What I know about shells

1. General information
2. Sed
3. Shell, terminal, console, command line
4. .bash_profile vs .bashrc
5. Tty, pty, interactive vs non-interactive
6. Login vs. non-login shells
7. Use bashrc also on login shell
8. Parallelize xargs
9. Discrepencies between /bin/sh versions

General information

• Xargs for creating positionnals through stdin included_figures.sh | xargs git add

• Avoid the sub-shell problem when while read a while read a; do ...; done < <(ls) pour éviter d’avoir le while dans un sous-shell C.f. Process Substitution dans man bash

• Parameter Expansion (man bash -> /Parameter Expansion) ${a:-text} if $a isn’t set, the${} expression returns “text” ${a:+text} if $a isn’t set, the${} expression returns nothing ${a:=text} same but $a is also set to “text” ${a// /_/} substitutes all spaces to _ in the content of a

• iterate on each space-separated elements of a list for e in echo "a b c d e"; do echo $e done

• To know if a variable is empty or not (man test): -n STRING the length of STRING is nonzero -z STRING the length of STRING is zero

Sed

Substitute dots to _ in filenames:

for f in *; do mv "$f" "`echo $f | sed 's/\(.*\)\.\(.*\)\(\.[a-z]*\)$/\1_\2\3/g'`"; done

Shell, terminal, console, command line

• Terminal text input/output environment (also means a specific file in Unix world) ttys001, ttys002… are Unix “terminal” files. These are provided by the kernel
• Virtual/emulated terminal layer over the plain Unix terminal (iTerm2, etc)
• Console local physical terminal (with screen and keyboard)
• Shell when launching a terminal (ttys001…), it is the program that allows to launch other programs (commands like ls)
• Command-line shell A shell specialized for interpreting commands
• Bash one of the many command-line shells
• tty: Teletype, meaning interactive terminal, as opposed to tty-less/headless terminal.
• pty: pseudo-teletype is a tty that does not directly talk to a end-user terminal but instead talks to another terminal. Most terminal emulators such as iTerm2, xterm… use a pty instead of a tty.

What happens when I want to launch some bourne-shell commands:

1. Open a terminal emulator
2. The terminal emulator will open a ttys001 for me
3. The terminal emulator will then execute a command-line shell (/bin/sh)
4. then I can type my commands

.bash_profile vs .bashrc

• .bash_profile (and .profile) when login shell
• .bashrc when interactive shell (tty/pty)

A login shell is a shell where $0 == “-”

Tty, pty, interactive vs non-interactive

node -p -e "Boolean(process.stdout.isTTY)"
[ -t 1 ]

Login vs. non-login shells

An interactive login shell is the shell launched when using ssh, iTerm2 or xterm. In interactive login shell, .profile and .bash_profile are used. To know if you are in a login shell:

$ echo $0
-bash

Interactive non-login shell: .bashrc is used

$ echo $0
bash

Use bashrc also on login shell

What I did was to add a simple thing to .profile:

if [ -f ~/.bashrc ]; then
    . ~/.bashrc
fi

Parallelize xargs

/usr/local/bin/code --list-extensions | xargs -L 1 -P 4 /usr/local/bin/code-insiders --install-extension

Discrepencies between /bin/sh versions

On Ubuntu, /bin/sh is hard-linked to /bin/dash. On macOS, /bin/sh is hard-linked to /bin/bash. None of them are the original “Bourne shell”!

On top of that, make always uses /bin/sh, which means there are discrepancies between Ubuntu and macOS:

• bash knows about <<< (here-string), but dash doesn’t
• bash knows about <() (process substitution), but dash doesn’t
• bash knows about ${a/b/c} (glob variable content replace) but dash doesn’t

  '
  bar
  % dash -c 'A=foo; echo ${A/foo/bar}'
  dash: 1: Bad substitution
  

**Workaround**: force `make` to use a well known shell, e.g. in your Makefile:

```sh
SHELL := /bin/bash