that require special package installs -- so long as they are important enough to merit inclusion.
- To keep this to one page, content is implicitly included by reference. You're smart enough to look up more detail elsewhere once you know the idea or command to Google. Use `apt-get`/`yum`/`dnf`/`pacman`/`pip`/`brew` (as appropriate) to install new programs.
- Learn basic Bash. Actually, type `man bash` and at least skim the whole thing; it's pretty easy to follow and not that long. Alternate shells can be nice, but Bash is powerful and always available (learning *only* zsh, fish, etc., while tempting on your own laptop, restricts you in many situations, such as using existing servers).
- Learn at least one text-based editor well. Ideally Vim (`vi`), as there's really no competition for random editing in a terminal (even if you use Emacs, a big IDE, or a modern hipster editor most of the time).
- Know how to read documentation with `man` (for the inquisitive, `man man` lists the section numbers, e.g. 1 is "regular" commands, 5 is files/conventions, and 8 are for administration). Find man pages with `apropos`. Know that some commands are not executables, but Bash builtins, and that you can get help on them with `help` and `help -d`.
- Learn about redirection of output and input using `>` and `<` and pipes using `|`. Know `>` overwrites the output file and `>>` appends. Learn about stdout and stderr.
- Learn about file glob expansion with `*` (and perhaps `?` and `{`...`}`) and quoting and the difference between double `"` and single `'` quotes. (See more on variable expansion below.)
- Basic file management: `ls` and `ls -l` (in particular, learn what every column in `ls -l` means), `less`, `head`, `tail` and `tail -f` (or even better, `less +F`), `ln` and `ln -s` (learn the differences and advantages of hard versus soft links), `chown`, `chmod`, `du` (for a quick summary of disk usage: `du -hk *`). For filesystem management, `df`, `mount`, `fdisk`, `mkfs`, `lsblk`.
- Learn to use `apt-get`, `yum`, `dnf` or `pacman` (depending on distro) to find and install packages. And make sure you have `pip` to install Python-based command-line tools (a few below are easiest to install via `pip`).
- In Bash, use **ctrl-w** to delete the last word, and **ctrl-u** to delete all the way back to the start of the line. Use **alt-b** and **alt-f** to move by word, **ctrl-k** to kill to the end of the line, **ctrl-l** to clear the screen. See `man readline` for all the default keybindings in Bash. [There are a lot](http://ss64.com/bash/syntax-keyboard.html). For example **alt-.** inserts last argument of previous command, and **ctrl-x *** [expands a glob](http://superuser.com/questions/215950/how-to-expand-on-bash-command-line).
- To see recent commands, `history`. There are also many abbreviations such as `!$` (last argument) and `!!` last command, though these are often easily replaced with **ctrl-r** and **alt-.**.
- If you are halfway through typing a command but change your mind, hit **alt-#** to add a `#` at the beginning and enter it as a comment (or use **ctrl-a**, **#**, **enter**). You can then return to it later via command history.
- Use `xargs` (or `parallel`). It's very powerful. Note you can control how many items execute per line (`-L`) as well as parallelism (`-P`). If you're not sure if it'll do the right thing, use `xargs echo` first. Also, `-I{}` is handy. Examples:
- In Bash scripts, use `set -x` for debugging output. Use strict modes whenever possible. Use `set -e` to abort on errors. Use `set -o pipefail` as well, to be strict about errors (though this topic is a bit subtle). For more involved scripts, also use `trap`.
- In Bash scripts, subshells (written with parentheses) are convenient ways to group commands. A common example is to temporarily move to a different working directory, e.g.
- In Bash, note there are lots of kinds of variable expansion. Checking a variable exists: `${name:?error message}`. For example, if a Bash script requires a single argument, just write `input_file=${1:?usage: $0 input_file}`. Arithmetic expansion: `i=$(( (i + 1) % 5 ))`. Sequences: `{1..10}`. Trimming of strings: `${var%suffix}` and `${var#prefix}`. For example if `var=foo.pdf`, then `echo ${var%.pdf}.txt` prints `foo.txt`.
- In Bash, redirect both standard output and standard error via: `some-command >logfile 2>&1`. Often, to ensure a command does not leave an open file handle to standard input, tying it to the terminal you are in, it is also good practice to add `</dev/null`.
- Use `man ascii` for a good ASCII table, with hex and decimal values. For general encoding info, `man unicode`, `man utf-8`, and `man latin1` are helpful.
- Use `screen` or [`tmux`](https://tmux.github.io/) to multiplex the screen, especially useful on remote ssh sessions and to detach and re-attach to a session. A more minimal alternative for session persistence only is `dtach`.
- It can be useful to make a few optimizations to your ssh configuration; for example, this `~/.ssh/config` contains settings to avoid dropped connections in certain network environments, use compression (which is helpful with scp over low-bandwidth connections), and multiplex channels to the same server with a local control file:
- A few other options relevant to ssh are security sensitive and should be enabled with care, e.g. per subnet or host or in trusted networks: `StrictHostKeyChecking=no`, `ForwardAgent=yes`
- To get the permissions on a file in octal form, which is useful for system configuration but not available in `ls` and easy to bungle, use something like
- For interactive selection of values from the output of another command, use [`percol`](https://github.com/mooz/percol) or [`fzf`](https://github.com/junegunn/fzf).
- For running a command with privileges, use `sudo` (for root) or `sudo -u` (for another user). Use `su` or `sudo bash` to actually run a shell as that user. Use `su -` to simulate a fresh login as root or another user.
- To locate a file by name in the current directory, `find . -iname '*something*'` (or similar). To find a file anywhere by name, use `locate something` (but bear in mind `updatedb` may not have indexed recently created files).
- For Amazon S3, [`s3cmd`](https://github.com/s3tools/s3cmd) is convenient and [`s4cmd`](https://github.com/bloomreach/s4cmd) is faster. Amazon's [`aws`](https://github.com/aws/aws-cli) is essential for other AWS-related tasks.
- Know that locale affects a lot of command line tools in subtle ways, including sorting order (collation) and performance. Most Linux installations will set `LANG` or other locale variables to a local setting like US English. But be aware sorting will change if you change locale. And know i18n routines can make sort or other commands run *many times* slower. In some situations (such as the set operations or uniqueness operations below) you can safely ignore slow i18n routines entirely and use traditional byte-based sort order, using `export LC_ALL=C`.
- Know basic `awk` and `sed` for simple data munging. For example, summing all numbers in the third column of a text file: `awk '{ x += $3 } END { print x }'`. This is probably 3X faster and 3X shorter than equivalent Python.
- Know `sort`'s options. For numbers, use `-n`, or `-h` for handling human-readable numbers (e.g. from `du -h`). Know how keys work (`-t` and `-k`). In particular, watch out that you need to write `-k1,1` to sort by only the first field; `-k1` means sort according to the whole line. Stable sort (`sort -s`) can be useful. For example, to sort first by field 2, then secondarily by field 1, you can use `sort -k1,1 | sort -s -k2,2`.
- If you ever need to write a tab literal in a command line in Bash (e.g. for the -t argument to sort), press **ctrl-v****[Tab]** or write `$'\t'` (the latter is better as you can copy/paste it).
- The standard tools for patching source code are `diff` and `patch`. See also `diffstat` for summary statistics of a diff. Note `diff -r` works for entire directories. Use `diff -r tree1 tree2 | diffstat` for a summary of changes.
- To convert text encodings, try `iconv`. Or `uconv` for more advanced use; it supports some advanced Unicode things. For example, this command lowercases and removes all accents (by expanding and dropping them):
- For web debugging, `curl` and `curl -I` are handy, or their `wget` equivalents, or the more modern [`httpie`](https://github.com/jakubroztocil/httpie).
- To know disk/cpu/network status, use `iostat`, `netstat`, `top` (or the better `htop`), and (especially) `dstat`. Good for getting a quick idea of what's happening on a system.
- For a more in-depth system overview, use [`glances`](https://github.com/nicolargo/glances). It presents you with several system level statistics in one terminal window. Very helpful for quickly checking on various subsystems.
- To know memory status, run and understand the output of `free` and `vmstat`. In particular, be aware the "cached" value is memory held by the Linux kernel as file cache, so effectively counts toward the "free" value.
- Java system debugging is a different kettle of fish, but a simple trick on Oracle's and some other JVMs is that you can run `kill -3 <pid>` and a full stack trace and heap summary (including generational garbage collection details, which can be highly informative) will be dumped to stderr/logs.
- Know about `strace` and `ltrace`. These can be helpful if a program is failing, hanging, or crashing, and you don't know why, or if you want to get a general idea of performance. Note the profiling option (`-c`), and the ability to attach to a running process (`-p`).
- Use `/proc`. It's amazingly helpful sometimes when debugging live problems. Examples: `/proc/cpuinfo`, `/proc/xxx/cwd`, `/proc/xxx/exe`, `/proc/xxx/fd/`, `/proc/xxx/smaps`.
- For deeper systems and performance analyses, look at `stap` ([SystemTap](https://sourceware.org/systemtap/wiki)), [`perf`](http://en.wikipedia.org/wiki/Perf_(Linux)), and [`sysdig`](https://github.com/draios/sysdig).
- It is remarkably helpful sometimes that you can do set intersection, union, and difference of text files via `sort`/`uniq`. Suppose `a` and `b` are text files that are already uniqued. This is fast, and works on files of arbitrary size, up to many gigabytes. (Sort is not limited by memory, though you may need to use the `-T` option if `/tmp` is on a small root partition.) See also the note about `LC_ALL` above and `sort`'s `-u` option (left out for clarity below).
- Use `grep . *` to visually examine all contents of all files in a directory, e.g. for directories filled with config settings, like `/sys`, `/proc`, `/etc`.
- Use `xargs` or `parallel` whenever you can. Note you can control how many items execute per line (`-L`) as well as parallelism (`-P`). If you're not sure if it'll do the right thing, use xargs echo first. Also, `-I{}` is handy. Examples:
- Say you have a text file, like a web server log, and a certain value that appears on some lines, such as an `acct_id` parameter that is present in the URL. If you want a tally of how many requests for each `acct_id`:
-`sponge`: read all input before writing it, useful for reading from then writing to the same file, e.g., `grep -v something some-file | sponge some-file`
- Be aware MacOS is based on BSD Unix, and many commands (for example `ps`, `ls`, `tail`, `awk`, `sed`) have many subtle variations from Linux, which is largely influenced by System V-style Unix and GNU tools. You can often tell the difference by noting a man page has the heading "BSD General Commands Manual." In some cases GNU versions can be installed, too (such as `gawk` and `gsed` for GNU awk and sed). If writing cross-platform Bash scripts, avoid such commands (for example, consider Python or `perl`) or test carefully.
With the exception of very small tasks, code is written so others can read it. With power comes responsibility. The fact you *can* do something in Bash doesn't necessarily mean you should! ;)
