Rico Sta. Cruz
GitHub
commit
c65dc4811d
88
bash.md
88
bash.md
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@ -78,9 +78,9 @@ See: [Functions](#functions)
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{: id='conditionals-example'}
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```bash
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if [ -z "$string" ]; then
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if [[ -z "$string" ]]; then
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echo "String is empty"
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elif [ -n "$string" ]; then
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elif [[ -n "$string" ]]; then
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echo "String is not empty"
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fi
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```
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@ -239,7 +239,7 @@ done
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### Reading lines
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```bash
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cat file.txt | while read line; do
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< file.txt | while read line; do
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echo $line
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done
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```
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@ -285,7 +285,7 @@ myfunc() {
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```
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```bash
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result=$(myfunc)
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result="$(myfunc)"
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```
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### Raising errors
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@ -321,19 +321,21 @@ Conditionals
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### Conditions
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Note that `[[` is actually a command/program that returns either `0` (true) or `1` (false). Any program that obeys the same logic (like all base utils, such as `grep(1)` or `ping(1)`) can be used as condition, see examples.
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| Condition | Description |
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| --- | --- |
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| `[ STRING = STRING ]` | Equal |
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| `[ STRING != STRING ]` | Not Equal |
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| `[ -z STRING ]` | Empty string |
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| `[ -n STRING ]` | Not empty string |
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| `[[ -z STRING ]]` | Empty string |
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| `[[ -n STRING ]]` | Not empty string |
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| `[[ STRING == STRING ]]` | Equal |
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| `[[ STRING != STRING ]]` | Not Equal |
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| --- | --- |
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| `[ NUM -eq NUM ]` | Equal |
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| `[ NUM -ne NUM ]` | Not equal |
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| `[ NUM -lt NUM ]` | Less than |
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| `[ NUM -le NUM ]` | Less than or equal |
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| `[ NUM -gt NUM ]` | Greater than |
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| `[ NUM -ge NUM ]` | Greater than or equal |
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| `[[ NUM -eq NUM ]]` | Equal |
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| `[[ NUM -ne NUM ]]` | Not equal |
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| `[[ NUM -lt NUM ]]` | Less than |
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| `[[ NUM -le NUM ]]` | Less than or equal |
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| `[[ NUM -gt NUM ]]` | Greater than |
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| `[[ NUM -ge NUM ]]` | Greater than or equal |
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| --- | --- |
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| `[[ STRING =~ STRING ]]` | Regexp |
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| --- | --- |
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@ -341,43 +343,55 @@ Conditionals
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| Condition | Description |
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| --- | --- |
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| `[ -o noclobber ]` | If OPTIONNAME is enabled |
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| `[[ -o noclobber ]]` | If OPTIONNAME is enabled |
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| --- | --- |
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| `[ ! EXPR ]` | Not |
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| `[ X ] && [ Y ]` | And |
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| `[ X ] || [ Y ]` | Or |
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| `[[ ! EXPR ]]` | Not |
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| `[[ X ]] && [[ Y ]]` | And |
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| `[[ X ]] || [[ Y ]]` | Or |
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### File conditions
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| Condition | Description |
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| --- | --- |
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| `[ -e FILE ]` | Exists |
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| `[ -r FILE ]` | Readable |
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| `[ -h FILE ]` | Symlink |
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| `[ -d FILE ]` | Directory |
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| `[ -w FILE ]` | Writable |
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| `[ -s FILE ]` | Size is > 0 bytes |
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| `[ -f FILE ]` | File |
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| `[ -x FILE ]` | Executable |
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| `[[ -e FILE ]]` | Exists |
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| `[[ -r FILE ]]` | Readable |
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| `[[ -h FILE ]]` | Symlink |
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| `[[ -d FILE ]]` | Directory |
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| `[[ -w FILE ]]` | Writable |
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| `[[ -s FILE ]]` | Size is > 0 bytes |
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| `[[ -f FILE ]]` | File |
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| `[[ -x FILE ]]` | Executable |
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| --- | --- |
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| `[ FILE1 -nt FILE2 ]` | 1 is more recent than 2 |
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| `[ FILE1 -ot FILE2 ]` | 2 is more recent than 1 |
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| `[ FILE1 -ef FILE2 ]` | Same files |
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| `[[ FILE1 -nt FILE2 ]]` | 1 is more recent than 2 |
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| `[[ FILE1 -ot FILE2 ]]` | 2 is more recent than 1 |
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| `[[ FILE1 -ef FILE2 ]]` | Same files |
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### Example
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```bash
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if ping -c 1 google.com; then
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echo "It appears you have a working internet connection"
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fi
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````
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```bash
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if grep -q 'foo' ~/.bash_history; then
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echo "You appear to have typed 'foo' in the past"
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fi
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```
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```bash
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# String
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if [ -z "$string" ]; then
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if [[ -z "$string" ]]; then
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echo "String is empty"
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elif [ -n "$string" ]; then
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elif [[ -n "$string" ]]; then
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echo "String is not empty"
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fi
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```
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```bash
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# Combinations
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if [ X ] && [ Y ]; then
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if [[ X ]] && [[ Y ]]; then
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...
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fi
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```
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@ -393,11 +407,13 @@ if [[ "A" =~ "." ]]
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```
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```bash
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if (( $a < $b ))
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if (( $a < $b )); then
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echo "$a is smaller than $b"
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fi
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```
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```bash
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if [ -e "file.txt" ]; then
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if [[ -e "file.txt" ]]; then
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echo "file exists"
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fi
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```
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@ -542,7 +558,7 @@ python hello.py &>/dev/null # stdout and stderr to (null)
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```
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```bash
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python hello.py < foo.txt
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python hello.py < foo.txt # feed foo.txt to stdin for python
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```
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### Inspecting commands
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@ -664,3 +680,5 @@ pwd # /home/user/foo
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* [Bash-hackers wiki](http://wiki.bash-hackers.org/) _(bash-hackers.org)_
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* [Shell vars](http://wiki.bash-hackers.org/syntax/shellvars) _(bash-hackers.org)_
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* [Learn bash in y minutes](https://learnxinyminutes.com/docs/bash/) _(learnxinyminutes.com)_
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* [Bash Guide](http://mywiki.wooledge.org/BashGuide) _(mywiki.wooledge.org)_
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* [ShellCheck](https://www.shellcheck.net/) _(shellcheck.net)_
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