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active/software_bash/bash.md

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+# Bash Tips
+
+- [Bash Tips](#bash-tips)
+  - [bashrc](#bashrc)
+    - [Eternal History](#eternal-history)
+    - [Global bashrc](#global-bashrc)
+    - [User Environment](#user-environment)
+    - [User Aliases](#user-aliases)
+  - [Check if executable exists](#check-if-executable-exists)
+  - [Check if file exists](#check-if-file-exists)
+  - [Check if last command exit code was zero](#check-if-last-command-exit-code-was-zero)
+
+## bashrc
+
+### Eternal History
+
+```bash
+# Eternal bash history.
+# ---------------------
+# Undocumented feature which sets the size to "unlimited".
+# http://stackoverflow.com/questions/9457233/unlimited-bash-history
+export HISTFILESIZE=-1
+export HISTSIZE=-1
+export HISTTIMEFORMAT="[%F %T] "
+# Change the file location because certain bash sessions truncate .bash_history file upon close.
+# http://superuser.com/questions/575479/bash-history-truncated-to-500-lines-on-each-login
+export HISTFILE=~/.bash_eternal_history
+# Force prompt to write history after every command.
+# http://superuser.com/questions/20900/bash-history-loss
+PROMPT_COMMAND="history -a; $PROMPT_COMMAND"
+```
+
+### Global bashrc
+
+```bash
+# Source global definitions
+if [ -f /etc/bashrc ]; then
+    . /etc/bashrc
+fi
+```
+
+### User Environment
+
+```bash
+# User specific environment
+if ! [[ "$PATH" =~ "$HOME/.local/bin:$HOME/bin:" ]]; then
+    PATH="$HOME/.local/bin:$HOME/bin:$PATH"
+fi
+export PATH
+```
+
+### User Aliases
+
+Allows custom aliases in ~/.bashrc.d
+
+```bash
+# User specific aliases and functions
+if [ -d ~/.bashrc.d ]; then
+    for rc in ~/.bashrc.d/*; do
+        if [ -f "$rc" ]; then
+            . "$rc"
+        fi
+    done
+fi
+unset rc
+```
+
+## Check if executable exists
+
+```bash
+if command -v aws 2>&1 >/dev/null
+then
+    echo "aws command found"
+else
+    echo "aws could not be found"
+fi
+```
+
+## Check if file exists
+
+```bash
+FILE=/etc/resolv.conf
+if [ -f "$FILE" ]; then
+    echo "$FILE exists."
+else 
+    echo "$FILE does not exist."
+fi
+```
+
+## Check if last command exit code was zero
+
+```bash
+cat /i/do/not/exit
+
+if [ $? -eq 0 ]
+then
+  echo "Success."
+else
+  echo "Failure." >&2
+fi
+```